Glossary · Controls, energy efficiency and insulation
What is flow temperature?
Flow temperature is the temperature of the water leaving a boiler or heat pump on its way to the radiators or underfloor heating. A lower flow temperature usually means the heat source works more efficiently, but it needs larger radiators or underfloor heating to deliver the same warmth to each room.
Key points
- It is the temperature of the water sent out from the heat source, not the temperature of the room.
- In England a new or fully replaced wet heating system should be sized to work at a maximum flow temperature of 55°C, under Approved Document L.
- BASI says wet underfloor heating wants around 35 to 45°C, against 65 to 75°C for a radiator system.
- Where 55°C is not feasible in an existing home, the system should be designed to the lowest temperature that still meets the heating needs.
- Heat pumps are designed with a flow temperature chosen to optimise their efficiency.
How flow temperature works
A wet heating system moves hot water from a heat source, such as a boiler or heat pump, around pipework to radiators or underfloor heating. The water leaving the heat source is the flow. The cooler water coming back is the return. Flow temperature is how hot that outgoing water is.
The warmth you feel from a radiator depends on how hot its surfaces are and how large it is. A hotter flow lets a smaller radiator do the job. A cooler flow needs bigger emitters or a larger floor area to give the same output. That trade-off is why designers think about flow temperature when they size a system.
Typical ranges and the rules
BASI says wet underfloor heating wants a low flow temperature, around 35 to 45°C, against the 65 to 75°C a radiator system runs at.[2]BASI Heating, Underfloor heating That difference is why underfloor heating suits a heat pump well, and why BASI says it can still work with a modern condensing boiler through a mixer manifold.[2]BASI Heating, Underfloor heating
The building regulations for England set a design rule. Approved Document L says that where a wet system is newly installed or fully replaced, all parts, including pipework and emitters, should be sized to meet the dwelling’s heating needs at a maximum flow temperature of 55°C.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
| Situation | What Approved Document L says |
|---|---|
| New or fully replaced wet system | Size to work at a maximum flow temperature of 55°C |
| Existing home where 55°C is not feasible, for example not enough space for larger radiators | Design to the lowest temperature that will meet the heating needs |
| System with no demand for heat | Appliance and pump switched off |
The 2026 edition, which takes effect on 24 March 2027 for most building work in England, goes further. It says a heat pump emitter circuit’s flow temperature should be designed to optimise heat pump efficiency, and allows the appliance to be set back to a reduced output temperature, rather than switched off, when there is no demand.[3]Ministry of Housing, Communities and Local Government, Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition)
A control that varies the flow temperature with the weather is covered under weather compensation. For boiler settings, see the page on the best boiler temperature setting.
Example
Daniel is adding underfloor heating to a new kitchen extension in Halifax while keeping radiators in the rest of the house. The kitchen circuit needs a much cooler flow than the radiators. The installer designs a mixer manifold so the underfloor circuit gets the lower temperature while the radiators keep theirs, the arrangement BASI describes.[2]BASI Heating, Underfloor heating
What people get wrong
People assume a hotter flow is always better. A hotter flow is only needed when the emitters are small for the room. Where emitters are large enough, a lower flow temperature can deliver the same comfort.
The other mistake is treating the flow temperature as a safe thing to alter inside the boiler. Settings behind the casing, and anything involving gas or the flue, are work for a Gas Safe registered engineer. Only use a setting your manufacturer’s user guide tells you to use.
The bottom line
Flow temperature is the heat of the water sent to your emitters. Radiators traditionally run hotter and underfloor heating runs cooler, and Approved Document L asks designers of new or fully replaced wet systems to work to a maximum of 55°C or lower. Lower is usually kinder to efficiency, as long as the radiators or floor can still heat each room.
Common questions
Is flow temperature the same as the thermostat setting?
No. The room thermostat sets the temperature you want in the room. Flow temperature is how hot the water is when it leaves the boiler or heat pump on its way to the heat emitters.
What flow temperature do underfloor systems use?
BASI says wet underfloor heating wants a low flow temperature, around 35 to 45°C, against the 65 to 75°C a radiator system runs at.
What is the 55°C figure in the building regulations?
Approved Document L says a newly installed or fully replaced wet heating system should be sized to meet the home's needs at a maximum flow temperature of 55°C.
Can I change my boiler flow temperature myself?
Only if your boiler manufacturer's user guide says users may and shows how. Anything inside the boiler casing is gas work for a Gas Safe registered engineer. Ask your engineer if you are unsure.
Sources
We cite the original source for every figure: government, regulators, standards bodies and manufacturers' own documents. Each one shows the date we last checked it.
- Ministry of Housing, Communities and Local Government. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments. Checked 11 November 2026. ↩
- BASI Heating. Underfloor heating. Checked 11 November 2026. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition). Checked 11 November 2026. ↩