Why Are My Heat Pump Radiators Only Lukewarm? Common Causes Explained

Many homeowners contact us because their heat pump appears to be running normally, yet the radiators never feel particularly hot or only ever seem lukewarm. Unlike traditional gas boilers, heat pumps operate at lower temperatures, so radiators may feel warm rather than hot to the touch.

However, if the house struggles to reach temperature or the system runs constantly without improving comfort, there may be an underlying system issue worth investigating.

1. Heat pumps run at lower temperatures

A gas boiler might run at 70–80°C, while heat pumps typically operate around 35–50°C.

This means radiators often feel lukewarm compared to gas systems, even when the system is working correctly.

2. Poor system circulation

If the system has poor flow through the radiators, it may struggle to circulate heat effectively

Common causes include:

• Incorrect pump settings

• Sludge or debris in pipework

• Long pipe runs restricting circulation

• Incorrect buffer tank connections

• Strainer/ filter blocked

3. Radiators undersized for heat pump temperatures

Heat pumps rely on larger radiators or underfloor heating.

If the original radiators from a gas boiler system were kept, they may not emit enough heat at lower temperatures or be properly sized for heat pump operation.

4. Incorrect system setup

In some cases the issue is simply configuration:

• Flow temperatures set too low

• Weather compensation incorrectly adjusted

• Controls cycling incorrectly due to poor commissioning or setup

• Zone valves restricting flow

Zoning and wiring faults can cause entire areas of a home to receive no heating at all — we documented one such situation in our underfloor heating not working zoning wiring fault case study, which explains how the fault was traced and resolved.

5. When to seek a technical review

If your heat pump is running but your radiators remain lukewarm, or your home isn’t reaching the temperature you expect, it’s usually a sign that something within the system design, flow, or configuration needs attention.

These issues are often not obvious without reviewing how the system has been set up and how it’s operating in practice.

👉 If you’re experiencing this, you can request a full heat pump performance review, where we assess flow rates, radiator performance, system setup, and overall configuration to identify exactly what’s causing the issue — and how to fix it.

Related reading: How Dirty Filters Can Cause Heat Pump Problems