Sizing a Heat Pump for Cambridge's Colder Winters

How sizing works

A heat pump is sized to match your home’s heat loss on a design day: a cold day set by MCS for your location. If the house loses 7 kW when it’s near freezing outside and 21°C in the living room, it needs a heat pump that can deliver about 7 kW in those conditions.

Cambridge sits in one of the colder, frostier parts of lowland England. The NIAB weather station records around 52 air-frost days a year and an average January overnight low just under 2°C. The design temperature for a Cambridge postcode is therefore a little colder than for a city in the South West, which pushes heat loss — and heat pump size — slightly higher for the same house.

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Typical sizes for Cambridge homes

Cambridge home	Typical heat loss	Typical heat pump
Modern flat or apartment	2.5–4 kW	4–5 kW
Two-up two-down artisan terrace, Romsey or Petersfield	4–5.5 kW	5–6 kW
Three-bed Victorian terrace with extension	5.5–7.5 kW	6–8 kW
1930s semi, Chesterton or Cherry Hinton, filled cavities	6–7.5 kW	7–8 kW
1960s bungalow, village edge	6–8 kW	7–9 kW
Large Victorian villa, De Freville or Newnham	10–13 kW	11–14 kW

Treat these as a rough guide only. Your survey will give the real figure.

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Why bigger isn’t safer

It’s tempting to think a bigger heat pump gives you a margin for the coldest nights. In practice, oversizing causes problems:

• Short-cycling. In Cambridge’s many mild, dry days in spring and autumn, an oversized unit switches on and off repeatedly, losing efficiency and wearing components faster.
• Higher cost for the unit itself.
• More noise and a bigger footprint, which matters in a small terraced yard where the noise limit is 37 dB(A) at the neighbour’s window.

A correctly sized heat pump is designed to cope with the design day. Modern units also modulate, running at lower output most of the time.

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Why undersizing is just as bad

An undersized unit can’t keep up on the coldest days and relies on a backup electric heater or immersion, which costs about three times as much per unit of heat. In a Cambridge cold snap, that shows up on the bill quickly.

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Things that change the number in Cambridge homes

• Solid gault-brick walls lose more heat than filled cavities.
• Suspended timber floors in Victorian terraces let in cold air from below.
• Sash windows — original sashes leak air unless draught-proofed.
• Mid-terrace vs end-terrace — an end-terrace has an extra exposed wall.
• Extensions — rear kitchen extensions with three outside walls and a flat roof are often the coldest rooms.

Draught-proofing and loft and floor insulation before the survey can take a kilowatt or more off the requirement, potentially dropping you to a smaller, cheaper unit. See fabric first in Cambridge.

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Defrosting and sizing

On frosty, damp mornings, the outdoor coil ices up and the heat pump briefly runs a defrost cycle. A good design allows for the small capacity lost during defrosts, which is one reason an accurate local design temperature matters. It doesn’t mean you need a much bigger unit; it means the calculation should be done properly.

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Two smaller units?

Since 29 May 2025, detached houses can have two heat pumps under permitted development. For a large villa or rural property, two smaller units can be easier to site and quieter than one big one.

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Next steps

• Questions to ask during the heat loss survey
• Cambridge heat pump prices by house type
• Heat pumps in cold weather