Glossary · Controls, energy efficiency and insulation
What is a thermostat?
A thermostat is a control that senses temperature and switches the heating on or off to hold the temperature you set. A room thermostat measures the air in a room. A programmable thermostat adds a daily timetable, so the heating follows your routine instead of a single fixed setting.
Key points
- A room thermostat senses the air temperature where it is fitted and switches the heating on or off to hold your setting.
- A programmable room thermostat adds a time schedule. Approved Document L accepts it, or a time switch plus a room thermostat, as time and temperature control for some systems.
- In England a new heating system, or a replaced boiler, should have thermostatic control of each room or heating zone.
- Radiator valves should not be used in the same room as the main room thermostat. The 2026 edition of Approved Document L adds that radiators there should have manual controls.
- A smart thermostat with automation and optimisation is one of the accepted energy efficiency measures when a gas combi boiler is installed in an existing home.
How a thermostat works
A thermostat has one job. It measures a temperature, compares it with the figure you set, and tells the heating to run or stop. A room thermostat does this with the air temperature in the room where it is fitted. When the room reaches your setting it calls for the heating to stop, and when it cools it calls for heat again.
A programmable thermostat adds a timetable. Approved Document L, the building regulations guidance on energy efficiency in England, treats a programmable room thermostat and a time switch plus room thermostat as alternatives for some systems.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments If you already have a separate central heating programmer, a basic room thermostat works alongside it.
The types
| Type | What it does |
|---|---|
| Room thermostat | Holds one temperature in the room it senses, with the heating timed elsewhere |
| Programmable room thermostat | Holds temperatures on a daily schedule |
| Smart thermostat | Adds app control and automation. Approved Document L names a smart thermostat with automation and optimisation as an accepted energy efficiency measure |
| Cylinder thermostat | Controls the stored hot water temperature. A replacement hot water cylinder should have an electronic temperature control such as this |
BASI covers one smart option on its Hive smart controls page.
What the regulations expect
For a new heating system, or when a boiler is replaced, Approved Document L says each room should have thermostatic room controls. These should adapt the heating output room by room, or by heating zone where that is justified.[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 under heating zones.
One way to meet this is a thermostat in a room the circuit serves, plus a thermostatic radiator valve on each emitter in the other rooms. The guidance adds that valves should not be used in the same room as the thermostat.[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, also says heat emitters in that room should have manual controls.[2]Ministry of Housing, Communities and Local Government, Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition) The guidance also says that when work is done on a system with a boiler, a boiler interlock should be installed.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments Working on the boiler itself is for a Gas Safe registered engineer.
Example
Chloe has a 1990s house in Ilkley with one wired room thermostat in the hall and radiator valves upstairs. She gets a new boiler. The installer keeps the hall thermostat, leaves the hall radiator on a manual valve and fits thermostatic valves on the others, which follows the guidance above.
What people get wrong
People often set the thermostat to a high number to heat the house faster. A thermostat only decides when to stop. It does not make the boiler produce heat more quickly.
The other mistake is putting a valve and a thermostat on the same radiator in the same room. They then fight each other, which is why the guidance keeps valves out of the room with the thermostat.
The bottom line
A thermostat holds a temperature, and a programmable one adds a schedule. In England the regulations expect thermostatic control by room or zone when you fit a new system or replace a boiler. A smart thermostat is an accepted option, not a requirement. Choose the type that fits how your household actually uses its heating.
Common questions
What is the difference between a thermostat and a programmer?
A programmer decides when the heating may run. A thermostat decides whether it needs to run to reach the temperature you set. A programmable thermostat combines both jobs in one unit.
Where should the room thermostat go?
Approved Document L says it should be in a room that the heating circuit serves, with thermostatic radiator valves on the heat emitters in other rooms and none in that room. The 2026 edition adds that radiators in the thermostat's room should have manual controls.
Do thermostatic radiator valves replace a room thermostat?
No. Under the guidance, a thermostat in one room is paired with a thermostatic radiator valve on each emitter outside that room. They share the job. A valve alone does not tell the boiler to stop.
Does a smart thermostat meet the regulations on a new combi boiler?
It can. For a gas combi boiler in an existing dwelling, Approved Document L lists a smart thermostat with automation and optimisation as one of four accepted energy efficiency measures.
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. ↩
- Ministry of Housing, Communities and Local Government. Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition). Checked 11 November 2026. ↩