Air source heat pumps are a risky choice for householders trying to save money and CO2 emissions. This piece looks at the experience of one householder in the south of England who has kept detailed meter readings over the last few weeks. The findings are disturbing. The recent low temperatures (early February 2012) have shown that the costs of running a heat pump can be unacceptably high in cold weather. Anybody considering this new – and apparently eco-friendly technology – should be very wary indeed about their energy bills in deep winter. In fact, they should consider turning off the pump and going back to electric radiators when temperatures drop.
The date in this article come from a home of about 90 sq metres (approximately 1000 sq ft), which is about 20% larger than the UK average dwelling. Because the house is detached, with a larger exposed wall area, energy bills are likely to be higher than a terraced house or a semi-detached of the same size. But the householder has done substantial eco-renovation on the house, including filling the cavity wall and insulating the floors and loft. The windows are double-glazed. His final action was to install a new air source heat pump, put in place by specialists. He knew that a heat pump could only possibly be effective in a well-insulated house but he thought his work would mean that his family would benefit finacially from the new heating system. So far, this hasn’t been the case
My rough calculations suggest that this well insulated house probably loses about 200 watts per degree of temperature difference between the inside and the outside. That is, if it’s 10 degrees outside and 20 inside, it will need a heating system that provides 2000 watts, or 2 kilowatts. The key question : is a heat pump a good way of providing this?
The big advantage of this relatively new technology is its potential ability to use relatively small amounts of electricity to create larger amounts of heat. (No – this doesn’t break the laws of thermodynamics, see here). The effectiveness of using heat pumps to cut our energy bills depends crucially on how much heat you get out for every unit of electricity you put in. Manufacturers will usually quote ratios of three or four. This householder’s experience suggests that the real figure may be as low as 2 or below.
At that level it makes no sense in cash or carbon terms to use a heat pump. Even for homes with cheap rate meters (‘Economy 7’) for night electricity, the average 24 hour price of power is about 8.5p per kilowatt hour at the moment. Mains gas – which isn’t available around the home whose electricity usage I am reporting here – is about 3.5p per kilowatt hour. In other words, a heat pump which converts one unit of electricity into only two units of heat costs more than 2 units of gas. The carbon dioxide emitted at the average power station to produce a unit of electricity is also over twice as much as the direct emissions from burning gas in a home boiler. If the figures at this home are typical, heat pumps don’t work well in the UK. (This is a strange finding – they really do work well in some other countries such as cold Sweden and nobody seems to be sure why things aren’t the same in the UK).
The failure of many air source heat pumps to save money in Britain must, I suspect, be down to poor expertise among installers. Heat pumps are fiddly to operate and require delicate adjustments. Unfortunately, until this problem is solved, no householder will be prepared to be the guinea pig for a technology that often seems to struggle in (relatively) cold weather. Some sources suggest that the problem arises because the pump ices up – but this doesn’t explain why the same problem doesn’t occur in colder countries
The numbers
We’ve had a wide range of external temperatures over the last couple of weeks. It started quite warm but the last few nights have been very cold by UK standards, with the thermometer dipping to as low as minus 7 degrees in the local area. As the chill worsened, the efficiency of the heat pump dropped dramatically.
| Dates of measurements | Average amount of heating required over 24 hours* | Estimated heating need for the house over 24 hours** | Typical daily electricity use*** | Implied ratio of electricity input to heat output (‘coefficient of performance’) |
| Around 23rd January | 10 degrees | 50 kilowatt hours | 25 kilowatt hours | 2 |
| Around 28th January | 15 degrees | 75 kilowatt hours | 50 kilowatt hours | 1.5 |
| Around 3rd February | 20 degrees | 100 kilowatt hours | 100 kilowatt hours | 1 |
*The difference between the average external and internal temperatures
** The average heat loss from the house’s walls, windows, door, floors and roof per degree of temperature difference multiplied by the average temperature difference.
** The metered use of electricity over a typical 24 hour period
In the early part of this short study period, the electricity consumption figures were poor but not excessively so. The family was getting 10 degrees of heating of his house from the pump for about 25 kilowatt hours a day. This meant the ratio of heat output to power input was about 2, well below the level promised by the manufacturer but still nearly enough to justify using a heat pump. But as the thermometer fell, the bills went up. He was getting about 100 kilowatt hours of heat for each 100 kilowatt hours of electricity he used. This means that in cold weather the unlucky householder is spending eight or nine pounds a day on electricity (multiplied up, £250 a month) but, even more strikingly, he would be better off if he simply installed a few electric heaters in the main rooms. In fact, if I were advising him, I’d say he should turn off the pump whenever outside temperatures fall below about 7 degrees.
The householder has been worried about the performance of his expensive new heat pump since it was put in. He’s had the people who installed it round, as well as the main contractors for the insulation improvements, just in case they could find out whether the house had major temperature leaks. His concerns seem warranted because his pump is costing far more than it should do. This story is repeatedly heard across the UK – it’s now time to really find out why many of the heat pumps installed in houses come nowhere near achieving the benefits claimed by manufacturers.
-
Your article raise several issues to do with insulation, efficiency and controls, etc. It does not mention energy storage.
Air source heat pumps draw energy from a highly variable temperature thermal store with very poor heat storage characteristics – air. Leaving aside questions of basic system efficiency etc, gas, base load storage heaters and ground source heat pumps all use much more readily storable forms of energy. Thus they are far less prone than air source heat pumps to losing efficiency just at the moment when you want it most.
Tony Day
-
A worrying tale and one not dissimilar from a study conducted by the Energy Saving Trust. However, I do have a number of questions and comments on this.
Air source heat pumps (ASHP) would be a good choice for a new build where the heating system has been specified at the design stage and matters such as up to date insulation, underfloor heating and low air permeability have been taken into account.
For a retrofit, an ASHP would not be recommended (currently) where main gas is available. So I don’t think your price comparison with mains gas is fair. Heating oil or LPG would be a fairer comparison.
There is no mention of how the heating is being delivered, underfloor heating or radiators and whether the radiators are of the type suitable for heat pumps. Neither is there any mention as to whether the heat pump is delivering hot water as well.
The fact that you had to do some calculations yourself suggest that a SAP 2009 assessment was not done for the property. I guess reputable contractors were used for the insulation work and the heat pump installation. So I would check about the SAP 2009 assessment.
The coefficient of performance (CoP) for ASHPs are quoted for standard conditions, usually an input temperature of 7 degrees and an output temperature of 35 degrees (suitable for underfloor heating). It is well known that the CoP will drop off markedly for input temperatures below 7 degrees.
I am puzzled as to why turning the heat pump off and using electric radiators would be cheaper. I am under the impression that the minimum CoP for a heat pump is 1. Therefore it is not going to be any worse than electric radiators but can be much better. The annual cost of running the heat pump would be a better figure.
What is quite clear, though, is that this is a more intricate technology with finer tolerances than the traditional boiler /radiator system, which is likely to be way over specified.
I suggest you have a look at John Cantor’s book on heat pumps.
http://www.heatpumps.co.uk/index.html
Mike
Disclaimer: I do not work for a heat pump company/installer and I do not have any investments in this technology. I am actively looking to install a heat pump in our own homer.
-
Mike Lloyd
Thank you for the comments.
a) Mains gas comparison. My point – not clearly put – was that ASHPs are not better than gas in cost terms. More relevantly, for those households off the gas grid, it would be cheaper to use storage radiators working from off peak electricity. Sorry for the clumsy writing on this. So if is cold, and the COP is only 1, then it would better to use resistance heating.
b) A SAP test was not carried on the property. My conversations with the householder, including discussion of the techniques used in the renovation, seem to me to imply that an ASHP was a reasonable choice for the heating system. (For most UK homes, of course, a heat pump would NOT be appropriate).
c) The heat pump is delivering hot water as well. The householder complains that his family is unable to get adequate hot water at the same time as the house is being heated.
d) Some of the house has underfloor pipes. The other radiators are new, and have large surface areas as they should do.
Chris Goodall
-
Chris,
Thank you for your response to my comments.
I have to say that without the SAP assessment it is very difficult to see how the heat pump would have been adequately sized or the appropriate heat pump selected, or whether a biomass system would have been the better option.
Also, whilst it is true that the ASHP CoP will be low for very cold weather, an ASHP will perform very much better that direct electric heating in milder weather.
You state correctly that heat pumps perform well in other countries. You mention Sweden, and I understand they are popular in Canada.
I begin to suspect that the fault here lies elsewhere rather than blame the heat pump.
Mike
-
Mike,
I agree. This issue is not a problem with heat pumps per se, but an issue to do with the way it was installed/set up. My point is rather that it demonstrates that UK installers/supply companies are often, it seems, not yet able to make ASHPs work satisfactorily, implying that householders might be well advised to avoid buying one at the moment.
The need for a sustained investigation of *why* ASHPs often aren’t working properly is, I think, urgent.
Chris
Chr
-
Chris,
Any chance you could alter your headline to reflect the case for the need to investigate the cause of poor installations?
I seem to recall that problems with installation was a finding of the EST study.
Danger is here that there are those,who merely read headlines and have agendas of their own to push, will use your article to attack heat pump technology per se.
Mike
-
Chris,
Sadly, none of this will come as much surprise to anyone who has read the EST reportThe further research planned and referred to in it was cancelled according to my local EST helpdesk (due to budget cuts). Perhaps, something might come out of the data gathered as part of the Renewable Heating Premium Payment scheme where a sample of beneficaries were to get free heat meters and have to provide performance data to DECC.
-
Interesting post. We have a similar system to that described. This was installed in a new ‘one off’ build house. The house has all the expected insulation. I believe the installation of the system, its spec etc was as per requirements. The heat pump always supplies the hot water, in the winter it also supplies the heating to the house. There is a heat store in the loft and also a very small supplementary gas boiler in the garage which kicks in when very cold (to assist in heating the house and not the hot water). The house is an above average four bed detached.
Total fuel bills over the year (elec and gas), put in monthly terms, is currently £155 / month. We cook with gas.
I have not had time to evaluate efficiency etc at this time. In addition, over the last few years we have progressively been spending less time in the house during the day as my kids (one , then the other) started school etc. Therefore, if anything I expect to see the costs come down a little).
From say April to September the system ‘appears’ to be very efficient. The heat pump kicks in early in the morning and is only on for maybe one hour and that provides hot water for the rest of the day. The only gas we use is for cooking so we use peanuts during this time. I would have to say that I have never found the hot water satisfying. Its fine to wash up to and have a bath etc, but for me having a shower I really want it steaming and you just do not get that (55 deg C is max).
[We are silly too. In the summer, for hot water we should be running the pump in the middle of the hottest part of the day and using the heat store to hold that water for later. I believe it is currently set to run early in the morning and every evening].
In the autumn and winter everything is more of a struggle. The pump obviously needs to run longer and longer. At the moment (-9 deg outside) the pump is providing water at 45 deg C. The heating is about to go on (0740 on a Saturday morning) and the gas boiler will also need to fire. The pump could be running all day today. Every 50 minutes it will go into defrost mode which stops it working for about 5 minutes whilst it gets rid of ice etc. The house is plenty warm enough – and funny enough, whilst the gas boiler does not heat the water, the water actually seems hotter in the winter. This is probably to do with the central heating water / hot water being stored side by side in the heat store in the loft.
Overall, if I had my time again I would not have this system in place. However, we have it and we are stuck with it!
-
In the US southeast, air source heat pumps have been widely used for several decades. I have owned three homes and rented one since 1983 and every one of them has used a heat pump as the source of both heating and cooling.
In each case, they were economical compared to alternatives when comparing annual or even seasonal costs. (On really cold days, there were some challenges.) They also tended to be a cleaner alternative than gas, but that was because each of the areas where I lived a large portion of the power supply came from nuclear fission, not from coal or gas combustion.
That electric grid fuel source improved the economic value of my heat pumps.
In the areas where I lived, electricity prices were based on a regulated rate of return for the power company. Since the power company owned nuclear power stations that had been brought on line before the focused opposition dramatically increased costs and schedule delays and because they produced power for a low total operations and maintenance cost (including low cost fuel). the rates I have paid have been on the order of 4-8 US dollar cents per kilowatt hour – which is less than half of the cost per unit as in your example.
Natural gas in the US is currently very cheap and selling for about 2-6 USD cents per kilowatt hour for retail customers depending on location (gas transportation costs are significant). However, that has not always been the case. A few years ago (2006-2008), I was paying 6 cents per kilowatt hour for electricity that came from a mix of about 50% nuclear, 40% coal and 10% gas. My neighbors with gas heat were paying 15 cents per kilowatt hour for their gas.
I was making out like a bandit with my air source heat pump, even on the few really cold days we experienced in Maryland.
Perhaps cheap, low emission electricity is the reason why heat pumps work well in Sweden and Canada where the power mostly comes from a combination of nuclear and hydro power.
-
It’s not mentioned what brand of ashp it is – some, I believe, are much better than others. I think Mitsubishi have a good reputation.
If you are running an ashp into radiators, it’s better to use a buffer store to even out the heat flow, especially when the hp has to defrost in cold and damp weather. Another way of improving things would be to fit larger radiators.
An annoying thing about heat pumps in general is that they don’t tell you how efficiently (or not) they are running. Mostly all the info is there in the controller and with a bit more work, it could tell you. -
Dear Bob,
I didn’t include the name of the manufacturer of the heat pump. The owner is engaged in negotiations with the company over the poor performance of the machine and he didn’t want to make the discussions more difficult. However I can say that the firm is in the front rank of suppliers of ASHPs.
Chris
-
Forgive the ignorance, but how do you even roughly estimate those 200 watts of heat need per degree of temp difference between indoor and outdoor ?
-
Alex,
Sorry for the terseness of the original posting. I should have explained there.
We can estimate the rate of heat loss of a building using what is known as the U values of the component parts, that is the roof, floor, walls, windows and doors. The U value of, say, an insulated cavity wall, provides an estimate of the the amount of heat moving between the inside and the outside of the house per square metre of surface area per degree of temperature difference between inside and out.
The owner of the property about which I wrote gave me details of the materials used in each part of his house. I was able to determine, for example, that the U value of his double glazed windows was likely to be about 1.2 (1.2 watts per square metre of window per degree) and multiply this by the area of glazing.
The overall figure is approximate, of course, but the number I quoted in the post is likely to be fairly accurate because of the quality of the estimates provided to me by the householder.
Chris
-
I installed an ashp end of 2011, and it worked very well this winter.
The unit brand is SinoCredo, and it seems to be from China.
-
Chris,
by the way have you ever looked at the possibly differences between the high performance, high COP and more expensive ASHP and the cheaper and less efficient ones ?I think it’ s an important point because I have noted that today there are already commercially available ASHP with a nominal COP of ~ 5,5 or 6, even at a very high cost of about 1500-2000 euros. Obviously, this is not only a matter of high efficiency per se, but the very high likehood to have anyway a decent efficiency/performance even at very cold (and rare for most sites) temperatures
-
For istance, I meant this kind of heat pumps
http://www.logicool-ac.com/salesliterature/Summit/Summit%20RAS%20Wall%20SX8.pdf
costing in the UK market from 1100 to 1300 £
http://www.climatedesigns.co.uk/products.cfm -
(UK) Folk may note that the UK MCS installation standards for domestic (air and ground) heat pumps have been significantly revised, and have just come into operation. I suggest that any potential purchaser should ensure that their installer is aware of these revised standards. This should hopefully address some of the issues that are apparent in this discussion. One of the common problems with ASHPs in the UK appears to be undersizing of the equipment, as well as inappropriately high load side delivery temperatures, and poor control settings.
-
Hi,
I am an engineer and installer of ASHP’s. These systems have to be sized properly with SAP calc’s made for the property. The problem with the rough calculations made. We find as a company the problem is with the installers. I have been on many manufacture training courses, and the control systems and field settings are the problem most comissioning engineers struggle to understand. All good heat pumps come with a weather dependant setting which compensates for the ambient temperature, which is hardly ever activated.
Also you have to compare heating bills over a year to see the savings. They work very efficiently together with solar panels and are as reliable as your fridge freezer.
My advice, always use an MCS Accredited company.
Steve
Reply
18 comments
Comments feed for this article
Trackback link: http://www.carboncommentary.com/2012/02/08/2268/trackback