Introduction
If you’re considering a heat pump or already have one, you may have heard about the importance of flow temperature — the temperature of the water flowing through your radiators or underfloor
heating.
What many homeowners don’t realize is that the size of your radiators directly affects how efficiently your heat pump can run. In short: bigger radiators = better efficiency. This paper
explains why larger heat emitters matter, how they impact your system’s Seasonal Coefficient of Performance (SCOP), and what it means for your comfort and energy bills.
What Is SCOP?
SCOP stands for Seasonal Coefficient of Performance. It’s a measure of how efficiently a heat pump delivers heat over the course of a year.- A SCOP of 4.0 means the heat pump produces 4 units of heat for every 1 unit of electricity it uses.- Higher SCOP = lower running costs and lower carbon emissions.
SCOP depends heavily on how hard your heat pump has to work — and flow temperature is one of the biggest factors.
Why Lower Flow Temperatures Improve SCOP
Heat pumps are most efficient when heating water to lower temperatures — ideally between 30°C and 45°C.- At lower temperatures, the heat pump compressor works less and operates more efficiently.- Every extra degree above 45°C starts to lower efficiency significantly.- Some heat pumps lose up to 20–30% of performance when forced to run at 55–60°C (like traditional boilers).
This is where radiator size comes in.
Why Bigger Radiators Help
Radiators work by giving off heat to the room. The bigger the radiator (in surface area), the more heat it can deliver — even at lower temperatures.
Think of it like this:
- A small radiator needs hotter water to heat the room.
- A large radiator can achieve the same room temperature using cooler water.
- So by increasing radiator size, you allow your system to:
- Run at lower flow temperatures
- Maintain comfort in every room
- Maximize SCOP and efficiency
Real-World Example
Imagine a room that needs 1.5kW of heat to stay warm on a cold day:
- A small radiator might need 60°C flow temp to provide that.
- A larger radiator could deliver the same 1.5kW with just 45°C water.
The lower the flow temperature, the easier the job for your heat pump — and the more efficient the system becomes.
Benefits of Running at Lower Temperatures
- Lower Electricity Bills – Higher SCOP means more heat from less energy.
- Quieter Operation – Heat pumps running at lower temperatures often run more steadily and quietly.
- Longer Equipment Life – Less strain on the compressor and fewer on/off cycles.
- Better Comfort – Larger radiators give off gentle, consistent heat, rather than cycling between hot and cold.
- Future-Proofing – If energy prices rise or you switch to solar, low-flow systems are better suited for flexible, cost-effective operation.
Should You Replace All Your Radiators?
Not necessarily — but your installer should:
- Do a room-by-room heat loss calculation
- Check whether existing radiators can deliver required heat at 45°C or lower
- Recommend upgrades where needed, especially in high-heat-loss areas (large windows, exposed walls, etc.)
Sometimes just a few radiator upgrades can make a big difference to overall system efficiency.
Conclusion
With heat pumps, size matters — and bigger radiators are better. By increasing radiator surface area, you enable your heat pump to work at lower flow temperatures, boosting its SCOP and
saving you money in the long run.
If you’re planning a heat pump installation or struggling with high running costs, ask your installer: Are my radiators sized for low-temperature heating? A few upgrades today can lead to big savings over the life of your system.