Radiators decide it, not the cold

A radiator gives about a third of its rated output at 45 °C flow instead of 70 °C. Emitter size, not outdoor temperature, is what stops a heat pump working.

Why it works

Output falls with roughly the 1.3 power of the water-to-air temperature difference, and radiators are rated at 50 K above room temperature. A heat pump at 45 °C sits around 21 K above the room, so the same radiator delivers a third of its label. Installers who cannot enlarge the emitters raise the flow temperature instead, and every extra degree costs roughly 2.5% of the efficiency the heat pump was bought for. Scandinavia runs on heat pumps at temperatures Britain never sees — the cold was never the problem.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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Why do heat pumps fail in some houses?

A radiator gives about a third of its rated output at 45 °C flow instead of 70 °C. Emitter size, not outdoor temperature, is what stops a heat pump working. Output falls with roughly the 1.3 power of the water-to-air temperature difference, and radiators are rated at 50 K above room temperature.

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