Underfloor heating in bathrooms

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.

How energy-efficient should a heat pump be?

Heat pumps have always been cited as energy saving devices. The fact that they can give out a quantity of heat several times that of the motive power to drive them is proof enough. However, the sceptics have in the past rightly pointed to the inefficiencies of the source of that motive power – electricity, and many conclude that the inefficiency of the power stations coupled with the very high efficiency of a heat pump only just cancel each other out. i.e. why not simply burn the power station’s fuel directly in the home without the complexity of heat pumps?
It is clear that the range of achievable efficiencies that a heat pump system delivers can vary greatly. This is primarily dictated by the type of application, and secondarily by the engineering details or the system.  For designers to evaluate the net-worth of a potential system, a good grasp of the environmental issues are required.
CO2 seems to be the prime consideration here, and we can make simple mathematical comparisons to see if the real advantage of a heat pump is sizeable or only marginal: worth installing or not.
On the generation side of things, market forces and other factors drive the decisions that dictate how the National Grid buy and produce electricity. The UK’s generation ‘mix’ leads to a figure of how many kg of CO2are released for every kWh of electricity produced – on average, around ½ kg for each unit of electricity. To make things complicated, this varies over the day, over the seasons, and will vary year on year.
(Note: I ignore here any ‘green’ tariffs on the grounds that we would need a great deal of it to make a notable difference to the figures nationally).
Given the UK figure of 0.5kg/kWh, and a rough notion of expected future variations, we should be able to compare CO2 figures for direct electric heaters (100% conversion), with gas and oil heating.   We should now be able to consider heat pumps with various COPs, and arrive at some figures for the efficiencies that we might like to achieve.
(Heat Pump efficiency is measured using the Coefficient of Performance (COP). COP is the ratio of useful heat output divided by the power input. Seasonal Performance Factor (SPF) is the annual useful heat divided by the annual Electrical input. This should include use of any direct electric back-up heater)

The vertical (left-hand) axis shows pollution figures for direct electric heating, gas and oil. The graph shifts to the right with increased COP or SPF of a heat pump (COP1 is the same as an electric heater).   As we can see, it’s relatively easy to work out a break-even SPFs compared to common heating methods. The more difficult question is ‘How much better should a heat pump be’?
At this point it seems important to turn our attention to what efficiency levels heat pump technology can offer, then to seek the compromise that all such designs are based upon; cost / benefit.   If very high COPs are attainable, but excessively costly to manufacture and install, they are probably of little benefit. At the other end of the spectrum; cheap low-efficiency systems may bring no carbon saving at all.  We need to look for application that are practical and affordable to install, and show a good CO2 saving.  The question to ask here is – is the particular application a good one for this technology?  Is it one where a high COP can be achieved without excessive installation cost. If not, maybe a different technology should be adopted – eg. If a boiler can be installed for 1/3 the cost, the money saved may allow some very serious insulation. The outcome might give a better net energy saving.  A holistic view is needed, and for heat pumps to compete- they need to be energy efficient.
My perception of people’s expectations for SPFs (annual COP) is that they have had a knock over the last year or so.  I had hoped that SPFs might pan-out around 4 – given some improvement in the technology over the years, however, it seems that many systems have in recent years, fallen short of  what’s-possible.
It seems to me that the industry is all too willing to go down the road of ‘mediocre’ efficiency. Indeed, for the heat pump industry to survive at all, installations must be affordable, so some are only too keen that, for example, an SPF of 3 is perceived as good – and who can blame them?.   Taking this further, if systems can be made that are cheap and easy to install, then break-even SPFs (compared to gas) might become attractive. A heat pump could possibly become a ‘convenient heating method’ with no environmental advantage.
So what efficiencies do we need?  The game on the DECC website titled ‘2050’ is worth a look. It shows how difficult it will be to achieve our carbon abatement targets by the year 2050. If we are to get even near, we need as much COP/SPF as we can get!   
The recently produced Emitter Guide  (  has been developed as a ‘guide’, not a design tool (‘Emitter’ referring to ‘heat emitters’ – radiators or underfloor). If I understand it correctly, the guide was produced to assist in ‘steering’ designers and installers towards better systems.  It has a well laid-out flow chart for the installer/designer to consider both new and existing radiator, and should be commended highly for its recommendation to stand-back and consider implications of improvements to thermal insulation, or reducing ventilation losses. This is a welcome deviation from the old just-do-my-bit ways of the industry. However, as I scan down the expected SPF figures for the various options of radiator oversize or underfloor heating pipe-spacing, The question I am asking myself is – how does anyone know what SPF to aim for?  I have considered this intangible question for many years, and I am still much in the dark.
The general drop in COP expectation is not helped in my view by the range of SPFs quoted on the emitter guide which, for the air source system, has a mid-range of 2.85, and the lowest figure (2.1) relates to CO2figures worse than gas.  If one were totally in the dark, one would probably be ‘swayed’ to thinking that an SPF of 3 was quite reasonable.   It is also important to note that the figures relate to space-heating only and since the vast majority of installations also heat hot water, and since DHW heating can occur with a relatively low COP in the region of 2 to 2.5, it is clear that some of the figures quoted will be pulled-down by DHW heating in practice.   With this in mind, the 6 star option (flow temperature only 35°C) could be viewed as ‘normal’ as opposed to the ‘exceptionally good’ that it might currently appear, being top of the list. 
I wonder at this point if those with a good handle on the environmental issues should get together with those who know what real-live SPFs are practically achievable, and give some guidance on what systems are worth pursuing, and which ones are not.   System designers need to know want level of system efficiency to aim for, otherwise there is a danger that other figures will be found, e.g. minimum standards, and these will be used as those targets.

I have added this intesting graph sent to me by John Logan of Maine USA (East Coast).  Its interesting to see their experience. John is a pioneer and proponent of the ‘Standing Column’ borehole system, and their experience looks impressive.  They are experiencing average COPs of 4.5. This shows an application and a climate that match well – here we have very competitive running costs and very good SPF.

The graph also shows a 12RLS2 air-air split systems with COP almost 3 – obviously fits well in their climate.  

Home Size 1,500 sq.ft., HP COP = 4.5, 12RLS2 COP = 2.97, Electricity $0.15, Pellets $250, Oil $3.50, Propane $3.00