AIR SOURCE HEAT PUMPS
An air source heat pump can be an attractive proposition for domestic space- and water-heating, for the following reasons:
- Powered by electricity (can be off-peak for water-heating), which can be derived from renewable rather than fossil fuels
- Unlike ground source heat pumps, air source pumps do not need significant land area nor boreholes
- Payback can be over a relatively short timescale (c. 5 years)
It works rather like a refrigerator in reverse, with the energy of the outside air being extracted and used to heat water for domestic use and/or for central-heating.
Last autumn, we installed a 16kW model, which has a fan unit (approx 1m wide x 0.3m deep x 1.4m tall) outside and a ‘heatbox’ (the size of a wall-mounted boiler) inside. They tend to come in ‘modules’ of 8kW and some of the smaller models have only one component part.
We run our pump from 4 – 6am to heat the domestic hot water, with a top-up session at noon for 1 hr. This seems to meet all our needs. It can be programmed to handle the space heating to suit one’s needs. We already had a gas boiler, which has been retained and can be used to augment the heat pump if desired. But while last winter certainly tested the performance of the heat pump, it was seldom necessary to boost it with the gas boiler. Indeed, the main worry is that the gas boiler may malfunction through lack of use!
It should be noted that the air pump’s heatbox does incorporate an auxiliary electric water heater, which is programmed to operate once a week in to raise the water temperature to 70 degrees Centigrade, to prohibit the growth of Legionella in the system. This heater will also kick in if the external air temperature falls below a certain level – which could be seen as ‘cheating’! However, even in last winter’s extreme conditions, this very seldom happened,.
Although these conditions inevitably increased the consumption of electricity to greater than budgeted levels last winter, in general the increase in our electricity bill has been within the limits anticipated and has of course been hugely offset by the decrease in our gas bill.
As fossil fuels continue to diminish in supply and increase in cost, the speed of payback should logically increase.
Chris & Alison Gill.