It depends.
Most radiators in UK homes were originally designed to work with flow temperatures of around 70°C. This is because they were typically installed alongside traditional gas boiler systems, which operate at much higher temperatures than modern heat pumps.
Heat pumps are generally most efficient when running at lower flow temperatures — often between 35°C and 55°C. While this improves efficiency and reduces running costs, it also means your existing radiators may not emit enough heat to warm your rooms effectively, because the water passing through them is cooler.
A key factor in determining whether you need larger radiators is your system’s flow temperature, alongside the heat loss of each room.
For example:
- A heating system running at 40°C will usually require larger radiators
- A system running at 60°C may be able to use smaller or existing radiators
This is because lower temperatures reduce radiator heat output.
One of the most important concepts to understand here is Delta T. Heat pump systems commonly operate around Delta T30, whereas traditional gas boiler systems are often based on Delta T50 ratings. This is why many existing radiators can appear undersized when paired with a heat pump.
Where can I find my flow temperature?
Your heat pump installer should be able to advise you on your system’s flow temperature.
Alternatively, if you use a monitoring platform such as HeatpumpMonitor.org, your flow temperature data will usually be tracked and visible there.
If you’re unsure whether your current radiators are suitable for a heat pump, a room-by-room heat loss calculation is the best place to start. This helps determine the radiator size required to keep your home warm and efficient at lower flow temperatures.
