Home energy guide
Heat pump sizing, flow temperatures and radiators
The same heat pump can be cheap or pricey to run, depending on how it's designed. The secret is a proper heat loss survey and a low flow temperature, which might mean a few bigger radiators.
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Work out your own numbers with the heat pump calculator →
Why the design matters more than the brand
A heat pump works hardest when it has to heat your radiators much hotter than the air (or ground) it pulls heat from. The temperature of the water going to your radiators is called the flow temperature. Keep it low and the heat pump barely breaks a sweat.
Gas boilers often run radiators at 60–70°C. Heat pumps are happiest much lower, often 35–50°C. We haven't found an official source that sets one target flow temperature, so ask your installer for the design figure.
The Energy Saving Trust lists properly sized radiators, your tariff and how the heat pump is controlled as the main things that drive running costs.
What efficiency means for your bill
Efficiency over a year is called SCOP: heat out divided by electricity in. Higher is better. For a home needing 10,000 kWh of heat a year, going from a SCOP of 2.8 to 3.5 saves about £188 a year.
See the numbers
| SCOP | Electricity used | Cost at 26.32p (0% VAT) | Cost at 27.64p (5% VAT) |
|---|---|---|---|
| 2.5 | 4,000 kWh | £1,053 | £1,106 |
| 2.8 | 3,571 kWh | £940 | £987 |
| 3.5 | 2,857 kWh | £752 | £790 |
| 4.0 | 2,500 kWh | £658 | £691 |
Prices are the October–December 2026 cap. Electricity has 0% VAT from 1 October 2026 to 31 March 2027 (Ofgem); the 5% column shows the usual rate. A SCOP of 2.8 was the median measured in the Energy Systems Catapult's Electrification of Heat trial (not re-checked today).
Step 1: the heat loss survey
A good design starts with a room-by-room heat loss calculation. It looks at your walls, roof, floors, windows, insulation, draughts and how warm you like each room. That tells the installer how much heat you need on a cold day and over a year.
Be wary of a quote based only on the size of your old boiler. Boilers are often oversized, and an oversized heat pump can keep switching on and off, wasting energy.
Step 2: picking the flow temperature
The designer chooses a flow temperature and checks each radiator can keep its room warm at that temperature. Lower is more efficient but needs more radiator surface.
Step 3: radiators
Radiators give out much less heat when the water's cooler. Rooms with radiators that are too small need a bigger one, an extra one, or underfloor heating. Often it's just a few rooms.
Check your quote includes any radiator upgrades. EST says the installation cost partly depends on whether you need them. The Boiler Upgrade Scheme grant can go towards the whole job.
Pipes matter too. Very narrow "microbore" pipes can limit flow, and your installer may suggest replacing some.
Hot water
A standard heat pump doesn't give instant hot water like a combi boiler, so you'll need a hot-water cylinder. It usually fits in a cupboard at least 80 × 80 cm (EST). Short on space? A heat battery or a hybrid can help, but hybrids don't qualify for the grant.
Heating the cylinder hotter costs more, so do it in a cheap tariff slot (see our heat pump tariffs guide).
Insulate first?
Often, yes. Less heat loss means a smaller heat pump and a lower flow temperature. Loft insulation, draught-proofing and cavity wall insulation are worth doing first if they're cheap. Insulation in homes has 0% VAT until 31 March 2027 (HMRC).
Questions to ask every installer
- Can I see the room-by-room heat loss calculation?
- What's the design flow temperature, and what SCOP do you predict at it?
- Which radiators need replacing, and is that included?
- What size cylinder, and where will it go?
- How will the controls be set up, and will you show me how to use them?
- Are you MCS-certified, and will you handle the Boiler Upgrade Scheme application?
Then pop the predicted SCOP into the heat pump calculator to compare quotes on running cost, not just price.
Frequently asked questions
What flow temperature should a heat pump run at?
As low as the radiators allow while keeping rooms warm. Heat pumps are typically designed for much lower flow temperatures than gas boilers. Ask your installer for the design figure and the SCOP it gives.
Do I need new radiators for a heat pump?
Sometimes, but often only in some rooms. A room-by-room heat loss survey shows which radiators are too small at the design flow temperature.
Why does SCOP matter?
It sets your running cost. For 10,000 kWh of heat at 26.32p, SCOP 2.8 costs about £940 a year and SCOP 3.5 about £752.
Can I keep my combi boiler for hot water?
That would be a hybrid system, which is not eligible for the Boiler Upgrade Scheme. Most heat pump installations add a hot-water cylinder.
Related guides
- Heat pump vs gas or oil boiler: running costs
- The Boiler Upgrade Scheme: who gets the £7,500 heat pump grant
- Electricity tariffs for heat pumps: how time-of-use cuts running costs
- Air source vs ground source heat pumps: which suits your home?
- Do you need planning permission for a heat pump?
- All home-energy calculators
Sources (checked 27 October 2026 unless stated)
- Energy Saving Trust: Air source heat pumps: running cost factors, radiator upgrades, cylinder space
- Ofgem: price cap 1 Oct–31 Dec 2026 (published 26 Aug 2026): 26.32p/kWh, 0% VAT on electricity to 31 Mar 2027
- HMRC VAT Notice 708/6: energy-saving materials: insulation zero-rated to 31 Mar 2027
- GOV.UK: Boiler Upgrade Scheme, what you can get
- Energy Systems Catapult Electrification of Heat trial median SCOP 2.8: not re-checked today
- Typical boiler and heat pump flow temperatures: general engineering practice, not verified against an official source today