Our air source heat pump (ASHP) was installed in 2015 when we had to choose between replacing our 15-year-old gas boiler or to look around for something different. We live in a modern 4-bed detached house that was built in 1994 on a typical UK housing estate.
Prior to getting the ASHP installed we were advised to get more loft insulation and get our walls filled with cavity wall insulation, both of which we managed to get done for free through existing government grants.
When the system was designed there were three ASHP specifications available from the installers (Husky): these were 8kW, 11kW and 16kW units.
The energy performance calculation for our house showed that we would need 8.7kWh to heat our house, so we could install the cheaper 8kW unit which might run continuously on very cold days, or for a few hundred pounds more install the 11kW unit and over spec the system.
For longevity and redundancy we chose to go for the 11kW system. Another factor in this decision was the fact that we had 10mm microbore pipes leading to our radiators, which is standard in modern houses. We also upgraded 8 of our 14 radiators to higher output, which meant buying four new radiators and relocating four of the larger old ones to smaller rooms.
The system runs 24/7 (October to April). We have the heating set to a constant 22C in the lounge, which effectively means it always maintains the whole house between 21.5-22.5C. It’s been able to maintain this room temperature even when it’s down to -7C outside overnight for several days.
The main thing to understand when running an ASHP system, rather than a traditional mains gas boiler, is that the system water temperature is about 15-20C cooler, which means the radiators never get “hot” to the touch and the system reacts slower to sudden changes like when you switch it on and off using a timer.
We were advised by the installers that the most efficient and effective mode of operation is to leave it on 24/7 at a set temperature and that’s certainly been the way to go from our experience. The other important thing to do to make sure it’s going to work effectively is to insulate, insulate, insulate!
For the 12 months (2014/2015) leading up to getting the ASHP installed our mains gas bill for heating and hot water was £674. In the 12 months (2015/2016) after installing and using the ASHP for heating and hot water, it cost us £449 and that’s without taking into account any electricity our solar panels contributed to running the system. This last year (2020) cost us £451 to run the ASHP.
We are more than happy with our air source heat pump system. It’s worked out cheaper to run than mains gas for us even though its running 24/7 at 22C. However, I’m certain that the main contributing factor is the high level of insulation we’ve achieved with 270mm of loft insulation, full cavity wall insulation, high quality UPVC double glazing and good fitting quality external doors.