What's the best temperature to set a heat pump? Whichever one you leave alone

What’s the best temperature to set a heat pump? Whichever one you land on and then stop fiddling with. That’s the honest answer, and it’s not the one people want.

I get asked for a number most weeks. 19? 21? Someone always has a figure from a forum they half-trust. The number matters less than people think. What matters is that you pick it and leave the system to hold it, instead of nudging it up when you’re cold and down when the bill lands.

A boiler forgives that habit. It can go from cold to roaring in a few minutes, so blasting it on and off doesn’t cost you much beyond the noise. A heat pump doesn’t work that way. It’s built to run low and slow, gently topping up heat that’s already mostly there, not fighting to recover a cold house from scratch. Ask it to do the boiler thing, off overnight then full pelt at 6am, and you’ll get the worst version of the technology. Slow to catch up, and expensive while it tries.

I fitted a system for a bungalow near Sittingbourne last winter where the couple were convinced the heat pump was faulty. Bills through the roof, house never quite warm enough. Turned out they were doing exactly what they’d always done with the old boiler: off at 10pm, back on at half six, thermostat cranked to 23 to force the pace. I set the flow temperature down to 38°C, told them to leave the thermostat on one number and stop touching it, and their consumption dropped by nearly a third within the month. Nothing about the house changed. Only the habit did.

This is what weather compensation is for, and it’s the bit installers explain badly. The system watches the temperature outside and quietly adjusts the water flowing through your radiators or underfloor pipes to match, a bit hotter on a cold snap, a bit cooler on a mild day, without you doing anything. Your job isn’t to chase comfort hour by hour. It’s to set the curve properly once, at commissioning, and then get out of the way.

Should you turn it off at night? No. That’s the single most common mistake I see, and it comes from thirty years of boiler habits that don’t apply here. Turning a heat pump off lets the house cool down properly, which means the pump then has to work hard and run hot to claw the temperature back, which is precisely the expensive mode you were trying to avoid. Leaving it ticking over at a slightly lower night-time setting, one or two degrees down at most, costs less than the recovery does. The cheapest heat pump in Kent this winter won’t be the newest one. It’ll be the one nobody keeps switching on and off.

I know this feels wrong. We’re used to heating being something you turn on when you want it and off when you don’t, like a kettle. A heat pump is closer to a fridge. Nobody turns their fridge off overnight to save money, because everyone understands it has to work harder to get cold again in the morning. Same principle, just harder to feel because you’re not stood in front of it.

If you want an actual number to start from, most well-insulated Kent homes sit comfortably with a flow temperature somewhere in the mid-to-high 30s and a room setting around 20 to 21. But don’t take that as gospel for your house. Take it as a starting point, set it, and then leave it for a fortnight before you judge anything. The people I fit heat pumps for who complain least are, without exception, the ones who stopped treating the thermostat like a dial they’re allowed to argue with.