Getting the best from underfloor heating

[Note, this blog was published 2014.. the underfloor heating industry has developed a lot in the last 5 years]

Many years ago, when heat pumps were not so common, I found it a real struggle to get any of the main underfloor heating suppliers to embrace the need for low water temperatures for use with heat pumps.

15 years on, and I’m sure that things have changed dramatically, but I’m still stumbling across things that make me doubt that.
When I visited Germany over 10 years ago, I got the feeling they commonly use much closer spacing between floor pipes than we do in the UK, and I have read of very close spacing (50mm) in Austria.  I realise these are potentially cold countries, but even recently visiting a Spanish heat pump company, they were surprised that we don’t use closer pipe spacing (more pipes).

Over the last 6 months I have heard several references in the trade to; ‘never use closer than 150mm pipe spacing’.   However, CIBSE clearly give ratings for 100mm spacing, as does the MCS Emitter Guide, so why are some in the UK so resistant to putting more pipe into floors.
To basics     COP v heat pump water temperature
Heat pumps are more energy-efficient if the water temperature is low.
Coefficient of performance (COP) = heat output / electrical power input
This graph of an air source heat pump shows how beneficial a low flow temperature is.   Units like this with electronic expansion valves are particularly efficient at very low water temperatures.
How to get a low flow temperature with underfloor heating
The values for the example graph above have been extracted from CIBSE, and show a general trend for a fixed heat output.  It demonstrates what we all should know  – the closer the spacing, the lower the required water temperature for the same heat output.
Putting the two together we could plot pipe spacing v COP.  It would clearly indicate that closer spacing improves COP, so why do some seem reticent to embrace close spacing?  Is that added cost for more pipe really that much??
I have gleaned that some people have the notion that close spacing could lead to warm floors and an overly hot house.  The whole point here is that with closer spacing, we can turn the water temperature setting (heating curve) down, and get the same heat output, thus improving the COP.
I think some of the fears about hot  floors stem from boiler systems, many of which had plenty of extra output capacity for quick warm-up, and hot patches could result. However, heat pumps are generally slower response, with lower water temperatures, so are much more forgiving in this respect.  I recently visited a system where I had used 50mm spacing in an always-open loop in a bathroom.  I asked the owner if they had ever thought the bathroom floor was too warm.  The answer was no, never. 
The reason why I had used 50mm spacing was that we didn’t want a buffer cylinder, so I was trying to ensure there was adequate water quantity, and flow-rate in the floor – effectively using the floor as a buffer.  
Another hot (excuse the pun) topic here is floor coverings. People are now seeing that a carpet will drop the star rating for RHI, so there is an added financial incentive for having solid (tiled etc.) floor, as seems far more common in Germany, instead of carpet.
However, by designing a house where some rooms are tiled, and some are carpet, you are somewhat limiting any changes that future occupants might wish.  (e.g.  if 100mm spacing in a carpeted room, and 200mm in a tiled room).   Underfloor pipes are literally set in stone, so can never be changed.  The only solution that I could think of here would be to interlace the loops (given that there is often more than 1 loop per room).  There is always the option of turning off one of the interlaced loops.  I like multiple interlaced loops.
I have come across an installation where a new owner had fitted thick pile carpet in one room without thinking the affect it would have.  The result was a cold room, remedied only by turning up the water temperature, thus  increasing the running cost, and the reliance on room thermostats to limit heat to the tiled rooms.  
The general design approach for underfloor is to consider the heat required (e.g. watts/sq m) , but unless a buffer cylinder is fitted, we should also consider what heat is being delivered to the floor, given that some zones will be closed for some of the time, and the outside temperature is seldom at design temperature (-2C etc.).   For most of the time, we have far more available heat than the floor needs.  Even with modulating heat pumps, there can still be a tendency for the flow temperature to stray above the theoretical flow/return temperatures. This is another reason for favouring more pipe in the floor.  Furthermore,  MCS requires the heat pump to provide at least 100% of the design at -2C etc.  Due to models only being available in certain size jumps, the heat pump installed is often oversized, so this is an added reason for ensuring that there is adequate pipe in the floor.
For any thinking that buffer cylinders are the perfect answer – they may be an answer, but they are seldom perfect, as shown by this piece of monitoring.     (
In this example of a system with a simple buffer tank, the heat pump flow/return needs to be approx. 4 degrees hotter than UH flow/return.    This could reduce energy efficiency by 10%, plus the added energy to run a second circulation pump.
A buffer can usually be avoided IF enough zones are always on AND if there is plenty of pipe in the screed.
Another potential worry is the thermal mass of the screed.   I was recently involved in the design of a passive house where a buffer cylinder was not wanted, so I proposed  to use the screed as the buffer, and fitted 9 x 100m loops with average 100mm spacing in a  floor that was 200mm thick.  One might expect some temperature overshoot, and I may not have been so bold as to propose it if it were not for the fact that due to the interlaced nature of pipework,  we could shut off ½ the zones if we needed to.
The result was surprisingly good with all room zones open and control on one master thermostat. The whole house has very even temperatures.  This demonstrates the self-regulating nature of very low temperature underfloor heating.
Another concern I have heard of is the pumping power required  for such a lot of pipe.  In the above passive house example.  If the heat pump needs 10 lit/min to give a 5 degree flow-return dt , then the flow rate is about 1.11 litres/min for each pipe loop.  If we used a more standard 5 loops, the flow rate per loop would be 2 litres/ min.  For the same heat (kW) and same dt, more pipe actually means less pumping power since the heat output per m of pipe is lower, and the flow-rate for each loop  is lower.
Finally, I have also come across the notion that mixer valves and pumps are desirable, even for a heat pump.  Here is an example to illustrate the potential penalty of having a mixer even if the mixer never actually mixes (i.e. its fully open).
The arrangement here with recorded temperatures shows that the flow to the floor is always equal in temperature to the return to the heat pump. In this example, the flow from the heat pump is 7 degrees higher  than the flow to the floor.  The mean floor temperature  was only 23°C (average of 28 & 18).
If the mixing valve and pump were removed, the heat pump’s flow could go directly to the bottom manifold.  
To get the same floor heat output, the heat pump setting could be adjusted down so as to give a working flow of 26 and return of 20.  This is a reduction in flow temperature of 9 degrees, potentially (according to our first graph at top of page ) saving 13%.  
(see top graph – The COP at 35°C is 3.1, at 26C its 3.1, saving 13% in COP). 
All these little details can eat into potential savings. They should be dealt with at design stage.

Potential perils of plastic pipe

(Note, if you are reading this you might also be interested in my flow/pressure simulator)

A college tutor recently suggested to me that the current environment in the heating industry does not encourage analytical thinking.   Maybe I should therefore be less amazed when I come across plumbers who think copper and plastic pipe are interchangeable.

Before you think I am anti-plastic pipe… I am not. The right size plastic is absolutely fine.
Environmental considerations of the materal aside, a more direct issue is that of flow rate and pressure drops.  Bore size of plastic is considerably smaller than copper, and it seems that this is often overlooked.  If installers select plastic pipe instead of copper without checking the size correctly, this could have a negative effect on heat pump’s performance.
(If you drop-off to sleep ½ way through this – read the scenario at the end )
With respect to the materials of plastic and copper, it would appear that the total energy involved in mining and manufacturing copper is far greater than the total energy (including the crude oil to make the plastic) to manufacture plastic pipe.  Here is one site that discusses the topic  , though not necessarily without bias.
Another factor to consider, that mainly affects pipe runs to hot taps etc, is the heat capacity of the pipe material. Plastic has a relatively high specific heat, and the wall is thicker, but it’s light. The net thermal capacity of the two is fairly similar.
(The cold feel of copper is more to do with conductivity from the hand than heat capacity)
However, obvious factors aside, one of the biggest issues that could affect installations involving heat pumps relates to the internal bore diameter.  This could have a very noticeable effect on the energy-efficiency of the system.
All metric pipes are measured by their outside diameter. As can be seen, with common pipe sizes (outside diameters), equivalent plastic pipes have considerably smaller internal area to copper. This has a dramatic effect on flow characteristics.
 The graphs below illustrate examples of the relative internal dimensions of common pipes.
A brief note about smoothness – It seems a common belief that plastic is ‘smoother’ than copper, but the inner wall ‘smoothess’ of the two is about the same.  However, plastic can be one-piece with slow sweep bends. In a different sense of the word, this is certainly ‘smoother’ than copper with tight elbows.   Re inner surface, we can assume the two materials are about the same.
Whilst it is fairly easy to look-up the pressure drop resulting from a specific flow rate with a specific pipe, we can see from the 2nd graph at-a-glance the relative flow capacity since the cross sectional area loosely indicates flow capacity.
If a certain flow rate is required, then we can look-up the required pressure that is required across the pipe length (beginning of pipe to end of pipe length).  The internal bore must be chosen such that the circulation pump is not overly large and energy-wasteful.
This graph shows the approximate pressure required to maintain a certain flow rate for one specific fixed length of pipe example.  As can be seen, the pressure drop along the example pipe varies very dramatically, so the wall thickness makes very big difference. 
In this example, we can see that a 15mm copper pipe could be used with a common central heating pump (shown at 3.6m head, 36kPa). However, if plastic were chosen, then one would need almost 9m head to achieve the required flow – far beyond the capability of normal circulators. On the other hand, if 22mm plastic were chosen, the pressure requirements would be only 1m head (10kPa) which is likely to achieve very low circulation pump energy.
In real life, we tend to have a pump connected to a pipe system, and the flow rate that results is dictate by the balance between the pressure produced by the pump and the ‘restriction’ of the entire pipe work circuit.
There is a common concern about the restriction caused by the inserts (stiffeners) needed at joints. These restrict the bore, but they are so short that the the affect on flow is much less than it may seem.
For our final graph, we consider a pipe circulating with a fixed-pressure electronic pump (Alpha etc)
The above graphs show relative changes in flow rate that would result from a fixed pressure. If the pipe chosen were too small, then a larger circulation pump may be needed in an attempt to compensate for the extra restriction caused by the small internal bore.
The point here is that by choosing plastic instead of copper of the same nominal size, the system could potentially suffer unless the sizing is checked. There is of course no problem using plastic if it’s the right diameter.  Indeed, 28mm plastic may be an ideal choice for the connections from a heat pump simply to minimise noise transmission.  The best final solution is often a mixture of both plastic and copper for a multitude of reasons.
All this emphasises how dramatically the wall-thickness affects flow rates and pump pressure requirements, but how does this translate into reduced COPs?
Scenario(based on something I observed on a barn conversion)
Let us consider a radiator at a far distance from the heat pump.  The flow-rate relates to the pressure drop, which relates directly to the pipe-run length, and of course, the required flow-rate relates to the room size (bigger the heat demand, the more flow required).  In this instance the room is large.
The default pipe size choice would be normal 15mm (outside diameter), but if the sums are done, it may become apparent that the choice should be between either 15mm copper or 22mm plastic. 
How could a pipe with too small bore affect the COP? 
Radiators should be balanced, in general by throttling valves (lockshield)  on smaller radiators, and those with shorter pipe runs. However, this is actually quite a difficult thing to achieve with a heat pump because the temperature difference (water inlet to water outlet) may only be 5 degrees (°C).  (It’s much easier to measure and adjust when the temperature differences are larger).
It is not ideal, or easy, to have to throttle most other radiators on a system, and there is the added risk of the circulation pump not being big enough to cope.
The likely result of any ‘restrictive’ pipe-runs would be a reduced flow rate to the radiator.  This would result in a considerable area (the bottom) of the radiator being cool, and a reduced heat output to that room.
The obvious action to redress the short-fall of heat would be to increase the flow temperature by increasing the heating curve setting of the heat pump. i.e. increasing the flow temperate from say 40 to 45°C.    Now the heat pump has to heat ALL water to a 5 degrees higher level.  This is likely to reduce the COP by 10 to 12%.

This all indicates that one must never assume copper and plastic are interchangeable without considering the pressure drops and diameters.  That aside, we have also highlighted the importance radiator balancing.  Ideally the pipe runs to radiators would be laid out, and bore sizes chosen so that the flow rates are naturally about right without the need for much valve adjustment.  A little extra work on the design makes life much easier thereafter.

Anyone intersting in further reading on water flows – this site is very intersting

Here you can enter pipe sizes and find out the pressure requirements to achieve a certain flow rates.   And here

Stand-by power and air-source heat pumps

 I have been aware for some time that my 5kW air-source heat pump (ASHP) has been using about 3/4 of a kWh/day whilst doing ‘nothing’ over the summer (no circulation pumps running), but recently I have put energy monitors on other ASHPs and was shocked see that these known-brand units are using 2kWh/day when doing no heating at all – I bet the average family use less than that showering! (an 8kW shower for 15 mins = 2kWh).
Whilst in standby mode, is there any reason or excuse for it consuming so much? I think not.
There has, in recent years, been a general focus on standby power since this ‘constantly dripping tap’ can tot-up to a sizeable amount. Highlighting this problem has been very productive – the standby power of most new products has now plummeted.
Let us look at some figures to get a handle on the quantities. Here is what I recorded using pulses monitored from my kWh meter
ASHP on standby          31 watts (average),       0.75kWh/day,     272kWh/ year,      (£38 / year)
My A++ larder fridge   7.6 watts (average),     0.182kWh/day,        67kWh/year,      (£9.30/year)
Yes you have that right – my fridge, in use, used one quarter the power of the heat pump on standby!
Standby power is however quite hard to measure accurately using any instantaneous devices. This is in part because the current (amps) wave form from modern electronic devices bears no resemblance to a sine wave, and in part since the values are relatively small . For my own satisfaction, so I borrowed a calibration unit to verify the accuracy of the ‘real power’ OpenEnergyMonitor internet-connected devices that I am also using.
Here is a plot of the 9kW ASHP that is consuming 2 kWh per day (Y axis is in kW).
It alternates between about 35-40 watts and 200 watts.
 Below is a plot that shows an 8 hr period..  Scale is 0 to 5kW input power.
The buff-coloured area on this 8hr plot relates to energy use, and shows a hot-water cylinder heating ‘hump’ (above 18:44) . This illustrates at-a-glance that the buff-colour area of the standby ‘blips’ (on the bottom axis) forms a significant chunk of the total.
I have previously decried ground source units with controllers that use 15 watts, so why are these particular air-source heat pumps using so much?
The oil inside the compressor can absorb some of the refrigerant. This not only thins the oil, but it can cause oil-foaming at certain times – both can lead to bearing failure due to inadequate lubrication. One answer is to keep the oil warm, and a small electric compressor heater (crankcase heater) can be used for this purpose.
Ground source units are situated indoors, so usually stay relatively warm. I don’t know of any GSHP with a compressor heater. However, air-source are sited outside, and they can experience extreme ‘swings’ of temperature. The worse scenario would be a cold sub-zero night followed by a warm day. In this scenario, the lightweight heat-exchangers warm up, but the heavy compressor can remain the coldest part of the system. Some refrigerant can then migrate to the coldest part (in effect, it condenses in the compressor like water condenses on cold windows). If this cold refrigerant-laden compressor were to start, then there is risk of damage.
The picture shows the compressor with strap-around element heaters. The compressor was quite warm, but one wonders why the lagging around the shell was open at the top; possibly to aid compressor cooling for times when the compressor is working at it’s upper temperature limits?
What is the solution?
There is a lot of scope here.
Most of these Japanese-type heat pumps are very impressive, they are clearly very well developed and very reliable. Computer-aided fan and heat-exchanger design has led to highly efficient quiet products, and most defrost mechanisms are very well optimised, but are all aspects up to scratch?
It seems already (from graph) that the heater is pulsed, and not simply left on, so why cannot a little more intelligence be added to the controller so that the switching can be controlled more responsibly.
Lagging the compressor should also be possible, and should help, but now one might need to introduce some form of cooling to deal with overheating whist running. There are many options here, and it would take time for a designer to work out the best reliable and cost-effective option that did not compromise over all energy use. I have certainly spent many hours in the past designing crankcase heaters out of systems. The phrase ‘could do better’ springs to mind, and this sentiment should really be directed to the manufacturer’s design office.
A similar situation existed with washing machines many years ago. All for the sake of a thermostat, the hot water feed was blended with cold, just in case the hot feed happened to be at 70C , and could therefore potentially damage clothing. No manufacturer wanted to be blamed for harming any clothes so they all played-safe and kept it simple, to the detriment of energy-efficiency.
My feeling is that we are at the same point with some aspects of ASHPs. Whilst standby power is a small fraction of the total annual consumption, it is still a detail worthy of attention since I’m sure it could easily be improved significantly.
I can think of a simple solution. Make it mandatory to print the standby power on the product label. This would push the manufacturers to give it a little more attention, and sooner or later this unnecessary waste would disappear.

Legionella protection and energy

Legionella protection and energy demand
It is poignant that I write this whilst a hot debate continues in Doha, Qatar over the issue of climate change, which is more evident in some poorer parts of the world, and the proposal that the energy-guzzling West should compensate in some way. This seems to put my blog below into perspective.
I recently read the following on a brochure from a mixing valve manufacturer:
‘The hot water storage tank must be kept at a temperature of 65°C (149°F) or higher in order to control any growth of legionella bacteria’.
I don’t believe this is correct, nonetheless, there seems to be a growing general feeling that cylinders should be kept hotter and hotter.
The desirable temperature to ensure that hot water is ‘safe’, is debatable, and last week’s request from MCS for evidence on the topic confirms (I am pleased to hear) that the jury is still out.
My concern here is the energy required to achieve elevated temperatures. If this is achieved using a conventional boiler system, the extra energy required may not be excessive. However, for a heat pump it is a different matter since the COP varies dramatically as temperatures rise, and I’m not sure that everyone is aware how great the change is.
The concept of periodic pasturisation is a well established method, but the necessary frequency: daily, weekly or monthly, still seems debatable.
Let’s look at the energy efficiency relating to cylinder temperature. For a ‘high temperature’ model,(e.g. refrigerant 134A), and if a heat pump were to heat a cylinder to 50°C (122°F) with a COP of say about 2.3 (assuming evaporating at 0°C, 32F), then the energy-penalty for increasing the store temperature above 50°C for a typical heat pump, as shown by the blue line, could be 11%, 21% and 29% for store temperatures of 55,60 and 65°C respectively. (131,140 and 149°F)
(scroll compressor data including a pump load etc. Evaporating at zero C)
The blue line is bad enough, but for the conventional heat pump model (crimson), with an upper temperature limit of say around 55°C, 131°F, (heat pump water), then the resulting drop in energy-efficiency is significantly worse with elevated store temperatures, since some of the hot water will need to be delivered from an immersion heater, with a COP of only 1. If 65°C were really needed, then even a conservative estimation would halve the COP, and this is on top of the increased energy loss from the hotter cylinder (and pipes) of a hotter cylinder. All in all, the energy implication for heat pumps to comply with over stringent legionella protection could be very considerable.
I am of course being general here, and one could create quite a big spreadsheet attempting to ‘model’ this since there are many variables, and the percentage provided by heat pump/immersion will be affected by things like sensor height, time clock, quantity consumed and various other variables. I am mostly ignoring options of pre-heating or batch heating the water since not much of the installed ‘kit’ does this, but it can be achieved, in part, by the owner’s careful use of a time clock. There is a lot of scope to optimise the net COP here, and a lot to lose as the store temperature rises.
The current new-build requirement for mixing (safety) valves is a bit of a can-of-worms since some valves give a considerable and unnecessary ‘leak past’ of cold water. This forces cylinders to be maintained at unnecessarily high temperatures. Given the 2% drop in COP per degree rise, this tortures any heat pump involved.
Its worth noting here that it was not that many years back that at least one major German heat pump manufacturer suggested storing at 45°C (with occasional pasturisation).
I am yet to be convinced that the legionella risk is as high as it seems to be popularly cited.
Out of the countries millions of cylinders, I’m sure that a small (but significant) number of them are kept at a very low ‘frugal’ temperature. Furthermore, houses are commonly left empty, and cylinders could sit for extended periods with tepid water in them, and are not necessarily sterilised before use. Hosepipes sit out on warm summer days with water in them for for weeks. Car wiper bottles have had warm water in them since the 1960s, and I see no evidence for any significant numbers of serious legionella cases. Am I wrong? For large cooling towers, where warm water is sprayed into air it’s tragically a different matter.
A couple of anomalies strike me. Why is little attention given to open header tanks in lofts (a UK habit) these open-top tanks (hopefully with cover) sit in warm lofts in summer. One might expect that if a cylinder is fed from one of these, it might requre a different steralisation regime to a mains fed cylinder, and surely there should be at least as much concern from the loft tank that there should be from the hot cylinder kept at only 50°C for example.
Of course, this is a very emotive subject. Who would dare to suggest we should ‘ease off’ when life it potentially at stake. On the other hand, is it too radical to consider that the added energy needed for hotter cylinder temperatures could have a wider environmental impact. I see no evidence of DECC or anyone else attempting to quantify the extra energy required. I for one think it is relevant.
I’m not suggesting to take a slack attitude to the problem, but I don’t agree with the a broad-brush turn-up-the-thermostat approach given the energy penalty involved.
It’s quite a difficult balancing act. One has to weigh-up local health and safety with energy costs and CO2. If we debated this at Qatar, and considered the global health and safety, I’m sure that the line would be drawn in a different place.

Underfloor heating in bathrooms (from 2012)

A bathroom has a higher room temperature requirement, so heat requirements are generally higher, but the floor area available for underfloor heating is reduced due to the area taken up by the bath. I have recently come across a few instances of inadequate heating issues in bathrooms – not surprising. The point of this blog is to discuss the option of continuing the underfloor heating to the floor below the bath. This practice seems to be a no-no, and I’m reticent to suggest it’s a sensible approach. If the floor under the bath has wet underfloor heating, the space under the bath will be warmed, this will add heat to the room by a certain amount simply due to a slightly warmed bath and side-panel surface, and clearly a metal bath would be considerably better here than a plastic one. A roll-top, claw-foot bath even better! I have no idea if the quantity of heat is worthwhile. i.e. if the room temperature in the bathroom would be elevated by a worthwhile amount without the need to increase the underfloor water temperature (Very important when a heat pump is used). Many years ago I took a coil of micro-bore pipe, and wrapped it around the outside of my new plastic bath, and ‘bonded’ it with fibreglass. In this case, I was experimenting to see if the bath could became useful radiator area, and if the bath should stay warm whilst in use. The results seemed worse that expected, and no doubt there was little gain for a lot of effort. Given that bathroom loops are generally some of the shortest loops in the system, it strikes me that putting extra pipe below the bath would be easy, and advantageous. Is it the risk of drilling into a pipe when making a bath fixing? Is it the fear that the under-bath could overheat? Is it a daft idea with little benefit? I suppose I should lay an electric blanket under a bath and monitor some temperatures in various places to see how it performs.

Repairing a heat pump – How well is it carried out?

This post is a little specific, and discusses the need for care and consideration when carying out any ‘major surgury’ on a heat pump.  A little extra time setting the system up right will save considerable energy (and money) over its life.

A domestic fridge will generally complete its useful life without encountering any problems within its inner workings (the refrigeration circuit).  A refrigerant leak is almost unheard of.

Ground and air-source heat pumps should also be able to claim such a high accolade, however, due to their added sophistication, component failures can happen, but they are rare.
I was involved very recently with a system where a high-pressure sensor had ruptured. This caused a partial loss of refrigerant and resulted in a breakdown in the form of a low-pressure lock-out.
Heat pumps (and fridges) hold a specific quantity of refrigerant (the heat-transfer working fluid).  This is accurately weighed-in during manufacture, and thanks to good quality-control and all-welded joints, this weight should remain within the system throughout its working life.  Any weight over or under the required amount can cause a reduction in energy-efficiency.  
Refrigeration equipment is slightly different to a package heat pump. In the past equipment had many mechanical joints and couplings – potential sources for leaks, big or small. Topping-up of refrigerant was expected, and the norm, after several years.  Due to their design, refrigeration systems are generally less critical of refrigerant quantity – they have a ‘liquid receiver’ (a liquid storage vessel), and are generally topped-up until the sight-glass is clear of bubbles.
(Note:- since there is both liquid and vapour in the system, pressure cannot be used to gauge the quantity)
On occasions when I have advised someone to engage a local refrigeration engineer to carry out a major repairs to a heat pump, I always say “make sure that they weigh-in the correct amount, as printed on the manufacturer’s label”.   Recently, an engineer replacing the pressure transducer ignored this advice and simply topped-up until the sight-glass inside the heat pump was ‘clear’. 
The dilemma he could have had may relate to the time it would take to recover and evacuate what was currently in the system before weighing-in the correct amount. There is also a risk of contamination during recovery process. It gets even more complicated since the components of refrigeration blends (as per most current refrigerants; R407C & R410A) can leak at different rates, therefore the final percentage blend-mix of anything either left-in (or recovered) can be incorrect.
The point that I am angling around to here is that the mind-set required to carry out heat pump repairs is different to that required for refrigeration repairs – for the following reasons:
·        The energy-efficiency of a heat pump is paramount.
·        Any refrigerant added may be ‘sealed-in’ for up to 20 years, so ensuring that it’s ‘right’ (for highest energy-efficiency) is essential.
·        The refrigerant charge is generally more critical for heat pumps.
Whether the afore-mentioned engineer did the right thing or not, I cannot say. However, using a sight-glass alone can a little risky. If any other part of the system is ‘out of tune’, this could affect the verdict of the sight glass, and could result in a diminished performance. Whilst the risk of this might be low, I have come across more than one system that was grossly over-charged. This causes the condenser (the hot heat-exchanger) to be flooded with refrigerant- effectively leaving a very small working surface area for condensation resulting in damagingly high temperatures and pressures, and poor energy efficiency.
It is hard to say how many repaired systems out-there are given the correct charge, and hard to know if, or how much this translates to a reduction in COP, and I don’t want to worry those who have had repairs carried out; most engineers are both capable and conscientious. On the other hand, I wonder how long the old-school attitude of the refrigeration engineer will plague our heat pumps in the field.
Air conditioning comes somewhere in between refrigeration and heat pumps. Systems are fairly fussy about refrigerant quantity, but bizarrely, to my mind, the pipe connections used are still the old copper ‘flare’ screw-together type. Whilst often a 99% seal, they can leak, and can ‘weep’. Why on earth there is not a requirement to use a more secure pipe joint eludes me.  Surely due to this, there are many air-conditioners and air-air heat pumps that are operating well below of their optimum with a low refrigerant level.  It’s very difficult for the owner to know if a system is energy-inefficient.
To finish my dig at the old-school ways of refrigeration engineers, I fairly recently watched an engineer weigh-in the correct charge of refrigerant. Great I thought, but when he got to the required level (calculated in his head!), he added a bit for luck!  Why did he do this?!  Maybe being helpful to allow for any futures seepage. But this extra amount will potentially reduce the efficiency.  Maybe a fresh look at the importance of getting our systems optimised and energy-efficient is needed.
Since refrigeration issues are rare, it is quite common for a local engineer to carry out the repair on equipment that they have no experience of.  It would be helpful for all manufacturers to provide charts similar the example below to help any engineer to ‘gauge’ if the refrigerant charge is correct without needing to recover and weigh-in. It would also server as a performance health-check for the system. It should be tucked in a plastic wallet inside the unit.