Do air source heat pumps (ASHP) save householders money? And do they reduce CO2 emissions? In an earlier article on this site I suggested that potential purchasers should be cautious on both measures. I was a little too pessimistic: a recent visit to a home with an ASHP enabled me to get actual data on energy consumption before and after the pump was installed. Carbon and cash savings were better than I expected, although the ASHP still does not represent a good investment without subsidy. But the government’s proposed Renewable Heat Incentive will provide very substantial cash payment to encourage a rapid take-up of these devices.
Details on the home with the new heat pump.
The ASHP was installed at a ground floor flat in a very large Victorian semi-detached house in north Oxford. The floor area is about 140 square metres/1500 square feet, compared to the average UK property of around 85 square metres. The walls are solid brick, a feature that makes the house quite difficult to heat economically.
The owners of the property have kept records of all their energy bills. Before the installation of the ASHP in May 2009, the house typically used about 2,600 kWh of electricity and 24,000 kWh of gas. The gas provided the fuel for space heating, water heating and cooking. These numbers are in line with UK averages for housing of this size and type.
Energy use in year before installation of ASHP
| Electricity | 2,680 kWh per year |
| Gas | 24,950 kWh per year |
| Total energy use | 27, 630 KWh per year |
The ASHP provides space and water heating. So some gas is still needed for cooking. After installation of the ASHP, the first twelve months energy use was as follows
Energy use after installation of ASHP
| Electricity | 8,481 kWh per year |
| Gas | 950 KWh per year |
| Total energy use | 9,431 kWh per year |
As expected, the total energy demand went down very considerably. If we assume that underlying electricity use (lights and appliances) stayed the same as before installation, the ASHP provided the house with heat using 5,801 kWh of electricity. This replaced about 24,000 kWh of gas (this excludes my estimate of about 950 kilowatt hours of gas used for cooking).
At first sight, these numbers look suspiciously good. Heating the house uses only one unit of electricity where four were used before. Most estimates of the efficiency of ASHPs suggest that they only produce about 3.15 units of heat for each unit of electricity. The explanation is that this house had an old and inefficient gas boiler. So although 24,000 kWh of gas were used for heating only about 18,000 would have produced usable heat. Divide 18,000 kWh by 5,801 kWh of electricity and the underlying performance of the ASHP was actually very slightly less good than predicted. (For fans of this index, the Coefficient of Performance was about 3.10). This is to be expected; a hard winter will marginally affect the efficiency of a heat pump.
So there was a real improvement in energy use. This is why many countries are strongly encouraging heat pump installation as a way of reducing the demand for fossil fuels. But what about the cash savings? Electricity is much more expensive than gas. My calculations used British Gas’s lowest online tariffs for Oxford and showed that the houseowners will have saved about £145 a year by buying a heat pump rather than replacing their old boiler with a new and much more efficient model.
The CO2 saving can also be calculated. Generating a kilowatt hour of electricity in the UK causes emissions of about 0.5 kg of CO2 compared to about 0.2 kg from burning a kWh of gas. The transition to an ASHP in this house saved about 1.1 tonnes of CO2 a year, or just over a fifth of total emissions from heating. This is a good saving and will rise as electricity generation moves to lower carbon sources such as wind and new nuclear. In other words, the government is right to push us into using ASHPs.
But heat pumps are costly. The one whose electricity use I measured cost about £4,000 more than a good new condensing boiler. So cutting your energy use in this way doesn’t come cheap. The government’s proposed Renewable Heat Incentive (RHI) will therefore pay householders an amount each year to compensate for the high cost of installing a pump. The RHI will make an estimate of a reasonable heat demand for a house (based upon its size and whether it has cavity walls) and then pay 7.5p for each kWh of heat that the ASHP is ‘deemed’ to replace. In the case of the house whose energy use I measured, the deemed heat use will probably be about 15,000 kWh – a little less than the actual figure before the ASHP was installed. So the yearly subsidy payment (for 18 years) will be about £1,125. This payment (which may be adjusted downwards before being finally implemented in April 2011) clearly provides a real incentive to spend an extra £4,000 when replacing an old boiler. In fact, you might say the payment was too generous: the rest of us will all be paying a little bit more on our gas bills for the next few decades to cover the cost.
What about the other worry I expressed in my earlier article? Did the house stay warm in winter? The owners were more than pleased, saying that their home was comfortable even during the coldest nights. My nervous scepticism was wrong; even in a old brick build house a good ASHP of the right size can provide all the heat we need.
Tags: air source heat pump reviews, coefficient of performance, CoP, Ecodan, Renewable Heat Incentive
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Chris,
We design and manufacture Air Source Heat Pumps in Lytham, Lancashire. Your figures are excellent although we are getting a Seasonal Performance Figure (SPF) of nearer four. This is a better figure to take as it gives a seasonal adjustment of your COP.
Personally I got the savings at £438 for replacing the old boiler and £280 comparing to the new gas boiler assuming it is 90% efficient. However both figures are based on a Gas Price which we both think is going to seriously rise over the next year or two. Of course the savings now against electric and oil heating are massive.
The RHI is chugely important to us to kick start our industry which unfortunately has stalled since the election and will make the installation of ASHP even more wortwhile.
Finally I am delighted that you are becoming an ASHP convert and if you want one specifically designed and manufactured in the UK for our climate let me know. -
Heat pumps are generally an excellent solution for space heating but less good for hot water due to the higher temperatures required. We are seeing great interest in combining our SOFC fuel cell based mCHP device called BlueGen with a heat pump. BlueGen has such high electrical edficiency that the heat output is very low, sufficient for hot water only and therefore ideal for use with a heat pump or in highly insulated homes where no space heating is required (PassivHaus). The RHI will certainly be important for the adoption of heat pumps just as the FIT is for mCHP. By using BlueGen to provide the electricity for the heat pump you reduce carbon twice over, both on normal electricity use and on the electricity used for the heat pump. In the longer term we hope also to see solutions which will allow our products to become zero carbon rather than low carbon but even in the short term they can make a big difference.
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Interesting point about the RHI: Over the 18 year expected life of the ASHP, you’d make over £16,000 even after deducting the £4000 initial capital cost, which would be paid back after 4 years. Generous indeed!
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So, any chance of the rest of the numbers, to give a full picture? How much did the electricity cost them, compared to the gas? Is the hot water in a hot-water tank? Do they have Economy 7? How long’s the guarantee on the ASHP, and what’s its expected lifetime? Did the house already have 3-phase supply; did it need it installing, if not; and how much extra did that cost? Do they have a backup heating system for when temperatures drop below -10 deg C?
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A few more questions. What outlet temperature was required to supply the central heating in cold weather? Usually this would have to be at least 55C unless the radiators were oversized (assuming that there was no underfloor heating) or the house was well insulated. What kind of refrigerant was used in the heat pump: HFC or CO2? HFC heat pumps are efficient only for underfloor heating (low outlet temperature), and CO2 heat pumps are efficient only for domestic hot water (low inlet temperature). As far as I know, no one has yet produced a heat pump that can simply replace an ordinary gas boiler where most of the heat demand is for space heating through radiators (high outlet and inlet temperatures).
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As part of my PhD studies, I have two households that have installed GSHPs in older (’50s, 60′s) houses, kept their radiators and still been reasonably happy. As far as I can see, the weather-compensation controls mean that the radiators don’t have to heat higher than 55C, and frequently stay lower than that. They do use some auxilliary, direct heat, but only when it gets really cold. The weather compensation seems to maintain a very steady indoor temperature.
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The worse insulated a house is the better the seasonal COP of an ASHP will be because it will be doing more of its heating in warmer weather. This is in a sense the ideal building to use one on, but really we should be trying to reduce the heat demand as a first step (where possible).
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I am currently getting quotes for a high-temperature airsource heat pump, as part of one of the pilot areas for pay-as-you-save. The Daikin Altherma HT is designed as a drop-in replacement for a conventional boiler using existing standard radiators – as the flow temperature is 80c (thereby eliminating the need for a supplementary heater for water). Therefore, one does not have to replace existing rads with new oversized rads. Typical COP is 3. If required, this can also be combined with Daikin’s own thermal panels for hot water (though not strictly necessary with this particular heat pump).
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Can I suggest you explore the Sanyo CO2 air to water heat pump, which uses CO2 as a refrigerant totally contained in the outdoor unit. Only water pipes connect the Heat pump with the internal Thermal store. Flow temperatures of 65°C can be acheived so existing radiators can be used, CO2 has a GWP of 1 compared to HFC of which R410A is the most common which requires F gas regulation with a GWP of 1800. The heat pump can operate in temperatures down to -26°C, HFC systems dramatically drop capacity when the outdoor temperature is below – 5°C somewhat of a frequent occurrence last year. The Heat pump can also provide Domestic hot water and does not have to use imersion heaters to Pastuerise the tank against Legionella but can do this on the Heat pump allone thus saving even more energy.
You can find out more here
http://www.oceanair.uk.net/index.php?substance=heating_products
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I have recently fitted a Sanyo 4.5 kw ASHP to my newly built, super insulated house. The system has kept us all warm during the coldest period in Scotland since records began. However, it consumes over 7 kws per hour by day and over 4 by night. In the last week it has used over 800 kws. I feel as if I have bought an expensive immersion heater.
Is there anyone out there who has a similar ASHP which is working economically? If so I’d love to hear from him.
Perhaps I have an issue over the sizing or the installation of my unit. -
Dear Joe,
The efficiency of air source heat pumps falls in cold weather. Nevertheless, 800 kWh sounds very high. Our house (detached, new but not well insulated) used 45 kWh of gas heating per sq metre in December (Oxford – colder winters than most places in southern England because of the distance from the sea). Even in very cold temperatures, a heat pump should offer a 50% saving on energy use by gas or oil, so although Scotland was very cold indeed, I would think about getting the installer to check if your electricity usage was more than, say, 25 kWh a sq metre for the month.
Any further info on what you find out would be much appreciated by readers.
Chris Goodall
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Dear Joe
Its very interesting the manufacturer stating he gets a COP of 4, I guess this is at an ambient temperature of 7 degrees c and a water temperature of 35 degrees c thats only any good for under floor heating great for new build but not for retrofit. So that blows his COP of 4 out of the water for most of the country’s housing stock. Wow! how we bang on about our COP which is stated differently on every manufacturers literiture confusing the customer and installers into believing its the higher COP that matters, Some standard has to be put in place that manufacturers have to comply with.
When using the new SAP2009 calculations ASHP get a big kicking there stated COP gets reduced to a much less desirable figure of 2.5 Unless there Appendix Q rated which means you can use the manufacturers stated COP otherwise there not worth touching for your new build that will be having to comply with the code for sustainable homes. Have a look theres not many!.
I design these and installers fit them which brings me on to my next gripe the industry has to be trained better fact to reduce them energy bills and not increase them by lack of knowledge. Go on a heat pump course and its pritty much a sales pitch with not much emphasis on actual installation and theory training. Can we not have a governing body that relates to governing individuals as well as firms and not the laughable MCS accreditation thats availiable to any one thats got 500 quid.
Also Chris your heat pump has been going flat out trying to maintain heat all month and its been pritty cold which means your heat pump will have been going into defrost mode more than usual to stop it from icing over some models use the heating water from the house to defrost so you can imagine it putting a lot of strain on the heat pump to maintain temperature, sure they work to -20 but they have to be sized correctly. One manufacturer stated to fit an in-line 6 kw immersion for those cold days when the heat pump could not cope with demand on top of its 3 kw built in immersion. 4.5 kw is probably at ambient temperature of 7 degrees c so your heatpump at -10degrees say would probably turns your machines output to not much, maybe 2kw output. Your immersions being running your house. Tally that up for last month and you have a great big bill thank God for the RHI to pay for all the in built immersions that will be going next winter, Roll on de-carbonisation 2030!
Lee Ellerby
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It seems I should’ve fitted a 9kw Sanyo unit; nevertheless, since January energy usage has improved to, currently, 37 kw hrs a day (24 hrs). Extrpolating the 6 month data to a full year, my spend on electricity will be about £1,400; not bad for a 320 sq m house.
I expect better results next year because I will move to an economy tarriff and also because the house (which was newly built last Nov) will have had a year to dry out. I suppose there was a lot of thermal mass to heat up in the house’s first winter.
My conclusion is that ASHP, properly installed and sized, is fine where there is no mains gas. -
Just out of curiosity, how does the COP for an *air to water* heat pump is exactly definied ?
We well know that for an *air to air* heat pump the COP is usually calculated for an outdoor temp of 7 °C to have an indoor temp of 20 °C. But what about the air to water heat pumps counterpart, considering that, as a lot of users here pointed out, hot water for heating has to be produced at a leaving temp of ~ 60 °C to be usefull (unless to have an underfloor heating system)
Moreover, at the same conditions for example that cited 7 °C outdoor temp, how does the nominal COP change in the two cases (air to air vs air to water heat pump) ?
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Hi Chris et. al.
Does anyone have updates regarding their experiences using ASHPs? We’re looking at getting a Victorian terraced property at the moment in East Oxford and I’ll need to do loads to get it up from it’s current High E energy rating. It is off the gas network at the moment and is all electric heating… so theoretically should benefit the most from this tech (especially if we rip up the tiles and do some underfloor heating).
I’m keen for any comments and experiences from those that have used these in practice! We’re looking at the EcoDan and Altherma systems.
Cheers,
Will
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Dear Will.
I don’t whether you have seen the post I did last week detailing the experiences of a second ASHP owner who kept detailed electricity consumption records over late January and early February. The problem he identified (very poor performance in cold weather) seems typical. The comments below the post are very helpful.
I suppose it is a good general rule that ASHPs will not work effectively in any house that isn’t very well insulated. Your Victorian home might need a great deal of work – including solid wall insulation!
Chris
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