Glossary · Controls, energy efficiency and insulation
What is weather compensation?
Weather compensation is a heating control that varies the temperature of the water sent to your radiators according to the weather outside. An outdoor sensor, or online weather data, lets the system run cooler on mild days and hotter on cold ones, instead of using one fixed flow temperature all year.
Key points
- It adjusts the flow temperature using outdoor conditions, measured by an external sensor. The 2026 edition of Approved Document L also allows online weather information.
- The flow temperature is varied either by modulating the heat source output or by using a mixing valve.
- Approved Document L requires a heat pump to have weather compensation or internal temperature control, plus a timer or programmer.
- For a gas combi boiler in an existing home, weather compensation is one of four accepted energy efficiency measures.
- It differs from load compensation, which responds to how the home itself reacts rather than to the weather.
How weather compensation works
A standard heating system sends water out at a set flow temperature whatever the weather. Weather compensation changes that. Approved Document L (2021 edition) defines it as a system that lets the operating flow temperature be varied, with an external sensor communicating with one inside the boiler.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments The 2026 edition, which takes effect on 24 March 2027 for most building work, adds that the boiler can instead access online weather information.[3]Ministry of Housing, Communities and Local Government, Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition)
The effect is simple to describe. When it is mild outside, the home needs less heat, so the water can be sent out cooler. When it is cold, the flow temperature rises. The aim is to match output to demand more closely. The government’s evidence review of compensation controls looked at the mechanisms by which weather compensation improves the efficiency of heating systems, and at its benefits and disadvantages.[2]Department for Energy Security and Net Zero, Heating controls: compensation and time proportional and integral (TPI) controls
How the temperature is varied
There are two ways, according to Approved Document L. One is direct acting, where the heat source modulates its own output. The other uses a mixing valve to adjust the flow temperature going to the emitters.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
| Control | What it responds to |
|---|---|
| Weather compensation | Outdoor conditions, from a sensor (the 2026 edition also allows online weather information) |
| Load compensation | The measured response of the heating system, which keeps the home at its internal temperature |
| Room thermostat | The temperature in one room |
Where the rules mention it
For heat pumps, Approved Document L says the controls should include weather compensation or internal temperature control, and a timer or programmer for space heating.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments For a gas-fired combi boiler installed in an existing dwelling, at least one of four measures should be installed. Weather compensation is one, alongside flue gas heat recovery, load compensation and a smart thermostat with automation and optimisation.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments Read more about the air to water heat pump and the modulating boiler.
Example
Mark lives in a Keighley semi and is having a combi boiler replaced. His installer offers a weather compensation kit as the measure to meet the efficiency guidance. The boiler is set up so that on a mild February day the water to his radiators runs cooler than on a freezing morning. Mark keeps his room thermostat as before.
Setting up weather compensation involves the boiler controls, so it is done by the installing engineer, who for gas work must be Gas Safe registered.
What people get wrong
People expect weather compensation to make the house warmer. It does not. Its aim is to hold comfort while using less energy, by running the system cooler when the weather allows.
A second mistake is thinking it replaces the room thermostat or the programmer. It works alongside them. Approved Document L still expects a timer or programmer for a heat pump, and thermostatic room controls on a new system or a boiler replacement.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
The bottom line
Weather compensation adjusts the flow temperature to the weather, using an outdoor sensor or, under the 2026 edition, online data. It is an accepted energy efficiency measure for a combi boiler and a standard control for heat pumps. It tunes the system rather than replacing your other controls, and it should be set up by the engineer who installs it.
Common questions
How does weather compensation know the outside temperature?
Approved Document L (2021 edition) says an external sensor communicates with one inside the boiler. The 2026 edition adds that the boiler can instead access online weather information. The system then varies the flow temperature.
Is weather compensation the same as a room thermostat?
No. A room thermostat reacts to the temperature indoors. Weather compensation adjusts the flow temperature using outdoor conditions. Many systems use both together.
Is weather compensation compulsory?
Not on its own. Approved Document L lists it as one of four accepted measures when a gas combi boiler is fitted in an existing home, and requires a heat pump to have it or internal temperature control.
What is load compensation?
Approved Document L defines it as a control function that maintains internal temperature by varying the flow temperature from the heat generator relative to the measured response of the heating system.
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. ↩
- Department for Energy Security and Net Zero. Heating controls: compensation and time proportional and integral (TPI) controls. 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. ↩