Flow temperature
The water temperature leaving a boiler or air-to-water heat pump toward the emitters, central to United Kingdom and European system design. Lower flow temperatures raise heat-pump efficiency but require emitters sized to suit them.
Also called flow temp, leaving water temperature
What is flow temperature?
Flow temperature is the temperature of the water leaving a boiler or heat pump toward the radiators or underfloor loops. Lowering it raises heat pump efficiency and lets a boiler condense.
Flow temperature: how it works
A radiator gives out heat in proportion to how much hotter it is than the room, so a cooler flow temperature means a given radiator delivers less. A heat pump has the opposite preference: the smaller the lift between outdoor air and the water it is heating, the more heat it moves per unit of electricity. Matching the two is what emitter sizing is for.
Where you meet the term
- On a heat pump controller, as the target the system is working to, either fixed or weather compensated.
- In a heat loss survey, where the design flow temperature decides which radiators need replacing.
- In a boiler efficiency discussion, where a lower flow temperature keeps return water cool enough to condense.
How to check it
- Read flow and return on the controller. Both are displayed on most heat pump controllers. The difference between them shows how much heat the system is actually giving up to the building. Owner check
- Lower the setting gradually and watch comfort. On a boiler, stepping the flow temperature down and observing whether the house still reaches temperature is a reversible change with a direct effect on gas use. Owner check
- Check emitter output at the design temperature. Radiator output tables are published at stated temperature differences. A radiator sized for 70 C delivers a fraction of that output at 45 C, which is why a heat pump conversion can need larger emitters. Technician
Figures you will see
| Condition | Figure | What it tells you |
|---|---|---|
| Underfloor heating design | Around 35 C | The large emitting surface allows a low flow temperature |
| Heat pump with sized radiators | Around 45 to 55 C | Requires radiators selected for that temperature rather than boiler-era sizing |
| Condensing boiler threshold | Return below about 55 C | Above this, the secondary heat exchanger recovers no latent heat |
Flow temperature: what it is not
- A higher flow temperature does not heat a house faster in any useful sense. It reaches temperature sooner and costs more doing it.
- Flow temperature is not the same as the room set point. One is the water leaving the appliance, the other is the air in the room.
Flow temperature questions
What flow temperature should a heat pump run at?
The lowest the emitters can work with while still meeting the heat loss on a cold day. That figure comes from the survey and the emitter sizing rather than from a general rule.
Will lowering my boiler flow temperature save money?
It lets a condensing boiler recover latent heat from its flue gases, which it cannot do while return water stays hot. The house takes longer to warm up, which is the trade-off.
What is the difference between flow temperature and delta T?
Flow temperature is how hot the water leaving the appliance is. Delta T here is the difference between that and the water coming back, which shows how much heat the emitters extracted.
Where this definition comes from
Written from government guidance material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Energy label and ecodesignThe European framework behind SCOP, seasonal space heating efficiency, and the product energy label.