Heating Simulator

This is fantastic! I have wanted to do this sort of thing for ages, and now thanks to my niece Ellie who constructed it, it’s working!    Next one on the drawing board – pressure drops and pipe sizes to play with.

Click here for a full screen version of the Simulator below

What is the purpose of this simulator?

What happens when we slow-down the flow rate?   What would happen if we double the radiator size?   What happens to the water temperatures and the deltaT if we change these things?

To get a deeper and intuitive understanding of heat, water flow rates and temperatures, you can ‘play’ with this Simulator to find out.

Fault finding has changed over the years. In many ways servicing has become more efficient. Diagnosis has become very product-specific and often done to laid-down procedures.  A downside of this is that engineers are not given the opportunity to go ‘off piste’ and learn for themselves.

Further to a teaching aid to get people thinking and talking, you could also use this as a tool to estimate how much heat your radiators could emit if working at lower (heat pump friendly) temperatures.  You just need to list out your radiator types and sizes and compare temperatures they would operate at given different heat outputs

So, there you have it.   If you wish, you can simply change the input values and see what happens.

Points to note

  • Your system configuration and controls must be set right if you are to achieve the low operating water temperatures that you are aiming for.
  • In practice, you might tend to get uneven temperatures (cool patches) with very big radiators. E.g., a 2.5 sq.m. radiator might act more like 2.2 sq.m. due to these ‘cool’ patches
  • Long rectangular radiators give out a bit more heat per sqm than square ones.
    (say 5-10% variation??)
  • The values are good non-optimistic estimates, but check manufacturers data for more specific values.
  • The deltaT (difference in temperature) we are considering here is the difference between flow and return temperatures.   The same term is often used to describe the difference between room temperure and radiator temperature (e.g. dt 50°).  Don’t confuse the two.


If you want suggestions to try… re-load the page to get default mid-range values.   Now observe the resulting water temperatures that we would see in real life.

Now try changing any one of the above and see what happens to the temperatures.

  • With more power input, the radiator gets hotter, and more heat is dissipated to the room.
  • If the radiator is bigger, it dissipates the same heat to the room, but at a lower temperature
  • When the water flow-rate reduces, the difference between the flow and return (deltaT, dT) increases. The flow-temperature rises and return-temperature drops. Watch the thermometers. Try increasing the heat input and also doubling the flow rate. Watch the dT.
  • If the room temperature changes, the radiator temperature changes too.  This is simply because the heat delivered depends on the difference in temperature between the radiator and the room air temperature.   So, if the water temperature is fixed, from a boiler of buffer tank, the heat emitted increases if you open a window and cool the room.


If you have an existing radiator system, and are thinking of adding a heat pump, and are concerned about energy efficiency, then you may want to operate on a relatively low temperature to keep the heat pump efficient

  • Go to each radiator and enter the approximate area and radiator type
  • Now change the heat input (Kw) until you get an average radiator temperature of say 40°C (e.g. 42.5°C flow and 37.7° return temperatures)  (40°C = 104°F)
  • Now adjust the flow rate until the dt is about 5 degrees  (9F)
  • Note down the heat (kW) and flow rate (lit/min) for this radiator
  • Now do this for every radiator in your house
  • Total them all up

From this you have estimated how many kW’s of heat you can ‘pump’ into your radiators so that they will operate at an energy-efficient temperature for a heat pump.

You may of course choose to run your heat pump at warmer temperatures, and accept that your COP may not be as good.

Be mindful that some heat pumps are relatively large, so not all systems would work efficiently at such low operating temperatures, due mostly to them stopping and starting a lot.

For anyone still reading.. I thought hard about the inputs and outputs on the simulator.  i.e. should the inputs be area and temperature, giving kW output? or
should the inputs be area and kW heat, giving radiator temperature result.
I concluded that the 2nd option is a better way of thinking about it from a learning point of view.   e.g. the circulating water temperature is the final end-result of the various inputs (kWs, area etc)

Radiant temperature.  Elephant in the room?

In an un-heated exhibition room at the Centre for Alternative Technology many years ago, there was one of those good-for-your-posture kneeling chairs.  Surrounding this chair was a curved hardboard sheet covered with Aluminium foil. You were invited to sit, surrounded by a shiny foil surface. The effect was quite dramatic, you could instantly feel the warmth reflected back from your body.

This chimed well with claims from various underfloor heating manufacturers that for the same level of comfort, you can have a slightly lower air temperature. This is due to the radiation emitted from the floor

The theory is that every surface emits radiant heat, and this depends on the surface’s absolute temperature (e.g., no radiation at -273°C). Things absorb it too, so radiation bounces back and forwards between objects.  E.g., between you and a wall.

I experienced this when I first installed some underfloor heating in a hall in Scotland. Having finished the floor (with wooden covering) in 1 day, I slept there and assumed my thermometer was incorrect because I was comfortable in a 16°C room.  It was only later that I realised that the floor’s radiation had distorted my sense of comfort.  Not that the floor felt particularly warm, but it wasn’t cold.  This was my first experience of underfloor heating.

I also came across this slide about human body cooling at an energy exhibition a long time back.  I find this one hard to believe but if it is even half true, it is showing how dramatic the effect of radiation is on comfort. Cold stone floors in very hot countries certainly seem to help at midday.

‘Radiant temperature’ is a thing, and is documented in CIBSE and ASHRAE.. There is plenty to read on the topic to support the notion.  Indeed, to quote https://www.hse.gov.uk/temperature/thermal/factors.htm
“Radiant temperature has a greater influence than air temperature on how we lose or gain heat to the environment.

Why therefore is the topic almost unheard-of in general heating speak?   And why are the standard ‘comfort’ conditions suggested to be 21°C air temperature etc. with no mention of surrounding radiation?

This particular topic came up for me while advising on the heating at a village hall. The hall is infrequently used and un-insulated, so if they choose a quick ‘blast’ of heat from cold, the cold walls and cold floors would have a negative effect on comfort.  Occupants would radiate themselves, but get little back from the surroundings.  To compensate for the lack of radiation, and to restore comfort, the air temperature may need to be set warmer. They could choose to keep the hall at a medium temperature all the time, this may add to comfort by removing the cold surfaces, but it is also a continuous loss of heat to the outside.   I started to feel that I knew nothing about heating since I had no idea how to work out how much ‘discomfort’ could be caused by cold walls’, and how much warmer the air temperature needs to be to achieve comfort.  I then quickly realised that nobody else knew either.

I got around to thinking that in constantly-heated houses, it isn’t really a problem.  The walls and surfaces can get up to around 20°C, and these all emit radiation, so there is no ‘shortfall’ of radiation.

For intermittently heated buildings, and those with stone walls and stone floors in particular, then there could at times be a considerable lack of radiation and this could lead to discomfort.

There is of course a well-established, simple and effective way of addressing this ‘shortfall’, and that is to fit infrared heaters overhead.  Indeed, they are used frequently in churches, which must be the hardest of all buildings to heat, being infrequently used and with no insulation.

However, my experience of such heaters is that they are usually sized generously by installers wanting ‘its’s nice and toasty warm’ feedback from occupants.   But many such community buildings are on a limited budget, so maybe the design brief should be – low running cost with acceptable warmth.

Any direct electric heating does however seem inefficient, and when comparing heat pumps with any direct electric heating, it is common to assume that any electric heater has a COP of only 1. i.e., it’s 100% efficient. But this is not the whole story. If a small amount of electric radiant heat can mean we can drop the air temperature say 1°C, then our heat pump could have a reduced input of 10%. (i)  So, using some radiant heat only at times of occupation, or before the wall surfaces have warmed, could be a net benefit.

The more I think about it, maybe every old intermittently-heated building should have at least a small amount of radiant heat input, but finding an installer to fit a small infra-red heater might be hard….  like asking a good chef for just one plain cheese sandwich!   And controlling it is another matter.  Radiant globe sensors do exist, but you won’t easily find them.  Hopefully, as time goes on, controls will get better.

I was pondering the development and fine-tuning of heating systems.  For cars, fuel economy is so easy to measure, so there is a natural evolution towards better efficiency, but for heating, it’s extremely difficult to measure net-effectiveness of installed heating systems.  The individual products are being developed, but on the installation front, there seems to be little incentive to drive forward little incremental improvements.  There still seems to be a long way to go.


Heat pump controls, we need better

These blogs are usually triggered when I come across issues (several times) that bother me… here goes.

Theory and practice show that the best performance for any (1) heat pump is achieved if the average water temperature that the heat pump ‘sees’ (the water temperature passing through it) is kept low (2), whilst still achieving the required level of comfort.

It is all very well installing underfloor heating or radiators that can give high heat-outputs with low circulating water temperatures.   However, the actual temperatures may deviate significantly from the theoretical, and this is highly dependent on the control system provided.  It is often assumed (by both installers and users) that the controls are capable of doing what you need… not always the case.

Whilst the SCOP rating and the predicted flow/return temperatures would seem to have the greatest bearing on energy-efficiency, it really hangs in how well it is controlled.

I am hopeful that over this decade we will see far better control systems. It strikes me that we are in a ‘middle’ phase with some claims of ‘intelligent’ control, but actually its not always helpful in practice. Furthermore, many programmers and controls are still user-unfriendly. No wonder that many are not programmed as they should be.

Whilst things have got better, I still come across installations that are clearly not ideal. E.g. inadequate controls, or sometimes too many and too complicated.  It is often very hard to estimate how worthwhile (cost-effective) any modifications would be. This would ideally involve some performance monitoring over time. So, annoyingly, many systems remain in a non-ideal state.

I don’t want to worry people unnecessarily here.  Most installers know what they are doing!  Many systems are well optimised, and do just what they say on the tin, but people still need to be mindful that it’s not always the case.

The below are some typical general examples, but there are many other offerings and options out there.

Some typical control methods

When things are simple, there are fewer potential pitfalls

Here we have an as-simple-as-it-gets system.  In this case the room control is the ‘target temperature’ or ‘auto adapt’ type. This modulates (variable inverter) the flow temperature according to room temperature so the actual water temperatures can adjust itself from anything below 30°C to above 40°C depending on the demand.  The average water temperature over the year can be surprisingly low, giving low running costs.   In this case, the Room control is supplied with the heat pump.

This is a good system for open-plan houses, and extremely well-insulated houses, but when we get into larger houses with different rooms used at different times, and radiators upstairs with underfloor downstairs, we can start running into trouble…. To be clear… It will heat adequately, but the running costs could become higher than they should.


The next level of complexity may involve a buffer cylinder.  This is simply a tank of water. It might store something in the order of up to 20 minutes worth of heat.   It is generally needed in buildings where there are many zones (many rooms), and where you may at times want only one zone on.
The simple principle is this –
* The heat pump heats the buffer cylinder to a certain temperature (variable over the year).
* Heat is pumped from this cylinder to rooms as required.

Unlike the first example where the controller ‘learns’ the lowest (and most efficient) water temperature possible, in the buffer tank case, a pre-determined temperature has to be set, and this usually uses a ‘weather compensation curve’ (put simply 50°C when very cold out, 30°C when mild, and a sliding scale between the two).
The ‘curve’ setting is usually found by trial and error, but as I am all too often finding, this is often set higher than it actually needs to be. E.g. in my example it is heated to 50°C.  Water at the radiator may (due to mixing) be at 45°C, and the thermostat in the room will ‘cycle’ the pump with on and off periods.   The radiator at a constant 35°C might suffice.

Compared to our first example, the heat pump ‘sees’ water at a considerably higher temperature, so the average COP is worse.

Ideally, the set point of the buffer would not only depend on the outside temperature, but it could reduce for certain periods of the day, and also reduce depending on the house temperature.  Many manufacturers have a facility for this, but I very rarely see it being used well.   Some controls allow two curve settings over the 24hr period.  Most don’t.

Underfloor and radiators on the same system

A next level of complexity is where mixing valves are used.  This is typical when both underfloor and radiators are used on one heat pump.

Here we can see an underfloor circuit that only needs water at the lukewarm temperature of 35°C.  Whilst our heat pump would heat this very efficiently, unfortunately the system takes all water up to 50° (less efficiently), only to mix some of it down to 35°C.   If our control was ‘cleverer’, it could drop the buffer set temperature for certain periods of time when only the floor needs heating. However, few currently available controls will cater for this.

These examples are only a few of many options and combinations. Of course, the manufacturers have thought this all through, however, it would be impractical to make controls for every circumstance.. it could be too complicated, so they tend to evolve a ‘system’ and hope it will suit most applications.

Intermittent and continuous heating

For houses that are unoccupied during the day, or offices that are vacated evening and night, there is no point keeping them warm all the time.  That said, if the heating is completely off for too many hours, then the required catch-up (heat-up) may be difficult for our heat pump.   In practice, a compromise is usually sought, with ‘occupied’ and ‘unoccupied’ temperatures.  This is of course dependent on the building, be it insulated or not, heavy stone or lightweight etc.

For constantly-heated homes, heat pump installations can be simpler.  But for intermittent heating, some extra complexity and extra capacity may be needed.

That said, for most of the year, there is spare capacity, so it may be perfectly acceptable in many situations to forego intermittency during the relatively brief coldest periods of the year. Again, good controls would be key to optimising this.     In the meantime, owners can set controls the best they can, but my feeling is that because programmers and controllers can be quite a chore to set correctly, many are not set well.

There seems still to be a lot to learn about the best way to operate heat pumps in different buildings with different situations, with respect to intermittency or constant.

A final gripe, .. for anyone trying to set a digital room thermostat to a frugal setting.  This could be in a workroom of maybe a big lofty room in an old house.  The ‘jump’ between settings of 16°, 17° or 18°C is great.  If energy-use is a minor issue, then you simply go 1° higher, but if you are keeping a tight rein on cost (e.g. a village hall), a finer control is desirable.    Manufacturers… please allow ½°C adjustment increments.  (It seems that 1°F increments are however about right). Gripe over.



  • (1)  The exception here could be a ‘transcritical’ heat pump using Refrigerant R744 CO2. These produce very hot water with relative efficiency, but don’t generally achieve the high COPs at low temperatures that other conventional types can.
  • (2)   How low can you go and still get an increase in COP? Whilst graphs show a trend of increased COP down to say 30°C, what happens below this?  I have done various ad-hoc test to the best I can, and found that some older fixed-speed designs tend to flatten off around 25-30°C.  But I have been surprised how efficient some inverter units work. This is could be to do with two things. i) better refrigerant control with an electronic expansion valve and ii) better motor power at very light loading.


Heating old buildings with heat pumps

It’s great to see how heat pumps are becoming mainstream, and increasingly the norm for newbuild.  However, I have noticed ongoing confusion relating to the use of heat pumps in old buildings.  The confusion can start with the ‘will it work’ question.  Let’s be clear – the right heat pump could be made to heat any building to any temperature we like. But the crux of the issue is the installation cost and the running cost.  The question that we should ask is – ‘can we heat old buildings and achieve acceptable energy-efficiency?’  Well… we probably can, and as time passes, it gets better.  Let’s drill into this.

On one level, it is quite simple, but on another level quite complicated.

The simple level relates mainly to the circulating heated water temperature required for poorly-insulated buildings, and this directly affects the energy-efficiency; the Coefficient of Performance (COP) – what heat you get out compared to what electricity you put in.  This has implications for the environment and also for running costs.

The complicated level relates to where you want the heat and at what times, and this is all wrapped up with the type of building and its use….   Does it have a lot of internal ‘mass’, brick walls etc i.e. is it slow to heat, and thereafter- slow to cool down.  Or is it ‘lightweight’ and mostly insulated internally – quick to heat and quick to cool.   Add to this is the occupancy; is it occupied all the time, or empty during the day?    Then there are off-peak tariffs and possible future ‘smart grid’ to consider.

Don’t put your hopes too high on finding definite answers here on how best to deploy heat pumps on old buildings for all situations, but you may glean something about where and how heat pumps are likely to be most worthwhile, and also where a good grasp of control settings may be needed.

Heat pumps can promise a fit-and-forget technology, but they sometimes need a modicum of user-engagement if the best results are to be achieved. Maybe more-so in poorly insulated buildings.

The easy stuff

As everyone should know by now, heat pumps are very efficient at producing lukewarm water – It’s an easy ride for them.  However, as the water gets hotter, the electrical power-input requirements rise too – the energy-efficiency drops and running cost rises.

The obvious solution to this is to use large-area radiators or well-designed underfloor heating. Both can often provide adequate heat with water temperatures well below ‘hot’.  (say 30 to 40°C,  85-105°F) for much of the winter.

Here is the nub of the issue relating to old buildings – it’s more difficult (and a more costly installation) to design a heat-emitter system (radiators or underfloor) to adequately heat an un-insulated building. Especially if using the low water temperatures that energy-efficient heat pumps require.  By contrast, well-insulated buildings require far less heat, so should not require overly large radiators. They are the ‘low-hanging fruit’ for heat pumps.

Conventional heating has generally been designed with high-output, for short periods.  From plumbers to householders, we are all used to ‘turning the heating on’ and feeling warm fairly quickly.  However, heat pumps are far happier operating with lower water temperatures, and this is where some confusion can start. Some patience, and also some confidence in the system is required when setting the required low water temperatures. This is because the system will respond very slowly, and you need to plan ahead and let the building to warm up gently. We are now erring towards a more continuously-enabled system, and contrary to instincts, a heat pump system operated like this can use less energy than one operated on an on/off time clock.

Intermittent heating / constant heating…… starting to get complicated

If the heating is required at a constant temperature, e.g. a retirement home, then things are very easy. The radiators may be able to operate ‘warm’ (not hot) all the time, resulting in very high energy-efficiency (high COP).

Keeping your house warm when you are out is generally seen as wasteful… why heat when you are not there?   However, if you do operate with a timer, and allow the house to cool down during the daytime or overnight, then you will need to operate the radiators at an elevated temperature so as to ‘catch-up’, and get the rooms back to a comfortable temperature.   The COP will be lower at this time, so you get less heat for your money whist operating ‘timed’.   But if timed, you need less total heat due to the lower average temperature of the house.   That said, heat is stored in the building’s fabric, so heating while you are out at work is not as wasteful as you may first think.

Both factors (on-all-the-time low, or intermittent and high) can tend to balance out. What you gain on one, you can lose on the other, and vice-versa.   The best efficiency is likely to be found with far longer run times than we are traditionally used to.    Instead of starting heating 1.5 hours before you get up, try 3 or even as much as 6 hours before, but adjust the circulating water temperature down first (or reduce the heating curve).

The thermal ‘weight’ (or mass) of the building will have a big impact here.    If the building is an old brick house (heavy), and a set-back (reduced) temperature is programmed for the night or un-occupied day, then the radiators may need to be considerably warmer to provide comfort in the evenings – not helped by the cold walls.   So, in this instance, like the race of the tortoise and the hare, the on-all-the-time tortoise can have the edge due to the good COP resulting from the constantly-enabled lukewarm radiators.

On the other hand, if the building has a lot of internal insulation, and few stone/brick inner walls, it will be thermally ‘light’. In this case the ‘catch-up’ to re-heat to a comfortable temperature should be acceptably short, so an amount of night set-back (lower room temperature at night). is likely to be advantageous.

Unfortunately, I don’t have data or evidence to say where the lines should be drawn, but have in the past been surprised how cheap to run some constantly-enabled heat pumps can be.

A word of warning here. Not all system configurations are that easy to ‘optimise’ for maximum energy-efficiency. The aim is to achieve a low operating water-temperatures and still get adequate heating to the building, if you have a weather-compensation heating-curve, it will need engaging with, and hopefully reducing. This should be easy to do, and don’t forget that the response time may be slow.    Be mindful that simply reducing TRV radiator valves is not addressing the issue here since the heat pump is not ‘seeing’ the lower water temperature that it likes.

The lowest running costs are likely to be achieved with some amount of night set-back.  However, it can be easier, with less risk of setting something wrong, to set the system to constant. You can then find the lowest circulating water temperatures that still keeps you adequately warm.  It can be worth experiment.   Try it and note down daily electricity use.

The bigger picture

The goal posts in the UK have changed greatly over the last 10 years (say 2010 to 2020).  Electricity generation is now twice as ‘green’ and beyond my expectations.

The increase in solar and wind generation, coupled with the phasing down of coal and advent of efficient gas power stations has resulted in the CO2 pollution halving in 10 years.   This means that lower COP heat pumps are more acceptable.   Added to this, heat pumps have bit-by-bit become more energy-efficient.  All-in-all, with a far better selection of energy-efficient products in a now well-established industry, we are far better positioned to install heat pump in all types of building.

Furthermore, there could also some positive advantages for using heat pumps with old ‘heavy’ buildings.  The storage of heat in the building’s fabric can mean that we could partly ‘shift’ (in time) the running of the heat pump to coincide with available PV solar, or to benefit from different time tariffs.  Future Smart Grids may, in a small way, use the slow-response of old buildings to advantage.  We could also ‘incentivise’ and air source system to run at the warmest time of day when its efficiency is high.   We are not quite there yet with the necessary ‘intelligent’ controls’ to be able us to control as in these ideas, but it’s coming.  I look forward to seeing developments in this area.

mygrid.com UK grid carbon CO2 history
MyGrid.co.uk Trend of UK CO2 from generation



Kilowatts per hour! A guide for the perplexed.

There is a little detail that annoys many people.  It’s the confusion that seems to arise over the commonly used units of energy and power

I came across this the other day and pondered why it’s a common error.  When we drill into the details, we can see why confusion could arise.

We are all used to lots of things that are – ‘per hour’   e.g.   Speed in miles per hour, water flow-rate in Litres per hour, wages in Pounds (££) per hour.

The unit of energy –   kwh, is the odd one out.  There is no ‘per’ involved.  We will come back to that in a minute.

If you think about it historically; miles, litres and money came well before any thought of needing the ‘per hour’ rate of these things.    E.g. We measured Miles well before we thought of needing a unit for speed…. Miles per hour.

Apart from in science circles (who got it right of course), I would think that the units of power and energy evolved historically the other way around.  The first unit required was power – the horsepower, the watt, or the kilowatt.  When we first started generating electrical power, it was horse-power or Kwatts that was the all-important unit.  E.g Watts as a measure of the intensity of a light bulb etc.

The next consideration would have been measuring the total quantity of power over time.  If we generate 1kW of power, then after 1 hr, we have a quantity of energy of 1 kW multiplied by 1 hour.   In a whole day, we would have 24 kWhrs (kW x Hrs), or 24 units of electricity.  Now a utility bill can be sent!!

The image below illustrates various examples of quantities of something on the left, with the corresponding rate to the right.

The first 3 above are easy to understand, but the last one is a little strange.     Given the common unit for power was the kWatt (on the right), then to find a unit of quantity we need to multiply power by time, not divide by time as for other units.

One important point to mention, that some people miss, is the  ‘/’. This means ‘divided by’ or ‘per’.    However if we want a ‘multiply by’ in our units,   what do we put??   We put NOTHING!   How confusing is that!

e.g.       Miles per hour or miles divided by hours   =    miles  /   hr.
Kilowatts multiplied by time                   =      kW         hr.
That gap in between the kW and h means ‘multiply-by’  !     I know its so obvious to scientists, but if we actually put something in the gap it might make it more understandable.  E.g. if we said ‘kW hrs’, but wrote kW.hrs,

Maybe it’s no wonder that there is confusion.

So here is how to never get it wrong again.

kWatt  is the amount of Power.
A 40kW car engine, a 2kW kettle, a 50W light bulb

kWatt hr.  is a quantity of energy.
One kWh is one unit of electricity and costs about 15 pence.  A typical household may use 5,000 kWh of electricity over a year. (costing £750)

kWatt/hr. is unheard-of

Distributing hot water to tap outlets

To my mind, the whole point of having a heat pump is to save energy, so it somewhat jars with me when I see one aspect of a system that is not in-line with that aim.

A heat pump always stores Domestic Hot Water (DHW) in a cylinder. The Cylinder is, we hope, sized and optimised for heat pump use so that the COP (and lowest running cost) is the best-we-can-get.  This detail should be taken care of by the manufacturers and designers.

The detail of distributing the water to the taps is however often done using rule-of-thumb and the installer’s past experience.  Energy-efficiency is often low down on the priority list, so may be given little thought.

Hot water must of course be available at a sufficiently high flow-rate, this may lead to choosing a large-bore supply pipe.   However, this large bore pipe holds a lot of water, so when a tap is turned on, there can be a long wait before the tap runs hot.  After the tap is turned off, the heat from this volume of hot water left in the pipe is slowly lost.  This is not good for energy-efficiency and low running costs.  This may in-part usefully heat the house, but often it is simply a waste.   It is certainly a waste of water.    I recently came across a pipe run that took such a long torturous long route that I filled a whole sink before the tap stared to run warm!

For sake of energy-efficiency, the distributions to taps should be;

  1. The smallest diameter to give adequate flow rate
  2. As short as possible – the most direct route.

Normally, a single ‘main artery’ is taken from the cylinder, and run to taps in turn. This distribution pipe will normally start big and, as each ‘tee’ off to a tap, it will at some point reduce in diameter. Furthermore, if several taps are on at the same time, then one flow may affect the other. E.g. the pipe needs to be big enough in diameter to maintain a good steady flow-rate for a shower.

The choice of pipe size is often a dilemma, so playing-safe and choosing larger pipes would at least safeguard against an inadequate pipe run. This would however tend to go against saving energy.

Running separate pipes from the cylinder.

The pipe run to the bath is usually large diameter. This is where the fastest flow may be needed so that the bath fills quickly. However, a bath is not used very frequently, so the losses as described above may not be as great here as envisaged.   Kitchen sinks however tend to be used many times over the day, so total losses of heat and water could be high.

The worst scenario could be a kitchen sink at the end of a large long pipe run via a bath.

A solution to this could be to run dedicated pipes directly to sinks.

Here are some suggestions for new-builds

  1. Position the cylinder as close to the taps that are most frequently used (e.g. the kitchen sink). Ideally find a central location to all taps.
  2. Early on in the build, organise the most direct pipe run routes from the top of the cylinder to the sinks.   (the ceiling is often shorter than running in the floor)
  3. Decide the best configuration, e.g. some sinks can share, others may benefit from their own dedicated supply pipe.
  4. Identify the most suitable diameter of pipe.
  5. Insulate frequently-used pipes, BUT this may give limited advantage since it may simply take longer to lose all the heat in the pipe.

Pumped secondary return loop

Hotels and large houses often adoped a pumped circulation loop so that the taps run hot almost immediately.  This saves water and is convenient to use, however, the hot loop can waste a lot of energy, even if well-insulated.  I would try to avoid a pumped loop if possible.  It may be better to have point-of-use direct electric heaters, than to have a pumped loop that is always wasting heat.

Copper v plastic

There is a bit of a myth that suggests that plastic pipe is ‘smoother’ than copper.  The most important factor for flow-rate is the actual bore size. The wall material makes little difference.  However, plastic can very easily be bent with sweeps, these are better than straight pipes with tight bends. In this sense, plastic can be ‘smoother’.

Anyhow, the difference between copper and plastic is quite dramatic due to its internal bore.

It is very important to allow for the internal pipe sizes, and it is possible to use a combination of the two, e.g. run plastic through the middle of joists if it’s possible, but use long straight runs of copper along a wall.

I have put two charts at the bottom of the page here    http://heatpumps.co.uk/types-of-heat-pump/domestic-hot-water-dhw/

There are figures here to give a rough idea of what sizes might work.  It is surprising how much more restrictive 15mm plastic is compared to 15mm copper.  This info goes along with a disclaimer since it can be hard to know all the factors involved in a system.  I am a great believer in getting a sample of pipe in a coil and doing a test on site with a bucket.

You can also play with this great little pressure-drop calculator   for theoretical pipe sizes

What could go wrong?

I have successfully run a kitchen sink via a 10mm plastic pipe.  The bore is only 6.7mm which is less than the diameter of a pencil.  I had no noise problem at all.   In general, more noise would come from part-open valve or tap than would come from a smooth pipe.   Remember to clip the pipe well to stop any annoying clonking that can occur with any pipe when a tap is shut-off quickly.

For further reading, see the AECB water standard



Pressure drops, flow-rates and LENGTH

(Note- if you are reading this.. you might also be interested in my new flow and pressure simulator)

What is the point of having a large diameter pipe run when the fittings on the heat pump are small?

I have heard that statement enough times to make me want to write a blog about it.  

It seems intuitive to think that any bottle neck in a pipe system is the limiting factor for the flowrate. At first thought, it seems like the ‘weakest link’ in the system.  I then started to wonder why this topic is so well understood by electricians and so misunderstood by some plumbers.

I also recall chatting to a Heating College lecturer who said “They have made these training courses so you don’t need to think anymore”. If that is the case no wonder the topic can be perplexing. However, it is far from rocket science.

So a little thinking …

For water to flow through a pipe system, a pressure difference is required. [between (A) and (C)]. This pressure is usually provided by a circulation pump that literally pushes the water through.

The flow-rate depends on two things, 1) the pressure difference between (A) and (C), and 2) the total restriction of the pipe.

Electrical circuits are almost the same.

In electrical terms, we have Voltage (the pressure), current[amps] (the flow-rate), and total resistance[ohms] (the restriction of pipe circuit)

Every electrician knows Ohms law intuitively –

v = i multiplied by     or             volt difference = amps x resistance[ohms].

However, heating does not have a commonly known equivalent
e.g. pressure difference = flow-rate x restriction

The reason why electricians have it so sussed is that they have the tools to know exactly what is going on in every circuit. A volt meter can show the voltages at all points in a circuit. A current clamp can show the amps that are flowing. We can easily know exactly what is going on in any cable.(e.g the voltages at points A,B & C)

Plumbers have no such luxury. We would need tapping ports at various points around a circuit if we want to know pressure differences.  We only know the static pressure at one point, which tells us nothing. Furthermore, we generally have no idea how much is flowing in each individual pipe run. There is also no easy way of measuring the resistance of a pipe system, so there is no easy resistance equivalent for pipes and fittings. All-in-all, the heating engineer is very much in the dark, and would have little idea what the pressure is at point (B).

Now let’s consider a simplified scenario that an electrician might face –

Here we have a PV panel at a long distance from a house. It will make perfect sense to any electrician to have a very thick long cable from (A) to (B), whilst having relatively thin cables at each end. The important requirement here is that the total volt-drop due to total cable resistance is kept low. For every metre of cable, there is a very small volt drop, and all these add together. The calculations can easily show the volt-drop. You can use a cable calculator on-line.

It is the LENGTH here that is the issue. If (A) and (B) are close, that section could be thin, but if the cable is long, it need to be fat. If the cable is very long, it needs to be very fat. However, making the short ends any fatter will make very little difference to the total volt drop.

Back to our water pipes,

The same thing applies. Every metre of pipe causes a pressure drop (when water is flowing), and every metre adds up. Again, it is the LENGTH that is a major issue here. For example, if you have say a 5m length of pipe and you need 1m head of water (0.1 bar) to make a certain flow rate, then you need twice the pressure if you have twice the length. 50M of pipe would need 10m head (1 bar) to give you the same flow rate. However, it you swap that 50m long pipe for one of bigger diameter, you will have far less ‘resistance’ in the pipe. The required flow rate can be achieved with far less pressure.

You can play with an on-line calculator to see what happens with different pipe sizes and lengths. See http://www.pressure-drop.com/Online-Calculator/

It is easy to overlook the pipe LENGTH, this has just as much affect on the flow and pressure drop as the diameter does. This is why a water meter with a very small bore does not affect the flow-rate very much – it is very short. Put 10 water meters in-line, one after another, and you have a big restriction, and a big problem.

So, back to our earlier assertion that any small-bore section of pipe bottleneck is akin to the weakest link;       The bottle-neck principle does not apply here in the same was as it would to traffic on a motorway. If cars acted like water molecules or like electrons, they would push bumper to bumper and shoot through a bottle-neck at extremely high speed!

I have added a page on this topic here http://heatpumps.co.uk/technical/pressure-drops-flow-rates/

An example

To finish, let’s do a little exercise on pipe sizing. Let’s look at a scenario with a 10m (30ft) high header tank is supplying a tap/valve via a large bore pipe. i.e. a good 1 bar pressure supply of water. Let’s consider extending this pipe horizontally and look at what flow-rate will emerge out the end of it. We are assuming 1 bar pressure at the start, and zero bar (atmospheric pressure) at the outlet.

We can firstly consider a 15mm (outside diameter) pipe (the most common pipe size for taps).
One 40m length of this plastic pipe would give us a flow rate of around 8litres/minute (About 2.1 US gallons/min or 1.75 UK gal/min.). This is a typical bath-tap flow rate.

If we use copper instead, the wall-thickness is thinner and internal bore is bigger. We could have well over twice the length of pipe and get a similar flow rate coming out of the end.

As you can see from the grid, you could go as far as 720m if a 22mm copper pipe were chosen.

If we go in the other direction and fit a small pipe, we can see that a 10mm copper micro-bore pipe that was about 14m long would still give us 8lit/min. If we now look at 10mm plastic with its small internal bore of only 6.6mm (pencils are fatter than that) then it could be no longer than 3m if we want the same flow rate to fill a bath.

This simply illustrates how the diameter and length could be adjusted to give the required flow rate given a certain available pressure.     In summary, a short thin pipe may act similarly to a long fat one.

Selecting the right Heat Meter.

This topic is VERY IMPORTANT
I am aware that the pressure requirements of heat meters is often overlooked – this can be detrimental to a heat pump system.
A heat pump will normally be connected to a radiator or underfloor heat-emitter circuit.  A circulation pump (circulator) is required to ‘force’ the water around this circuit.  Each part of the pipework and each components acts as a restriction to flow. The greater the restriction, the bigger the circulation pump needed.  Indeed, many systems have 2 or more circulators, whilst it should be possible to only have one.
Heat meters  (reading heat in Kw’s) are often required to calculate RHI payments. The main component of a heat meter is a device fitted in a pipe to measure the liquid flowrate through the system. This device will cause a restriction to flow. However, it would be ridiculous for our heat-measuring device to impair energy-efficiency by either causing an inadequate liquid flow-rate, or by increasing circulation pump energy demand.
Let us get a ‘feel’ of the pressures involved.   Metres head (height) of water is a good understandable unit of pressure measurement, but several other units are used;
1 bar pressure (about 1 atmosphere)  = 10.2m head of water = 1,000mB = 100kPa
Most common circulation pumps can produce a maximum of 5 or 6 metres water head  (0.5 to 0.6 bar), however, this is at the pump’s low flow-rate.  For energy-efficient operation, we should operate them at mid-pressure range, ideally no more than a pressure of 2.5m head of water to work against.
Some heat pumps have one circulator to circulate through the heat pump and the emitters.  If the heat pump requires say 1m water head for its required flow-rate, and the emitter circuit requires a further 1m head, then the circulation pump needs to exert a total pressure of 2m head. There is little pressure left  to overcome restrictions for a the heat meter, so the heat meter should not be too restrictive.    Even if your system has 2 circulators, the head meter should not cause a significant restriction so you must SELECT THE CORRECT METER.
At their rated flow-rate, some  heat meters will require high pressure for circulation.   Some cheaper options can require up to 2.5m head pressure. This is a lot.   However, if a larger heat meter is chosen, its pressure drop can be very low since, for example,  if you halve the flow-rate, you quarter the pressure requirements.   Modern meters can retain high accuracy even at very low flow rates.
E.g. if your system flow is 1 cubic m / hour, your best low-pressure option might be to fit a 2.5 cubic m/hr device.
The example chart  below show how the pressure drop can be very low if the meter is used at a flow-rate well below its maximum.   Note also that some meters are inherently more ‘restrictive’ than others.   The colours are my own opinion, and relate to adding a heat meter to a heat pump system.
The Vortex Flow Sensors have exactly the same issue, and there are fewer pressure ranges to choose from.  However, in the region of 25% of their rated capacity, the pressure drop is usually nice and low.
The message here is – Don’t fit a meter without assessing the pressure drop at your expected flow rate.  Don’t necessarily trust the meter supplier since they don’t all understand the energy loss implications related to heat pumps, and may want to sell you the cheapest option.    It could be perfectly acceptable using a high/medium pressure drop heat meter  on a biomass district heating system feeding radiators, but for heat pump applications, we could be running into trouble unless the pressure drop is low.
Oddly, I see little mention of pressure drop on many heat meter data sheets.


Heat Pump Performance Monitoring Examples

This is an old blog but may be of interest to a few
OpenEnergyMonitor are going strong. See 2020 update here
Summary of a brief presentation at the Ground Source Heat Pump Expo at the Ricoh Arena on the topic ‘Energy or Performance Monitoring’, so its timely to do a little blog here to elaborate on some of the examples I will be showing from some OpenEnergyMonitor dashboards I have been using
Note; The below are just examples for this blog, and don’t necessarily show the whole story.
I have been working with OpenEnergyMonitor for some time, and now have various installs using OEM kit from Megni.co.uk
In brief, most of the systems I have installed use around 8 temperature sensors, CT power measurement with voltage sensing (real power including power factor), and/or pulse counting from standard electrical kWh meter.  I have used Grundfoss VFS flow sensors, but we are currently working on the direct interrogation of a Kamstrup heat meter, giving heat output
Data is sent via Ethernet to www.emoncms.org and displayed on dashboards (as samples below). These real-time graphs give a fantastic tool for the installer and the home owner. They show exactly what is happening now, and what has happened over the previous hour/week/month or year. The information can be used to improve the design of a system and also be used to fine tune the user settings.
Let’s start with a SIMPLE dashboard example
This type of dashboard can be accessed on any internet-connected computer using www.emoncms.org    e.g.  www.emoncms.org/example
The dashboard above shows a bar graph of daily energy input to the heat pump. This can be checked periodically for unusual values. It is showing high use on 15th March. By mousing-over the graph we can see that on this day, 24.2 kWh were used. The reason for this high use could be investigated.  The time period can easily be changed to anything you wish by zooming in or out.  Below this is the outside temperature. This might be interesting on its own right, but may be interesting if compared to energy used per day.   To the right are a few useful dials and figures –  cylinder temperature, room and outside temperature – things any home owner might like to know.    We can see that as from 6th April, the system is switched off.
This type of simple dashboard is ideal for the home owner, but we can make as many dashboards we like, of varying complexity and detail.  These are very useful for installers and designers, and a far more in depth analysis can be made.
Let’s start with a good example of a GSHP connected to underfloor heating.
This is a 12kW (max output) inverter-drive GSHP operating for a 40 minute period here. The green area represents electrical input and the purple represents heat (direct reading from Kamstrup meter).  The ratio if these two areas gives the COP.  We can see the flow and return temperatures slowly ramping up to a final flow temperature of only 32°C.  Since this is September, the ground collector is exceptionally warm. This, along with the low flow temperature explains why the COP is currently almost 6.  Current conditions are ideal, but from tests earlier in the year, we expect to see average COPs for heating in excess of 4.
This graph is showing us a healthy flow-return dt of 6 degrees.  It is also showing how nicely the speed of the compressor drops in response to the rising flow temperature.
Below is another snip. This time from a fixed-speed GSHP. This one shows the source temperature too.
This shows a period of about 1/3rd of a day, and approx. 30min. run durations which is quite acceptable. The flow and return is nice and low, with average flow temperature around 30C.  The underfloor is a good design here, but he source is dipping below zero. This is not ideal, but a zoom-out of yearly temperatures and knowledge of total heat used would give better understanding.  In this case, since the underfloor is so good, it may be acceptable to have a slightly inferior ground source.
Next, a good example of a GSHP heating a domestic hot water cylinder.  The cylinder is copper with the heat exchanger coil in the bottom section of the cylinder. The heat pump is only 3.5kW and the coil is a nice large 3sq m.
This example is showing the heat pump electrical input power as the shaded yellow area  (no heat meter fitted). The heating period starts at about 1.3kw input and finishes at around 1.7kw.  It also shows four temperatures;  cylinder top and bottom, and the flow and return temperatures.
This graph is showing the early evening heating period having been off by a time clock.
As we can see, the top of the cylinder is still at a useable 50°C before heating, but the bottom has dropped to 40°C.  The 24 minute heating period shown here starts by heating the bottom water from 40°C.  Indeed, this system has been set up carefully to ensure the system heats from a lower starting temperature.  The heat pump ‘sees’ flow and return temperatures of only 45/40°C at the start.  The 40°C cylinder bottom (not very hot) ‘pulls down’ the heat pump working temperatures, resulting in a high energy-efficiency.  By looking up the heat pumps performance data, we can estimate the average COP with reasonable accuracy; here it is about 3.5 at the start of the heating-up cycle.
As the cylinder warms, we can observe the point just before 18:00 where the bottom is becoming warmer than the top, and natural convection causes the top of the cylinder to rise with the bottom. After about 25 mins the whole cylinder has reached about 53°C.  At the end, the heat pump ‘sees’ temperatures of 55/52°C.  This is getting quite hot, and getting close to the limit of the heat pump’s comfort zone. The COP here may be about 2.8.  (taken from heat pump data).  We can then look at the period of time the heat pump has spent at different COPs, and estimate the COP for the whole DHW heating session. Its somewhere around 3.05.
If the system were enabled 24/7, and the sensor position not optimised,  the lower cylinder would not  drop so far, so the cylinder would  heat  more frequently from a higher starting point. The average working temperature would be much higher, so COP would be lower.  At worse, the COP could be not much better than 2.8.  Added to this, losses from the pipe run, and starting-up losses could result in worse performance.
We can therefore use the monitor to enable us to set the system to operate at a low average temperature, but for the cylinder top to remain at a useful temperature (e.g. say 47°C).
We can also see how nice and close the final cylinder temperature (53°C) is to the maximum flow temperature of the heat pump (55°C). This minimises the need for immersion heater (with COP of only 1).  In this case, the compact copper heat exchanger is exceptionally large compared to the heat pump size, and the coil is also only occupying the lower section of the cylinder.  This gives exceptionally good results, and allows us to heat some of the water  in a ‘batch’ from a colder starting point.
For the next example we have a complete contrast. This is a very inefficient system!


This one is a 14kW  ASHP.  The heat pump is fine, and function exceptionally well,  but the cylinder heat exchanger is debatably a little small for this big heat pump.
Looking at the graph, heating starts when the middle of the cylinder is 48C.  the flow temperature s runs up to 60C within 15mins, at which point, the input power drops and the heat pump ‘tracks’ the 60C flow temperature.  After 30mins running, the cylinder is 55C.  The flow temperature here is considerable higher than our previous example. In part due to the heat pump NOT reducing its speed, and in part due to the smaller heat exchanger coil, but it has not done too bad.
However, the period after 55°C is clearly grossly inefficient.    We can see that the compressor switches off frequently and spends the next 3.5 hrs! attempting to achieve 60°C.  The other thing to mention here is that the distance between the heat pump and cylinder is around 15m. What is actually happening is that most of the heat is simply being lost from the pipe run.  The energy consumed is shown by the yellow area of the power plot.  The final 5 degrees (to 60C) uses several times the power (area) of the first section from 1 to 2.
The biggest problem here is poor use of the controls.Clearly, it would make a lot of sense to adjust the hot water setting to 55°C so that the heat pump stops.  .
The final ‘floor heating’ period is just as terrible as the DHW period. Here, only 1 or 2 underfloor zones are open so the flow rate is far too low, as can be seen by the large temperature difference between the flow and return.  This is in excess of 10 degrees. Again, heat dissipated by the floor is far too small for this large heat pump.
This is a clear case of an over-sized heat pump connected to a cylinder and emitter system. A smaller unit would work far better.
Finally, just to top anyone up with a little heat pump theory, I am adding a graph that I used to illustrate heat pump efficiency v the output temperature.   If there is one thing to learn about heat pumps – LEARN THIS.
Here we have the characteristics of 2 sample heat pumps.  The vertical Y axis shows the efficiency, the COP. A 3kW immersion gives 3 kW of heat. It has a COP of 1. Heat pumps give out more heat than they consume because they extract heat from outside.  The X axis shows operating output temperatures ranging from tepid on the left to very hot on the right.
I am showing 2 typical heat pumps. A typical (R407C refrigerant) unit can reach say 55°C, whilst a ‘high temperature’ (134A refrigerant)  unit may achieve 65°C.  Anyhow whatever type, we can see that to the left, where the water is lukewarm, the heat pump has an easy time, hence the COP is very high (1kw in for 4.5kW of heat). I liken this to driving a car up a slight incline. We should get good fuel economy here, maybe 50mpg .  However, it we heat up to 65°C, the temperature ‘lift’ is great, and this is a little like driving a car up a steep incline – we are in a low gear and the MPG is only 20!.    In the same way that you will NEVER get good fuel economy when driving up a very long steep hill, you will not get a good COP when heating to a high temperature.  That said, it should always be better than using an immersion heater.
So, knowledge of the performance of any heat pump should be understood, and data should be available for all models relating to output temperatures at specific ground source or air-source temperatures.
If you have a high temperature heat pump, it doesn’t mean you always have to operate it at a high temperature.  If you operate it at lower temperatures, the performance should be far better. It is however always a good idea to find out the working limits of you unit.
Some of you may have wondered about the flattening of the curve at very low temperatures.  I have drawn it that way because some heat pumps are not very good at the extremities (limits) of their performance. However, I am finding (and partly guessing) that most inverter heat pumps with electronic expansion valves work very well over a very wide range, so some heat pumps will easily exceed COPs of 5 in ideal conditions (usually late spring or autumn, when not much heating is needed) .Never forget mid-winter conditions – this is the time we need most heat, so the operational area to focus on.


So, it is with an understanding of the characteristics of heat pumps, performance monitoring can be used to great advantage. In general, we want heat pumps to spend as much time at lower output temperatures ( and high source temperatures) as possible.

Temperature sensing with OpenEnergyMonitor

Temperature sensors for monitoring heat pumps

I have been using OpenEnergyMonitor.org energy monitors with heat pumps for a few years now and thought I should briefly share my experience.  This covers the practical side of temperature sensors, and is based on my experience.
(If you just want advice about fitting temperatures sensors onto pipes, skip further down the page)

The sensors used in OpenEnergyMonitor.org modules are DS18b20. These are Maxim 1-wire digital sensors.
They actually have 3 connections;   0v, 5v and signal.   Since each sensor has its own unique i.d. code, multiple sensors are simply connected into the same 3 terminals on the monitor device.

The great advantage of these digital sensors is that there are no errors due to cabling.  Phone extension cable can be good enough.  Thermistor sensors (variable resistance, as PT100) rely on a specific cable resistance and furthermore some sensors could ‘drift’ out of calibration.   A further advantage of digital sensors is that they do not in themselves generate heat. They can therefore be used in still air with good accuracy.

If several sensors are strapped together (with an elastic band) and tested in a thermos flask over a few hours, most seem to read within 0.2 degrees C of each other.  If you have say 6 sensors, you can chose those that agree most for the most important sensing (e.g. flow and return).   Anyhow, these sensors are perfectly accurate enough for this type of monitoring, and there is comfort in knowing that if you are seeing 35.2C (for example) on a graph, then that sensor tip must be at 35.2 +/-, with a small error margin of say 0.15 degrees.  This is because the digital signal is generated inside the sensor.

What to measure on a heat pump?

Typical temperature measurement could include the following:
Ground source
Water flow and water return from heat pump (hot side)
Ground collector glycol  inlet and outlet from heat pump (cold side)
Outside (ambient) air temperature.
Hot water cylinder temperature, and at least one room temperature.
Air source
Water flow and water return from heat pump (hot side)
Outside (ambient) air temperature and air-off (cooled air leaving heat pump)
Hot water cylinder temperature, and at least one room temperature.
For either of the above there could be a buffer cylinder involved in the design, therefore the flow and return from buffer cylinder to emitter (e.g. underfloor or radiators) may need to be monitored.
There may also be mixing valves (sometimes unnecessary mixing valves) on underfloor manifolds. These may also need monitoring.

For more detailed analysis, the refrigerant internals of the heat pump can be monitored. These may include discharge, suction and liquid temperatures.   This is however a little intrusive, and could affect the warranty. It may be necessary to discuss this with the system installers.

Measuring air and liquids 


Measuring internal air temperature is relatively straight forward, but sensors must be positioned away from any source of heat or radiation source.  Sunlight and close-by appliances and even lighting can greatly affect readings.
The general rule for outside air measurement is to keep the sensor well away from sunshine, or areas that get sunshine.  The sensor can also drop below the air temperature if it can ‘see’ a clear sky.  A small polystyrene roof is a good shield protection from positive or ‘negative’ radiation (i.e. when radiation from sensor is greater than its surroundings).  It is also necessary to keep the sensor dry, unless you purposefully wish to cover it with a wet ‘sock’ to record Wet Bulb Temperature.  There is a lot of information available about this relating to weather stations.


Most heat pump monitoring involves the temperature measurement of water and liquids flowing in the pipes.
For very high accuracy results, as required by heat meter devices, an immersion pocket is required. (Heat Meters measure the difference between two sensors, so both must be very accurate)
The pocket usually involves a ‘tee’ fitting and a hollow pocket that is completely surrounded by the liquid. The sensor is inserted into the pocket.
An easier and cheaper method is to fix the sensor to the outside of a metal pipe. This method is used in all heat pumps for their on-board sensing.
Any sensor that is outside the liquid can be affected by the temperature of the surrounding air, but the error in the reading can be negligible if the sensor is mounted correctly.

There are various methods as follows-

1)  Sensor strapped to metal pipe or metal fitting
2) Sensor inserted in surface pocket (a pipe soldered to the pipe’s surface)

This shows 10mm pipes that have been soft-soldered to copper fittings. Due to the high conductivity of copper, the temperature of the inside of the small pipe is almost exactly the same as the temperature of the liquid inside the pipe. After insulating (lagging) this pipe, the accuracy of the sensor inserted inside the small pipe should be excellent.  Ideally the pocket internal diameter is close to the sensor diameter.  Conductive paste should be used before inserting the sensor
It is a good idea to fit several pockets like this in the right places during installation. Obviously the pipe must be empty.
Many heat pump manufacturers solder pipes on the outside of water or refrigerant pipes so that their sensor probes can easily be inserted to give accurate measurement

Strapping a sensor to a pipe (or a pipe fitting)Whilst not as good as surrounding the sensor with a conductive copper pocket, it is possible to simply strap the sensor to a pipe.   Strapping using a copper strap (as used in refrigeration) , or copper wire, will give better results.
The following shows one simple method.

Copper or brass conduct heat well.  The temperature difference between the inside of the pipe and the outside is tiny, so always fit sensors onto metals.  Plastic pipes are poor thermal conductors, so the effect of surrounding air can affect the sensor temperature. Furthermore, if the pipe is changing in temperature, the sensor may ‘lag’ in time behind. i.e. it will respond very slowly to temperature changes.  This may or may not be a problem.
This is not the best position for a sensor since there is 3mm of plastic between the liquid inside and the probe, however, the copper wire strap is making the best of a bad job.  Heat will conduct along the copper. There is a lot of copper in contact with the plastic, and this will conduct and will transfer heat to/from the ‘surrounded’ probe.  If this is insulated well on the outside, it may give good-enough results for ground source comparisons.  Time response may be a little slow, which may not necessarily bad.   Fitting the probe onto a metal fitting is a far far better bet.