Readers of this post may also like to consult a later article in which I do more detailed work on the impact of cold weather on the electricity use of ASHPs at http://www.carboncommentary.com/2012/02/08/2268. The results discussed in the post below may not be typical. 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.