Many existing radiators are too small for a heat pump because heat pumps run at lower water temperatures. A room-by-room heat loss calculation shows which ones need upsizing; usually it is some rooms, not all.
Many existing radiators are not big enough for a heat pump as they stand, because a heat pump heats the water to a lower temperature than a gas boiler, and a radiator gives out much less heat at lower temperatures. That does not mean every radiator has to go. A room-by-room heat loss calculation shows which rooms will fall short, and in most homes it is a handful of rooms rather than the whole house. The pipework, the water quality and the balance of the system matter as well.
Why Heat Pumps Need Bigger Radiators
A radiator's heat output depends on how much hotter the water is than the room. Heat pumps are most efficient at low flow temperatures, so the gap is smaller and each radiator delivers less heat. Our explainer on how a heat pump works covers the refrigerant cycle and why flow temperature matters so much.
Radiator outputs in catalogues are normally quoted at a mean water temperature of 70°C in a 20°C room, known as Delta T 50. Many gas systems were designed to run at around that temperature. A heat pump running efficiently might send water out at 45°C or lower, which cuts the temperature difference roughly in half. At that point a radiator can give out somewhere between a third and a half of its catalogue figure. A room that was comfortably heated by a boiler can end up short by a noticeable margin.
You can run a heat pump hotter to make up the difference, but efficiency drops as the flow temperature rises, which pushes up running costs. Sizing the emitters properly is what lets the heat pump run cool and cheap. Once the heat pump is in, keeping it efficient also depends on routine care; see how often a heat pump needs servicing.
How to Tell If Your Radiators Will Cope
The reliable way is a room-by-room heat loss calculation, compared against what each radiator can deliver at the planned flow temperature. Anything else is a guess.
The calculation looks at each room's size, walls, windows, floor, ceiling and ventilation to work out how much heat it loses on a cold day. Each radiator's output is then corrected down to the heat pump's flow temperature. Rooms where the radiator cannot match the heat loss need a larger radiator, a second radiator, or a different emitter. Installers certified under the Microgeneration Certification Scheme (MCS) are required to carry out this kind of calculation before designing a heat pump system. Our guide to radiator sizing and BTU calculations explains how room heat loss is worked out.
There is also a useful real-world test if you still have a condensing gas boiler. Over a cold spell, run the heating with the boiler's flow temperature set lower, using the control described in your boiler's user manual, and note which rooms struggle to reach temperature. Those rooms are the ones most likely to need attention. Leave anything beyond the user controls to a Gas Safe registered engineer.
Do You Need to Replace Every Radiator?
Usually not. Rooms with generous radiators, good insulation or low heat loss often cope as they are, while rooms with large windows, external walls on several sides or undersized radiators are the ones that need upgrading.
Typical changes are swapping a single-panel radiator for a double-panel or larger model in the same position, adding a second radiator in a large room, or fitting a radiator with a bigger surface area. Bathrooms and rooms in extensions or loft conversions are frequent problem areas, because their radiators were often sized for convenience rather than calculated. Our radiator installation service covers like-for-like swaps, upsizing and additional radiators. Doing this work while the existing boiler is still in place spreads the disruption and lets you check the result before the heat pump arrives.
Pipework: Microbore and Flow Rates
Heat pumps move more water, more slowly warmed, than a boiler does, so the pipework has to carry a higher flow rate. Thin microbore pipes can restrict this and may need replacing on some runs.
Systems plumbed in 15mm pipe to each radiator are often usable, depending on layout and radiator sizes. Microbore systems, which use 8mm or 10mm pipe, are more likely to need upgrades to at least part of the circuit. The primary pipework between the heat source and the rest of the system usually needs to be larger than on a boiler system. This is assessed as part of the design, not something to decide by eye.
Clean Water and a Balanced System
A heat pump is less forgiving of dirty water and poor balance than a boiler. Sludge reduces heat transfer and flow, and an unbalanced system leaves some rooms short when every degree of output counts.
Before a heat pump is connected, the system should be cleaned, flushed and dosed with inhibitor, and a filter fitted to protect the heat pump. Once running, each radiator's flow needs setting so that heat reaches every room as designed. On a heat pump system that balancing is set to the design temperature drop, which is typically smaller than on a boiler system. Our guide to radiator balancing explains how it works.
Fabric First: Cut the Heat Loss Before Upsizing
Every bit of heat loss you remove is output the radiators no longer have to find. Loft insulation, draught-proofing and better windows can bring a borderline room within range of its existing radiator.
In London's older terraces and conversions, solid walls and single glazing are common. It is worth addressing the cheap, simple measures first and then running the heat loss calculation, so that you are not paying to oversize radiators for heat that is escaping through a gap under the door.
What We Do, and What a Heat Pump Installer Does
Preparing the wet system and fitting the heat pump are separate jobs. We survey the existing radiators and pipework, upsize radiators where needed, clean and balance the system; the heat pump itself is designed and fitted by an MCS-certified installer.
If you are thinking about a heat pump in the next few years, getting the radiators and system ready now means the switch is simpler when it happens. If a heat pump is not right for your home, because the space, the heat loss or the budget does not work, we will say so. Book a free survey and we will tell you which rooms fall short and what it would take to fix them.
Frequently asked questions
Can a heat pump work with existing radiators?
Often it can with some of them. Radiators give out less heat at the lower water temperatures heat pumps use, so rooms where the existing radiator is only just big enough will fall short. A room-by-room heat loss calculation shows which radiators can stay and which need upsizing.
Do I need underfloor heating for a heat pump?
No. Underfloor heating suits heat pumps well because it runs at low temperatures, but correctly sized radiators work too. Most retrofits in London keep radiators and upsize the ones in rooms that need more output.
What flow temperature does a heat pump run at?
It depends on the design, but heat pumps are most efficient at low flow temperatures, often around 45°C or below. Radiators are then sized to heat each room at that temperature. Running hotter is possible but reduces efficiency and raises running costs.