Heat pump vs gas boiler running costs: it comes down to one dial

Heat pump vs gas boiler running costs is the wrong fight. People want a straight answer, electric or gas, and there isn’t one, because the same heat pump can come out cheaper than gas or dearer than gas depending on a single setting almost nobody asks about.

That setting is flow temperature, the heat of the water going round your radiators. A gas boiler doesn’t care what number you pick. Gas is gas, and whether the boiler’s firing water at 45°C or 70°C, the cost per kilowatt barely moves. A heat pump is the opposite. Push it up to boiler temperatures and it burns through electricity to get there, because it’s compressing refrigerant harder for every degree. Drop it down to 35–40°C and the same box does the same job for a fraction of the electricity.

I did a Tonbridge semi last spring where the old boiler cost roughly £950 a year to heat the house. Fitted a heat pump, same radiators, no upgrades, and the installer before me had left it running at 55°C flow because that’s what felt safe. Bills came in barely under the boiler. Nothing wrong with the unit. I turned it down to 38°C, let it run longer and gentler through the day instead of short hot bursts, and the same house came in near £600 for the year. Same walls, same radiators, same weather. The only thing that changed was that one number.

Here’s why. Electricity in Kent runs around 25p a unit and gas around 6-7p. On paper gas looks four times cheaper. But a heat pump at a low flow temperature can produce three, sometimes four units of heat for every unit of electricity it uses. Push the flow temperature up towards boiler levels and that ratio can fall to two, or worse. Do the sums at four-to-one and electricity beats gas comfortably. Do them at two-to-one and gas wins. The kilowatt price never changed. The dial did.

This is why the running cost question gets answered badly so often. Someone compares a heat pump quote against their gas bill, assumes like-for-like, and either gets talked into a system that’ll disappoint them, or talked out of one that would have worked. The honest comparison isn’t heat pump against boiler. It’s a well set up heat pump against a badly set up one, because that gap is bigger than the gap between either of them and gas.

None of this means gas is finished. A boiler in a house with small, high-output radiators and no room to lower the flow temperature can genuinely be the cheaper choice, at least until the radiators get sized up. I’ve told people to stick with their gas boiler rather than fit a heat pump that would have to run hot to cope, because a heat pump forced to behave like a boiler will lose to one on every bill.

So when someone asks me whether a heat pump or a boiler runs cheaper, my first question back is always about the radiators and the flow temperature the system will actually need, not the price of gas that week. Get that number down and a heat pump wins running costs most winters in most Kent houses. Leave it running like a boiler, and don’t be surprised when it costs like one.