Heat pumps

Heat pump flow temperature too high

Warning Do not attempt any electrical or gas work yourself. Always use a qualified, MCS-certified, or Gas Safe registered professional for repairs.

What you’re seeing (symptoms)

Your heat pump is running, and your radiators feel very hot (similar to a gas boiler), but your SCOP (efficiency) is low, and your electricity bills are higher than expected. The heat pump may be “cycling” (turning on and off) frequently.

What’s normal vs not normal

Normal:

  • Flow temperatures rising to 50-55°C during a hot water cylinder reheat.
  • Flow temperatures rising slightly on the coldest days of the year (e.g. -3°C outside).

Not normal:

  • Flow temperatures of 55°C or higher for central heating when it’s mild outside (e.g. 10°C).
  • Radiators feeling “scalding” to the touch.

Likely causes (ranked)

  1. Weather compensation not active: The heat pump is set to a “fixed flow” temperature instead of adjusting based on the outside weather.
  2. Poor heat loss calculation: The system was designed to run hot because the radiators are too small for the property’s heat loss.
  3. Installer default settings: The unit was left on factory settings (often 50-55°C) rather than being commissioned for efficiency.
  4. Incorrect curve settings: The “heating curve” is too aggressive, meaning it asks for too much heat for a given outside temperature.
  5. Conflict with TRVs: Thermostatic valves are closing and the heat pump is “fighting” to push heat into a restricted system.

Quick checks you can do safely

  • Check the controller: Look for the “Flow Temperature” or “Heating Curve” settings. Is it set to “Fixed” or “Weather Dependent”?
  • Open the TRVs: Ensure your main living area radiators are turned to max to see if the heat pump can lower its temperature while still keeping you warm.
  • Monitor the COP: Many modern controllers show an instantaneous COP (Coefficient of Performance). If it’s below 2.5 when heating, your flow is likely too high.
  • Lower the curve: If you have access, try lowering the heating curve by 2-3 degrees and wait 24 hours to see if the house remains comfortable.

When to call an installer

  • If you cannot find the settings to enable weather compensation.
  • If your radiators must be 55°C+ just to keep the house at 20°C (you may need larger radiators).
  • If the system is frequently displaying “Flow Temp Error” or similar.

Questions to ask your installer

  • “Can you help me set up a weather compensation curve for my home?”
  • “Are my radiators correctly sized for a 45°C flow temperature at the design limit?”
  • “Is the ‘Auto-Adapt’ or ‘Advanced’ weather logic enabled on this controller?”

Related NetZeroNow tools & guides

FAQs

Why does flow temperature matter? A heat pump is most efficient when it only has to lift the temperature a small amount. Moving heat to 35°C water is much “easier” (and cheaper) than moving it to 55°C water. Won’t my house be cold? No, as long as the radiators are large enough. Low-temperature heating works by providing a steady, gentle heat over a longer period, rather than “blasts” of high heat. What is Weather Compensation? It’s a feature where a sensor on your outside wall tells the heat pump how cold it is. If it’s mild, the heat pump automatically lowers the flow temperature to save energy. Can I use it with a smart thermostat? Yes, but the heat pump’s own weather compensation should ideally do the heavy lifting, with the thermostat acting as a “limit” or “setpoint.” What is a good flow temperature? For a well-designed heat pump system, you should aim for 35-45°C for radiators and 30-35°C for underfloor heating.\n

Written by NetZeroNow | Last updated on 2026-03-02