What flow temperature is
A wet heating system circulates water. The water leaves the heat source, runs through the radiators or underfloor pipes, gives up some of its heat to the rooms, and returns cooler. The temperature of the water as it leaves the heat source is the flow temperature; the temperature as it comes back is the return temperature.
Gas boilers have traditionally run hot, with water leaving them far hotter than a heat pump would send it. Heat pumps are designed to run cooler and for longer, because that is where they are most efficient.
Why lower is better for a heat pump
A heat pump's compressor lifts heat from the temperature of its source, the outside air or the ground, to the flow temperature. The bigger that lift, the more work the compressor does for each unit of heat, and the lower the efficiency. Lowering the flow temperature shrinks the lift, so every unit of electricity delivers more heat.
That is why the same heat pump can be very efficient in one home and disappointing in another. In a home designed to run at a low flow temperature, it does a small lift for most of the year. In a home where the radiators need very hot water, it does a large lift all winter. The field evidence reflects the spread: the Electrification of Heat trial's interim median for air source heat pumps was 2.80, with systems either side of it.
What it means in pence
On the price cap for 1 October to 31 December 2026, a heat pump achieving 3.5 over the season turns 26.32p electricity into heat at 7.52p a kWh; at 2.5 the same electricity becomes heat at 10.53p. Much of that difference comes down to flow temperature. The cost calculator shows the effect for your own tariff.
The catch: radiators give out less heat when cooler
A radiator's heat output depends on how much warmer it is than the room. Run it with cooler water and it gives out much less heat. So a radiator that kept a room warm with a hot boiler may not do so at a heat pump's lower flow temperature. The government's explainer says some radiators may need to be changed for larger ones, and this is why.
The answer is not always bigger radiators everywhere. A room-by-room heat-loss calculation shows which rooms need more emitter area at the planned flow temperature. Some radiators will be fine as they are, some will need replacing with larger or double-panel ones, and underfloor heating, which has a large surface area, already works at low temperatures. The government's explainer also quotes Energy Systems Catapult that heat pumps can operate efficiently at moderate and high flow temperatures, so a design does not have to chase the lowest possible figure in a home where that would mean replacing everything.
How the flow temperature is set
- The design flow temperature is chosen for the coldest day the system is designed for. It sets the radiator sizes.
- Weather compensation then lowers the flow temperature automatically on milder days, which is most of the heating season. See controls.
- Hot water is heated separately and needs a higher temperature, so the heat pump runs less efficiently while it does that. See hot water.
What to ask about it
- What design flow temperature has been chosen, and why?
- Which radiators change, and to what, to keep each room warm at that temperature?
- What seasonal efficiency is expected at that flow temperature?
- Is weather compensation set up, and how will the flow temperature change through the season?
Those four answers say more about how a heat pump will run than its brand or its label. The government's general figure of around 3 units of heat per unit of electricity is a good average; the flow temperature is what moves a particular home above or below it.