Heat pump in a 1930s semi in the UK — does it work out once the grant is in? Heat Pump

Bills first: mine went from roughly £1,100 of gas to roughly £950 of electricity for heating, on a house slightly tighter than yours, running a seasonal efficiency around 3.2. That's not the dramatic win I'd been promised. What made it work anyway was the Boiler Upgrade Scheme — £7,500 towards an air source unit, applied for by the installer rather than by you. There's also a temporary uplift to £9,000 running from 21 July 2026 for off-gas-grid oil and LPG homes, which won't apply to you on mains gas.

Your suspended timber floor is the actual project here. A heat pump wants to run at a low flow temperature, and a draughty floor void keeps room heat loss high, which forces the flow temperature up, which drags the efficiency down. Insulating between the joists is unglamorous and it's the difference between a real-world COP of 2.6 and 3.4 on the same equipment.

Get the heat loss calculation as a document and read it properly. If the installer sized the unit off floor area, or off the output of your existing boiler, walk away. Oversized units cycle, cycle badly, and then their owners write posts about how heat pumps are noisy and expensive.

Ask specifically what flow temperature the design assumes and what radiator changes are included. A quote that assumes 45°C with your existing radiators and no upgrades is either optimistic or you have unusually generous rads for a 1930s semi.

Slightly against the grain: I'd keep the 2011 combi until it actually dies. It's fifteen years old, not thirty. Doing the floor and the airtightness work now, with the boiler still in place, means the heat pump quote in three years will be smaller and better designed. Grants change, but so do install prices and so does the kit.

Whatever you do, check the noise limits and where the unit would sit relative to your neighbour's windows. That's the complaint that ends up in a formal letter rather than a chat over the fence.

The tariff matters as much as the pump. Moving to a time-of-use tariff with a cheap overnight rate, plus decent controls, shifted my numbers more than any efficiency tweak I made to the system itself.