Heat Pumps and Your Radiators
The compatibility question, answered honestly.
Yes, heat pumps work with existing radiators. Most homes need no changes at all. Some radiators may need upsizing. We check every single one during the survey, so you know exactly what's needed before any work starts.
The short answer
Heat pumps work with standard radiators. The difference is flow temperature. A gas boiler pushes water through your radiators at 55-65 degrees C. A heat pump runs at 35-45 degrees C. Lower temperatures mean higher efficiency, which is the whole point.
At lower flow temperatures, each radiator outputs less heat. In most homes, that's fine because the radiators are already oversized for the rooms they're in. But in some cases, a radiator that was borderline with a boiler won't keep up with a heat pump.
That's why we measure every radiator during the survey and calculate the heat output it delivers at heat pump flow temperatures. We compare that to the heat loss for each room. If a radiator needs upsizing, we tell you. If it doesn't, we leave it alone.
When radiators need changing
In our experience, most homes need zero to two radiator changes. The rooms that most often need upsizing are those with small, old radiators and high heat loss, such as large living rooms with older windows, or extensions with thin walls.
Usually fine as-is
Modern double-panel radiators (Type 22) installed in the last 20 years. Rooms with good insulation. Any room where the current radiator feels oversized (it's too hot at low boiler settings).
May need upsizing
Small single-panel radiators in large rooms. Old cast iron radiators with low surface area. Rooms with high heat loss (large windows, thin walls, poor insulation). Bathrooms with towel rails as the only heat source.
Upsizing a radiator is straightforward. It's not a major building job. In most cases, the pipework stays the same and just the radiator panel gets swapped for a larger one. It's a couple of hours of work per radiator.
Underfloor heating
If you have underfloor heating, a heat pump is the ideal match. UFH is designed to run at 35-40 degrees C, which is exactly where a heat pump operates most efficiently. You get maximum comfort with minimum energy use.
You don't need UFH throughout the whole house. A common setup is UFH on the ground floor (often already installed in kitchens and extensions) with radiators upstairs. The heat pump handles both, running different circuits at different temperatures.
If you're considering adding UFH to some rooms as part of the heat pump installation, it can be done at the same time. But it's not a requirement. Radiators work perfectly well.
Hot water
A hot water cylinder is included with every heat pump installation. If you currently have a combi boiler (no cylinder), we install one. If you already have a cylinder, we either keep it or replace it depending on condition and capacity.
The heat pump heats your hot water to 50-55 degrees C for normal daily use. Once a week, it runs a legionella cycle, heating the water higher to kill any bacteria. This is standard practice and happens automatically.
Practically, it works the same as any other hot water system. You turn the tap, hot water comes out. The cylinder stores enough for a typical family's daily use, and the heat pump tops it up as needed.
Cold weather performance
Air source heat pumps extract heat from outside air. They work down to -20 degrees C. The UK rarely drops below -5 degrees C, even in the coldest parts of Scotland.
For context: in Norway, Sweden, and Finland, where winter temperatures regularly hit -25 degrees C, heat pumps outsell gas boilers. The technology is proven in conditions far harsher than anything the UK experiences.
Efficiency does drop slightly in extreme cold. At 7 degrees C, a typical heat pump delivers a COP of 3.5 to 4.0 (350-400% efficient). At -5 degrees C, that might drop to COP 2.5 to 3.0 (250-300% efficient). That's still two to three times more efficient than a gas boiler at any temperature.
Getting the size right
Every property is different. The size of heat pump you need depends on your home's total heat loss, which is determined by floor area, insulation, glazing, ventilation, and hot water demand.
| Property Type | Typical Size | Typical System |
|---|---|---|
| 2-bed flat / terrace | 5-7kW | Small ASHP + 150L cylinder |
| 3-bed semi | 7-10kW | Mid-range ASHP + 200L cylinder |
| 4-bed detached | 10-14kW | Larger ASHP + 250L cylinder |
| 5+ bed / period property | 14-18kW | Large ASHP + 300L cylinder |
Getting the sizing right is critical. An oversized system wastes electricity by cycling on and off. An undersized system can't keep up on cold days. The only way to get it right is a proper heat loss calculation, done room by room, using the actual measurements and insulation values of your property.
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