If you have recently switched from a traditional gas boiler to an air source heat pump, you might touch your radiators and worry something is wrong. Instead of feeling scalding hot, they often feel lukewarm to the touch.

The short answer is: this is completely normal. Lukewarm radiators are actually a clear sign that your heat pump is operating efficiently.

Heat Pumps vs Gas Boilers: Temperature Differences

To understand why your radiators do not feel hot, it helps to look at how different heating systems deliver warmth to your home:

  • Gas Boilers: These are high-temperature systems that heat water to around 60°C–70°C. They operate in short, intense blasts, turning on and off throughout the day to heat rooms quickly.
  • Heat Pumps: These are low-temperature systems that heat water to roughly 35°C–45°C. Instead of short, hot bursts, a heat pump runs continuously at a lower temperature to maintain a steady, comfortable climate.

Because human body temperature is around 37°C, water running through a radiator at 40°C will feel only slightly warm—or even lukewarm—to the touch, even though it is actively keeping your room at a comfortable 21°C.

When Lukewarm Radiators Might Indicate an Issue

While mild radiator temperatures are standard, your home should still reach its target temperature. If your rooms feel genuinely cold, a few common factors could be at play:

  • Inadequate Radiator Surface Area: Because heat pumps run cooler water, radiators need to be larger (or double/triple panelled) to transfer enough heat into the room. Small single-panel radiators may struggle to keep up.
  • Turning the System On and Off: Heat pumps work best when left running gently in the background. If you turn your heating completely off when leaving the house, a low-temperature system takes significantly longer to warm the property back up.
  • Trapped Air: Just like with a boiler system, air bubbles in the pipework can prevent water from circulating evenly. Bleeding your radiators can resolve cold spots at the top of the panels.
  • Incorrect Weather Compensation Settings: Modern heat pumps adjust their water temperature based on the weather outside. If the heating curve curve settings are incorrect, the system might not push enough heat into the radiators on colder days.

Getting the Most Comfort from Your System

To ensure your home stays warm and your energy bills remain low, keep these best practices in mind:

  • Set It and Forget It: Avoid turning your thermostat up and down dramatically. Choose a comfortable temperature and let the heat pump manage the climate automatically.
  • Keep Radiators Clear: Avoid placing heavy curtains or long sofas directly in front of radiators, as this blocks warm air from circulating around the room.
  • Trust the Room Temperature, Not the Radiator: Focus on the temperature displayed on your wall thermostat rather than how hot the metal radiator feels to your hand.

If your radiators feel lukewarm but your home is comfortably warm, your heat pump is doing its job perfectly. If the house remains cold, a quick system check or heat-loss review by a qualified engineer will ensure your setup is properly optimised.