Flow Temperature, Design Temperature and Your Cambridge Heat Pump
Last reviewed: 2 October 2026
Two temperatures shape a Cambridge heat pump design: the cold outdoor day it's sized for, and the water temperature it sends to your radiators. This guide explains both and what they mean for your bills.
Two temperatures on your quote
A good heat pump quote states two temperatures. Together they decide how big the system is and how much it costs to run.
- Outdoor design temperature — the cold day the system is sized for, set by MCS for your location. Cambridge’s is colder than western cities’: the NIAB station sees around 52 air-frost days a year.
- Design flow temperature — how hot the water leaving the heat pump will be on that design day. Lower is better for efficiency; typically 45–50°C for a radiator system.
If a quote doesn’t state both, ask for them.
How flow temperature drives efficiency
The heat pump has to lift heat from the outdoor air up to the flow temperature. The bigger that lift, the more electricity it needs.
For a typical Cambridge semi needing about 12,000 kWh of heat a year, at around 26p per kWh:
- designed for 45°C: seasonal efficiency around 3.4, so about £930 a year
- designed for 55°C: seasonal efficiency around 2.8, so about £1,130 a year
That £200-plus a year difference, sustained over the life of the system, is why installers spend effort on radiator sizing.
Why Cambridge’s climate raises the stakes
On a frosty Cambridge morning the air may be −3°C. Lifting heat from −3°C to 55°C is far harder than lifting it to 45°C. A low design flow temperature keeps the heat pump efficient precisely when it’s working hardest. It also keeps the system comfortably within its capacity on cold mornings, so it never needs an expensive electric backup heater.
Weather compensation
Your design flow temperature only applies on the coldest days. The rest of the time, an outdoor sensor lets the heat pump lower the water temperature automatically. On a 7°C day, a system designed for 45°C might run at around 33–36°C. Cambridge’s dry, mild spring and autumn days are where this pays off most.
What to check after installation:
- Weather compensation is switched on. It should be by default; it’s worth confirming.
- The curve suits your home. If rooms are too warm on mild days, the curve can be lowered.
- Thermostatic radiator valves aren’t fighting it. Leave most of them fully open and let the heat pump control the temperature.
Getting to a low flow temperature in older homes
Cambridge’s solid-walled gault-brick terraces and Victorian villas lose heat faster than modern homes. The usual ways to keep flow temperature down are:
- Larger radiators in the coldest rooms (see radiator sizing)
- Draught-proofing sash windows and suspended floors
- Topping up loft insulation
Where several radiators can’t be enlarged, perhaps for conservation reasons, an R290 heat pump that runs efficiently at higher temperatures may be specified instead.
For landlords
Tenants often turn thermostats and radiator valves up and down as they would with a gas boiler. A short note explaining “set it and leave it” protects the flow temperature you paid to achieve.
Related: SCOP and COP explained · running costs through a Cambridge winter
See what a heat pump would look like in your Cambridge home
The mechanism is the same in every UK home; the design is specific to yours. Our team carries out a free in-home survey including a full room-by-room heat-loss calculation, identifies the system size and configuration that fits your property, and provides a written quote with the £7,500 BUS grant already deducted.
You'll see the actual figure you'd pay — not an estimate — and the projected efficiency for your specific install.