Skip to main content
Observer Ethical awards Winners 2011

YouGen Blog

Heat pumps and underfloor heating: perfect partners?

Posted by John Barker-Brown on 1 November 2010 at 11:01 am

Underfloor heating is often portrayed as the perfect partner for heat pumps. With its large heat emitting area and low input temperature requirements it seems to provide the ideal choice to offer the maximum efficiency from the heat pump. However, is that always the case?

With any underfloor system to be used with a heat pump, it is important that the system is designed correctly, for example due to the lower flow temperatures of a heat pump system the underfloor pipes are generally installed at smaller centres increasing the pipe density and the type of floor covering used will also have a major effect on performance. 

There are a number of different underfloor heating systems which can be installed in a property:

1) Underfloor heating mounted in screed: This is most commonly used on the ground floor, but can be used in first floor applications in buildings constructed using a beam and block design. As the underfloor pipes are mounted in a concrete screed with a high conductivity and lack of air gaps, a low flow temperature can be used and this is generally around 35C. This temperature should be adequate to meet the Building Regulations requirement of 21C in the living area, with an average of 18C throughout, at an external ambient temperature of -3C. As said above the lower the flow temperature, the greater the heat pump efficiency, and COPs (coefficients of performance) of around 4 can be achieved. The use of the screed can also act as a thermal store and so lower cost off-peak electricity can be used, lowering running costs further.

2) Joisted systems: There are a number of different joisted systems for underfloor and these are generally used for first floor and above underfloor systems. I have listed the 4 most common ones below:

i) Over-joist – This is where the pipes are laid on top of the existing floorboards and then covered with a wooden deck. You generally have to run the heat pump at a higher flow temperature to drive the heat through the ‘insulating’ floorboard and into the room. This will increase the required flow temperature and we have found this to be around 45C, meaning the COP is now around 3. With over joist systems there are a couple of further issues which also need to be taken into account. If the pipes are bedded in a dry sand/cement mix to provide a close contact and thermal store to enable the use of off-peak electricity, this has an associated weight which needs to be considered in the joist design. There is also an increase in floor level so staircases, skirting boards etc need to be designed with this in mind.

ii) Between Joist – The underfloor is now installed between the joists, keeping the floor level the same. The same structural issues apply as with over joist if a dry sand cement mix is used. With regards to flow temperatures again these need to be around 45C to drive the heat through the ‘insulative’ floorboard, lowering the heat pump efficiency. Although between joist seems a better solution it is more difficult and hence costly to install. 

iii) Plated system – This consists of aluminium plates placed between joists to which the underfloor pipes are clipped to and is probably the most common of joisted underfloor systems. It has the advantage of being quick and easy to install and hence lower costs. However there is no screed to act as a thermal store to maintain the heat into the room when the heat pump is turned off, so the heat pump generally has to run 24 hours a day. The system is still covered with floorboards so there is an effective insulation layer and the flow temperature of the heat pump has to be increased to around 45C with the corresponding drop in efficiency. 

iv) Air Void systems – This is where the underfloor is simply mounted between the joists, with no aluminium plates or screed. Due to the fact that as well as having to drive the heat through the floorboard, the heat now has to cross an air void, which is a very good insulator, the flow temperature has to be even high than the 45C. In fact the flow temperature is now so high that the heat pump generally cannot provide this and still maintain Building Regulations temperature requirements. Air Void systems are generally not advised to be used with heat pumps.

So although underfloor heating can be the ideal partner for heat pumps it does depend on how the underfloor heating is installed. For any joisted underfloor heating you have to run the heat pump at 45C, which is the same temperature that radiators need from a heat pump. So, why not install radiators on joisted floors with underfloor on screeded floors instead? It's a much cheaper solution.  Don’t get me wrong a radiator/underfloor mix is not perfect, there is a delay in the radiators getting warm as the screed downstairs takes all the heat and the radiators are generally oversized due to the low flow temperatures, however these are minor issues compared to the cost savings involved.

Photo by Bryn Pinzgauer

About the author: John Barker-Brown is special projects manager at British heat pump manufacturer Kensa Engineering.

If you have a question about anything in the above blog, please ask it in the comments section below.

Like this blog? Keep up to date with our free monthly newsletter


15 comments - read them below or add one


heatingdudeComment left on: 30 December 2011 at 11:51 am

Excellent article, something that put me off on my own underfloor heating installation was the additional start up cost that it incurrs.

 Readers please be aware that you will make the extra cost back in savings on your utility bills from the increassed efficiency, and added value to your property. Ive nearly had mine pay for itself due this already.

infact i set up a site (underfloor heating cost) dedicated to the issue, hope you dont mind me posting the link here - i feel its very relevant. 

report abuse

Kensa Heat Pumps

Kensa Heat PumpsComment left on: 3 December 2010 at 11:42 am

Conservatories are a very popular home improvement. A decade or so ago, they were seen as a key element in the design of passive solar houses, but more recently, the accepted view seems to regard conservatories as energy liabilities. In winter the larger window area results in a large heat loss, resulting in a cold room. Whatever heating system you employ has to overcome this heat loss to maintain a comfortable temperature within the room.

The bigger the conservatory, the more heating it will require. A large conservatory, even if it is only heated to 18°C, can cost more to heat than the well-insulated modern house it is attached to.

I’ve quickly done some simple heat loss calculations based on your post and some assumptions and you need to overcome a heat loss of 118W/m2 (based on an ambient temp of 0). The problem with underfloor is that even with 20mm of natural stone covering and a maximum temperature of 50C it will only supply 94W/m2, so it is not an effective heating system in this application.

The next problem you are going to have is a heat pump is a low temperature device, so producing the heat you require in this type of application is going to difficult if not impossible. Even if it does produce the required heat then its efficiency will be poor.

I’m not surprised that you are confused about the difference between air source heat pumps and air conditioning units, a number of manufacturers have simply reversed an air conditioner and marketed it as an air source heat pump. The main difference between the two is for a true heat pump the compressor needs to be designed and optimised for heating as opposed to cooling.

report abuse


JohnWLComment left on: 2 December 2010 at 1:04 pm


In response to your question with regard to heating a conservatory I guess the first point to make is that normally one would recommend that if at all possible all insulation methods that can be taken, are. (i.e. double or triple glazing and cavity wall insulation, if possible in the cavity of the two brick walls )

It obviously doesn’t matter which technology you are using to heat your conservatory, if there is less heat loss there will be less fuel, what ever it may be, used.

In terms of the choice between underfloor heating or radiators this depends on the way the room is going to be used. Underfloor heating allows for lower flow temperatures to be utilised, which allows a heat pump to work more efficiently, but in most cases requires some thermal mass which effectively means the response time is slower than radiators. If you are just using the room for an hour or so at a time the problem would be the heat you have in invested in the thermal mass of the floor would continue to be emitted even after you had left the room thus effectively wasting heat. If however at the other end of the scale you intend to use the room through out the day and night or for long spells, underfloor heating would provide a far more effective solution.

In terms of the difference between a Air Source Heat Pump and a Air Conditioning unit, I can see why it would be confusing. Essentially they both use refrigeration circuits to pump heat from one place to another, thus the term Heat Pump. Essentially the difference between the two is that the components in an Air Conditioning unit are selected to be optimised for cooling. In other words to get the maximum cooling capacity for the minimum power input, whilst heating optimised Heat Pump’s have their components selected to optimise their heating efficiencies. If you were to use more energy in the year cooling your building than heating, you should chose a  Air Conditioning unit, if however, as is normally the case in the vast majority of domestic premises in the UK, you use more energy heating your property than cooling then you should opt for a heating optimised heat pump.

report abuse


ajbusby10Comment left on: 1 December 2010 at 12:38 pm

Hi, as suggested by YouGen I've posted this as a comment to see if any ideas/advice arise....

I'm trying to work out the most green/economical way of keeping a conservatory heated when needed. We do tend to use it most of year except when it gets really cold (i.e. night frosts). Details: North Facing Only two out of the three sides are glass (the third is brick) We have high spec roof and window glazing already Dimensions approx length 4.4m, width 2.8m, height 2.3m All I've worked out so far is the dimplex electric panel heaters being used at the moment are too expensive/not very green. In particular the difference between an air source heat pump and an air conditioning unit appears to be confusing. Underfloor heating also appears to be an option. Happy to invest up front if it minimises the long term running cost as energy prices are only going one way. Any expert advice appreciated. Thanks, Andrew.

report abuse

Stirling Consulting Limited

Stirling Consulting LimitedComment left on: 4 November 2010 at 3:40 pm

The MCS is a gateway organisation to facilitate the award of grants to promote the adoption of, in this case, heat pump technology as a precursor to a larger government stated objective of reducing emissions by 2015. Like many self-interested groups it is, as you recognise, not infallible [and I refer you to the response to Cathy] but I would have hoped that the identification and promotion of all regulatory matters would carry the highest consideration.

Our independence from manufacturer allegiances enables us to highlight areas of concern which could reduce the uptake of any emission reducing technology. The irrefutable fact is that at some point the plant will fail and the associate costs of recycling and repairs will be, to many, an unacceptably high percentage of the installation cost.

I would refer you to the F-Gas Regulations and specifically the following: All parties are responsible for the refrigerants on site and are liable to substantial fines or the EPA being invoked for any leakages or records not being kept.’ Do I conclude from your comments that heat pumps can be left for extended periods without inspection whilst still maintaining compliance?

As the representative of a heat pump manufacturer I can fully understand your responses and the rationale therein but I hope you take the points forward in a positive manner and provide written statement of all requirements as a matter of standard practice. As a slight aside being relatively new to refrigeration [with only 25 years’ experience, ISO and Professional Memberships] where do you actually get warranty on a compressor without having conducted an acid test and replaced drier(s) in accordance with manufacturer’s instructions?

No doubt we will have occasion again to exchange views when I comment on the environmental aspects associated with heat pump technologies- I look forward to the exchange!

report abuse

Stirling Consulting Limited

Stirling Consulting LimitedComment left on: 4 November 2010 at 1:41 pm

Cathy, I can see how you might consider the comments becoming too technical so I will re-phrase the approach. The following is published by the Micro Certification Scheme [MCS]:

‘The performance of Microgeneration heat pump systems is impossible to predict with certainty due to the variability of the climate and its subsequent effect on both heat supply and demand. This estimate is based upon the best available information but is given as guidance only and should not be considered as a guarantee.’

The MCS also refer to compliance and yet fail to provide specifics [we have been in contact with the aforementioned and will be providing information for their steering group which will hopefully close this loophole].

I ask myself this simple question, would I invest anywhere between 6 & 20K, on an installation in light of comments previously made or the reproduced statement above?

I would like to think that the regulatory content of these threads, since they seem rarely quoted by manufacturers in full detail, are EXACTLY those which the website would encourage to facilitate the readers considered evaluation of heat pump technology.

report abuse

Cathy Debenham

Cathy DebenhamComment left on: 4 November 2010 at 12:32 pm

Thank you very much for such considered and detailed discussion - we welcome participation both from industry professionals and consumers. However, I agree with John that this discussion is probably getting a bit technical for this forum. The YouGen website is aimed at homeowners, and we aim to provide enough information to enable them to make good decisions about what might suit their house. This discussion seems more suitable for an industry forum...

report abuse

Kensa Heat Pumps

Kensa Heat PumpsComment left on: 4 November 2010 at 12:24 pm


Point 7) Maintenance is minimal for a heat pump. Slightly higher for air source than ground but minimal compared to an oil or gas boiler. Any repairs that need to be undertaken should be carried out via the appropriate trained individual.

Comparing cars to heat pumps is like apples and pears. A closer analogy would be a fridge and the maintenance people carry out on these, which if anything is minimal. Decommissioning or repair is complex, but doesn’t happen that often. The 3 or 4 times we have been required to do an acid test, the results have been negative. We have supplied over two thousand systems with no real problems. 

I agree that clients should not be mislead and provided with all the information they require to make an informed judgement. MCS although not perfect does go some way to achieve this. There is a lot of mis-information in the market place which can confuse clients and this needs to be removed as soon as possible.

report abuse

Kensa Heat Pumps

Kensa Heat PumpsComment left on: 4 November 2010 at 12:21 pm

Sorry but for some reason (maybe the thread is getting to long) I lost half of my reply. 

Please see the continuation below:-

Point 4) ASHP’s do outnumber GSHP and are forecast for a higher growth, however there are still thousands of GSHP’s installed and these are becoming the technology of choice for a number of social housing providers, simply due to the lower life costs. I think GSHP’s are less problematic than ASHP’s simply due to the smaller number of moving parts, not having to be installed outside, not having to work so hard, etc.

With regards to moisture, with any ground array the wetter the better. With areas such as tennis courts moisture also flows under the court from the sides. There is also the added benefit that the tarmac surface actually absorbs more warmth. The actual depth of a slinky or any ground array (except boreholes) should be below 1m, this is the general accepted depth in the UK, any deeper than this the benefit does not outweigh the installation costs.

For boreholes, the only way to determine the exact length of a borehole for a given load is to complete a thermal response test on the borehole. This involves a week of testing, and is somewhere around £7-£8K just for the test. It therefore means that for small domestic applications the driller will use rules of thumb, desk studies and basically over estimate (to be on the side of safety) the depth. For larger commercial applications the Thermal response test becomes more important and can result in a reduction of boreholes and hence a cost saving.

Point 5) When temperature difference is talked about within the heat pump industry it generally means the difference between the source temperature and the outlet temperature. The greater this temperature difference the lower the efficiency as the heat pump has to work harder.

Point 6) Not really, as MCS requires the installer to be competent to Design, Supply, Install and Commission Heat Pumps. To install a heat pump which will be eligible for grants and funding streams you need to be MCS accredited. Without this accreditation clients will simply not order from you. It goes through the whole process from design, quote, supply, etc. Although it is paid for via its members, it was developed by DECC and is similar to the Gas Safe scheme. Part of the scheme is that you need to be a member of the REAL assurance scheme, which deals with customers complaints, etc.

With regards to the EST report a large majority of the installs were carried out before MCS was truly embraced by the industry and not by MCS approved installers. In fact this is one of the reasons that EST feel could be the reason for the lower efficiencies.

Point 7)

report abuse

Kensa Heat Pumps

Kensa Heat PumpsComment left on: 4 November 2010 at 12:08 pm

I've got no problem replying to your comments in the public domain if you wish. I just feel that the thread was never relevant to the article and it is getting more in depth then really required for this forum.

Anyway please see my comments below:-

Point 1) This is a bit harsh as there are companies who provide more than enough information to clients and will only sell heat pumps in the correct application, however as always there are companies who are opportunistic and do not. The microgeneration certification scheme has been introduced to ensure that the installer should inform the client of all relevant information. The scheme is designed to provide reassurance to the client that the technology is not being mis-sold.

Point 2) Correct, however SAP is currently the best method we have and is endorsed by Government. In fact SAP is being considered for calculating the payments for the Renewable Heat Incentive. It is not ideal but it is the preferred method.

With regards to the Energy Saving Trust report and the lower efficiencies reported, this is a whole subject on its own. For example a number of the heat pumps had in-built immersion heaters (lowering overall efficiency), the efficiencies for the air source were also distorted as 6 results were estimated from data provided by the manufacture, (these results were within the top 7 efficiencies of air source). It must also be remembered that the report is only half complete, with phase 2 hopefully starting this year. If you wish to take part in the second part of the report EST will allow you for a fee to be involved.

Point 3) The South West was being used as an example, but I don’t think you can say because people are off grid they have extremes of climate. The UK does not have a climate which is extreme, however depending on the location of the property allowances should be made for the local climate in sizing. For example in Scotland we use an ambient temperature of -5 degC. -3Deg C ambient is generally what is used to calculate an acceptable peak load of a property in England. 

report abuse

Stirling Consulting Limited

Stirling Consulting LimitedComment left on: 3 November 2010 at 5:45 pm

I would have hoped that you continued the responses on this forum rather than behind closed doors for the benefit of anybody who might wish to follow the thread.

From our viewpoint we shall continue to provide free independent advice, for example on changes to F-Gas Regulations should they be forthcoming or other aspects relating to heat pump technology, or any other fields which might require our input.

report abuse

Kensa Heat Pumps

Kensa Heat PumpsComment left on: 3 November 2010 at 10:29 am

The article was written as a generic article on underfloor heating and heat pumps, I think the comments have somewhat drifted off the subject and seem to be getting longer and more in-depth. 

I'm happy to respond to your comments, but don't think this is really the forum. If you wish to e-mail me your contact details we can continue the discussion. 

report abuse

Stirling Consulting Limited

Stirling Consulting LimitedComment left on: 2 November 2010 at 1:28 pm

In response to comments received:

1) All of our comments were made to support an overall contention that the industry sector fails to disclose adequate information to enable prospective customers to make an informed decision.

2) You mention the adoption of SAP results, presumably provided from another source, and herein we may have one possible insight into the lower efficiency values, as defined in the Energy Saving Trust Report, than anticipated. The output of this process [SAP] or any others are absolutely dependent upon the quality of information obtained by a visual inspection and the only circumstances under which you might have 100% predictable results are with a new build.

3) Off grid refers to anybody who currently employs non-mains energy systems such as oil/propane or electric. The ambient conditions in the South West are better than that elsewhere but since our comments relate to the industry sector should they not be applied to the whole of the UK?

4) The observation on the bias towards Air to Water Heat Pumps [AWHP] reflects our experience in larger numbers of these units being installed over and beyond Ground Source Heat Pumps [GSHP] however these units can be equally as problematical and still retain refrigerant. For a GSHP to operate effectively the soil adjacent to the slinky or bore hole must remain moist. If the level of moisture was to reduce, as in the case of an installation under a tarmac tennis court, the soil contracts creating an air gap and as you commented air is a good insulator. Equally the prediction of actual depth of insertion for the slinky or the bore is not an exact science since ground conditions can and will change affecting ultimate performance values.

5) Heat transfers naturally take place from higher to lower values on which basis we heat our homes and offices. In the case of conventional boilers a flow temperature of between 76-80C is typical and given that an average room stat setting is 22C the temperature differential is some 58C which in turn warms the air. In the case of either AWHP or GSHP with a typical flow temperature of 45C the temperature differential is reduced to 13C which could result in extended periods of operation, which questioned the efficiencies being produced as confirmed in the Energy Saving Trusts Report, a factor which is amplified if the selection was borderline from the outset. 

7) The exemption of annual inspection of hermetically sealed systems is determined by weight of charge [6Kgs]. This does not however absolve the operator/maintenance/service company from compliance with other aspects of F-Gas Regulations. Individuals and companies must be accredited with one of a number of available bodies to maintain compliance [current provisions must be upgraded by 4th July 2011]. Refrigerant and Oil are defined as hazardous waste and need identified procedures for disposal [as stated previously] such that the examples indicated herein and those contained within the Act might affect the customer’s choice to install these systems.

For example if we can agree that new equipment operates at maximum efficiency by definition that which has operated for a period will be subject to a proportional decrease from its starting point. It continues that at some juncture the equipment will fail and require the attendance of an accredited engineer [a factor which might be accelerated in the absence of maintenance in much the same manner as that which could be anticipated from our car if service intervals are ignored]. In this example the compressor has failed so the engineer must recover the system charge-this produces a cleaning charge for the reclaim bottle. The compressor is removed and tested for acid levels- the test phials contain oil and therefore are included in the identified procedures quoted above- again adding to the cost. The replacement compressor is installed alongside a burn-out drier [to catch any acidic elements left within the system] alongside a new liquid line drier to capture any moisture which may be within the system. The aforementioned components are sealed and taken for recycling via the documented procedure with the results confirming this process supplied to the client to maintain their compliance. The system is evacuated/pressure/leak tested and then re-charged and re-commissioned prior to the engineer’s departure. Approximately two to three days later the engineer attends to check the acid levels within the system, again the phials must be sent for recycling, if the test proves the presence of acid both driers should be replaced [the process being repeated until the system is defined as free from contamination] with the aforementioned recycling considerations. In short when considering the total costs of the repairs they could exceed that of a new boiler!

As we stated at the start of this response the sector should, in our opinion, make clear these requirements, and others not listed herein, so that customers feel informed enough to make a considered evaluation. Failure to address this will probably result in a reduced uptake in this technology with its obvious implications for emission reductions.

report abuse

Kensa Heat Pumps

Kensa Heat PumpsComment left on: 1 November 2010 at 2:51 pm

Thanks for the comments. The article has been written trying to be generic regarding the type of heat pump provided, air, water or ground, however the points in your comments seem mainly to be air source based?

I completely agree with the comment about obtaining a correct heat load and how difficult this can be with regards to existing properties. I think in the case of this article where we are talking about installing underfloor (due to the disturbance involved) we would generally assume that this is a new build or complete renovation which would generally mean a SAP report is produced. This can be used to help calculate the required size of heat pump. Existing properties do require extra care when sizing. 

By off-grid I presume this is the electricity grid? If so the number of properties which are off-grid is very small and not the target market of heat pumps. If you mean off-gas, then I'm sure the people in the South West will disagree with you about extremes of climate. A large number of properties are off-gas and yet the climate is relatively mild.

Quoting 7C also seems to indicate you are talking air source. If you take ground source then you are looking at a pretty constant ground temperature, meaning the large drop in efficiencies you mention tend not to occur.

Also the last sentence 'maximum efficiencies are produced at maximum temperature differentials' I believe should be 'maximum efficiencies are produced at minimum temperature differentials'?

Comment 3 is completely true regarding air source but not ground source which do not have a defrost cycle. 

With regards to comment 4, although true regarding you do not need to be accredited to buy or install a heat pump, to access grants a MCS installer has to be involved in the design, supply, install and commissioning of the system. This will give the client access to the grants such as the Renewable Heat Incentive which will come into play next year (although it is planned that installations from 15th July 2009 will be eligible). F-Gas regulations will also state that hermetically sealed systems under 5kgs of refrigerant (which most heat pump systems are) do not need to be inspected, however if maintenance is required (very rarely) a F-Gas registered engineer is required.

report abuse

Stirling Consulting Limited

Stirling Consulting LimitedComment left on: 1 November 2010 at 1:45 pm

1) In the absence of pre-construction data or invasive inspection techniques the results typically incorporated within CIBSE tabulated forms to produce a definitive load are subjective on the basis of age and type of construction. It might well be argued that the results from the recent Energy Saving Trust confirm the problems in using generic data to provide specific determinations.

2) If we consider that the most probable application for renewables is off-grid it’s reasonable to anticipate that extremes of ambient might exceed the frequently quoted 7C or as in the case of this content -3C. At lower ambient conditions [-5C the COP can reduce to 1.8 manufacturers data]. Equally it must be recognised that maximum efficiencies are produced at maximum temperature differentials so if the equipment selection is less than ideal extended operational periods can result adding significantly to the costs.

3) Given the infinite ambient operational range within which heat pumps are required to operate the determination of defrost duration [ice removal from the coil] requires extensive calculations which in no way might be viewed as definitive. It is the method of removing the ice which again quantifies the overall efficiencies produced. One manufacturer has traded the use of previously generated heat to guarantee the removal of ice as opposed to others who rely upon hot gas bypass which in lower ambient may fail to clear the coil adequately reducing efficiency of the next generation period.

4) Under current legislation it is possible for anybody to purchase a heat pump and install the same [the requirement for accreditation is based on charge weight- the current level is 3Kgs or above] but is not permitted to service/maintain the same if company and personal accreditation [Refrigerant Safe Handling Certification] has not been achieved. Additionally by definition both refrigerant and oil are classed as hazardous wastes which require an audited procedure demonstrating correct disposal to maintain compliance [failure to comply results in the application of fines].

In Summary

Heat Pumps will as a result of their elevated outputs over and above more traditional alternatives produce reductions in emissions.

We wait to see if the costs from cradle to grave [including recycling of service items and the unit when replaced] and those of attending remote sites actually produce the complete savings which are quoted in certain publications.

Our current workload centres on mandatory compliances [EPC/DEC/SAP/ACI’s] and addressing sites which have failed to meet full expectations. This does not make us anti heat pump technology but it must be considered in full context.

In our opinion full disclosure of all aspects, some defined within this response, would represent a significant move forward which would enable customers to make an informed decision rather than the basis of advertised monetary savings.

report abuse

Leave a comment

You must log in to make a comment. If you haven't already registered, please sign up as a company or an individual, then come back and have your say.