Heat Pump Flow Temperature Settings Explained

Every degree lower your heat pump's flow temperature runs, the less electricity it uses and the more money you save. This is the fundamental truth of heat pump efficiency, and it is why understanding flow temperature matters so much. A heat pump running at 35°C is dramatically more efficient than one forced to run at 55°C — yet many homeowners (and some installers) set temperatures too high.

This guide explains what flow temperature is, why it matters, what yours should be set to, and how weather compensation manages it automatically.

What Is Flow Temperature?

Flow temperature is the temperature of the water your heat pump sends to your radiators or underfloor heating. It is the "outgoing" temperature — the water returns to the heat pump several degrees cooler after passing through the heating circuit.

A gas boiler typically runs at 60-80°C flow temperature. A heat pump is designed to run at 35-45°C. This difference is not a weakness — it is how heat pumps achieve their remarkable efficiency.

Why It Matters for Efficiency

A heat pump's efficiency is measured by its coefficient of performance (COP) — the ratio of heat output to electricity input. The lower the flow temperature, the higher the COP:

Approximate values — actual COP varies by heat pump model and conditions.

Running at 55°C instead of 35°C roughly halves your heat pump's efficiency. That translates directly into higher electricity bills.

Target Flow Temperatures

How Weather Compensation Manages It

Rather than running at a fixed flow temperature, a properly configured heat pump varies the flow temperature based on the outdoor temperature. This is weather compensation — the most important efficiency feature of any heat pump system.

On a mild 12°C day, the system might run at just 30°C flow temperature. On a freezing -5°C day, it increases to 45°C. This automatic adjustment prevents energy waste on mild days while ensuring comfort on cold days. The "heating curve" that defines this relationship is set during commissioning.

Impact on Radiators

At 35-45°C, your radiators will feel warm rather than hot to the touch. This is completely normal and does not mean the house is not being heated properly. The heat pump compensates by running for longer periods, delivering the same total amount of heat over time.

If a room is not reaching the desired temperature, the solution is almost always a larger radiator (more surface area to emit heat at lower temperatures) rather than increasing the flow temperature. See our guide on whether you need new radiators for more detail.

Homeowners with solar panels can maximise their savings by running the heat pump during daylight hours when solar generation offsets electricity consumption.

Frequently Asked Questions

What flow temperature should my heat pump be set to?

For radiators, 35-45°C, managed automatically by weather compensation. For underfloor heating, 30-40°C. Higher temperatures reduce efficiency significantly.

Why are my radiators not as hot as they were with a boiler?

This is normal. Heat pumps run at lower temperatures but for longer periods. If a room is not warm enough, the radiator may need upsizing rather than the flow temperature increasing.

Can I increase the flow temperature on my heat pump?

You can, but you should not without good reason. Consult your installer before making changes — the solution is usually larger radiators or better insulation, not higher flow temperatures.