The One Heat Pump Setting That Matters Most — Flow Temperature

What it is

Flow temperature is how hot the water is when it leaves the heat pump and goes to your radiators. Old gas boilers often send it out at 70°C or more. A heat pump works best at around 45°C, sometimes lower.

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Why lower saves money

Think of it like a car going uphill. The heat pump has to “climb” from the outdoor temperature up to the flow temperature. A smaller climb uses less fuel.

For a typical three-bedroom Leicester semi:

Flow temperature	Rough yearly heating cost (standard tariff)
40°C	about £830
45°C	about £860
50°C	about £905
55°C	about £1,030

These are illustrative, but the lesson is clear: keeping the temperature down keeps the bill down.

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What to aim for

• Interwar and post-war semis with filled cavity walls (Knighton, Western Park, Evington): 40–45°C is often achievable.
• Victorian terraces with solid brick walls (Clarendon Park, Highfields, the West End): around 45–50°C, usually with a few larger radiators.
• Big Stoneygate villas: 50°C or so, unless every radiator is upsized.

Your installer sets the design figure. Ask for it in writing.

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Your heat pump adjusts it for you

Most heat pumps use weather compensation: a sensor outside tells the system how cold it is, and it raises or lowers the water temperature to match. On a mild day it might run at 32°C; on a frosty one, nearer the design maximum. You don’t need to touch it.

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Habits that keep it low

• Leave it running. Turning the heating off and on makes the heat pump run hotter to catch up.
• Leave radiator valves open in rooms you use. Turning them all down forces higher temperatures.
• Don’t boost it. If the house feels cool, raise the room temperature by a degree rather than switching to a high-temperature mode.
• Keep doors closed to rooms you don’t heat, rather than turning those radiators off completely.
• Keep the outdoor unit clear so it can breathe easily.

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What if a room won’t warm up?

It usually means that room’s radiator is too small for the flow temperature. The fix is a larger radiator, not turning the whole system hotter. See do I need bigger radiators?

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Hot water is separate

Your hot water cylinder is heated to around 50°C on its own schedule. Leicester’s hard water can scale up the coil over time, making that harder, so mention any slow reheating at your annual service.

Related: SCOP and COP in plain English · running costs in Leicester