Installing an Air-source Heat Pump

By John Swindells 11 Jan 2025 | Comment
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This is a guide to help you decide on whether an ASHP (air-source heat pump) is right for your home. Are you sitting on the fence about whether to replace your ageing fossil fuel boiler with an all-electric solution? You can read a broad spectrum of views, from praiseworthy to highly negative, so it's hard to decide what the reality is and what you should do. I was in that position until last spring (2024) when I decided that my oil boiler and gravity-fed hot water system were destined for the scrap heap.

Costs

At the time there was a generous (£7500) government grant to install a heat pump. In my case this grant covers most of the cost, but you should expect to spend £4k+ of your own money to get the whole heating and hot water system updated. However, it could be a lot less if you already have a modern, unvented system.

Planning

My 2-storey house, built in the mid-90's, had a standard central heating system already, with a large cold water tank (and central heating expansion tank) in the loft, a hot water cylinder on the landing, an oil boiler in a giant hole cut into the kitchen wall, and a 2500L oil tank in the back garden. All of the radiators were basic types without any convection fins - so needed a lot of power to push the heat out.

The heat pump solution required replacement of the radiators with type 21 and type 22 (single and double convection fins) and a new hot water cylinder in an un-vented configuration. Without these efficiency improvements, the heat pump would probably be unable to work hard enough to warm the house and supply hot water.

Where to put the heat pump?? My initial plan was to put it on the rear patio, in a sheltered, south-facing location and with a short pipe run to the heating system indoors. Doing this would eliminate the possibility of putting a conservatory there, so not ideal. Then I looked at an unused space on the other side of the house, on the chilly north-east side. It would still have a decent pipe run distance, mostly under the main bedroom floor. My initial concern about it being in a "cold" location wasn't really valid - these heat pumps work very efficiently with a large range of inlet air temperatures.

Where to put the hot water tank?? The current space taken up by the hot water cylinder was insufficient for the new tank and extra gubbins. My simple plan was therefore to put the new hot water tank in the loft, where there's plenty of space, but I wasn't keen on this from a heat loss perspective and in terms of maintainability. It would also mean longer pipe runs for the hot water. I then thought that a new, larger space on the landing could work, taking a chunk out of the bathroom. This could work, but I was still concerned about pipe runs. I then had an epiphany - put it all under the stairs! This is a large, open space, fairly under-used with coats and bags, so would be a perfect fit and would make everything super-easy to get to. The pipe runs would also be much shorter, as it's positioned half-way between the upstairs and downstairs taps. Only issue was the "pressure relief pipe", a large pipe that takes scalding water out of the house should anything go badly wrong. The under-stairs location is right in the middle of the house and surrounded by doorways, but I had the clever idea of digging a narrow channel into the hallway floor to get the pipe outside and to a drain next to the front door. My chosen installer was happy with this setup, so the decision was made.

The important take-away here is that you need to think carefully about your options before going ahead with the work. Look at the indoor spaces available for the water cylinder, where the heat pump will go, and where the pipes will run.

Don't forget the consumer unit! The system needs a few mains electrical connections that will support the heat pump, control unit and immersion heater. (Yes, your new hot water tank will still have an immersion heater installed, as a manual source for hot water in case there's an issue with the heat pump. Do check your mains consumer unit to see what space you have; you will almost certainly need a spare slot for a 40A breaker for the heat pump.

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