Heat pumps

Radiators lukewarm with heat pump

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)

The radiators in your home feel warm or lukewarm to the touch, rather than the “piping hot” sensation you may be used to from a traditional gas boiler.

What’s normal vs not normal

Normal:

  • Radiators feeling warm (35-45°C) rather than hot (60-70°C).
  • The house remaining at the desired setpoint temperature despite the “cool” radiators.
  • Radiators being warm for long periods of the day.

Not normal:

  • The house failing to reach the temperature set on the thermostat.
  • Some radiators being cold at the bottom or top.
  • The heat pump running constantly but the room temperature dropping.

Likely causes (ranked)

  1. Low flow temperature design: Heat pumps are most efficient when they run at lower temperatures than gas boilers.
  2. Weather compensation: The system automatically lowers the radiator temperature when the outside air is milder.
  3. Air in the system: Air pockets can prevent warm water from circulating properly.
  4. Incorrect thermostat settings: The “setback” temperature might be too low.
  5. Undersized radiators: Radiators may not have enough surface area for the lower flow temperature.

Quick checks you can do safely

  • Check your room thermostat—is it actually reaching the target temperature?
  • Check for cold spots on radiators (may need bleeding).
  • Check that your curtains or furniture are not blocking the radiators.

When to call an installer

  • If your home is consistently below the temperature you’ve set on the thermostat.
  • If the heat pump is showing an error code or “low flow” warning.
  • If some radiators are completely cold while others are warm.

Questions to ask your installer

  • “Is the weather compensation curve set correctly for my home’s insulation?”
  • “Are the radiators balanced correctly across the whole house?”
  • “Do I need larger radiators in certain rooms?”

Related NetZeroNow tools & guides

FAQs

Why don’t they get hot? Heat pumps are designed to provide a steady “trickle” of heat at lower temperatures, which is much more energy-efficient than the “stop-start” high heat of a boiler.

Will lukewarm radiators actually heat my house? Yes, provided they have been sized correctly and the house is reasonably insulated. They just take longer to raise the temperature initially.

Can I turn up the temperature? You can, but it will significantly reduce the efficiency (COP) of your heat pump and increase your electricity bills.

Should I leave the heating on all the time? Generally, yes. Heat pumps work best by maintaining a constant temperature rather than trying to heat a cold house quickly.

Does this mean my heat pump is undersized? Not necessarily. Lukewarm radiators are a feature of a well-designed, efficient heat pump system.

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