Glossary · Controls, energy efficiency and insulation
What is a central heating programmer?
A central heating programmer is the timer that tells your boiler when to run. It lets you set the times the central heating and hot water switch on and off each day. It controls when heat is available. A room thermostat controls how warm the house gets while the heating is on.
Key points
- A programmer sets when the heating and hot water may run. A room thermostat sets how warm the rooms get during those times.
- Building Regulations guidance says boiler controls count as part of the boiler installation when a boiler is replaced.
- The guidance expects hot water and heating to have separate time controls, although a single existing controller may stay in place.
- A smart thermostat does the programmer's job through an app schedule, so the two are alternatives rather than a pair.
How a central heating programmer works
A programmer is a timer wired into your heating controls. You set the times each day when you want the heating on and, if you have a hot water cylinder, when you want hot water heated. During a switched-on period the programmer allows the system to run. Outside those periods it keeps the boiler and pump off.
Building Regulations guidance expects the controls to be wired so that the boiler and pump switch off when nothing is asking for heating or hot water.[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 programmer decides when the system is allowed to run. The thermostat and radiator valves then decide how much heat is needed inside those times.
On a system with a hot water cylinder the programmer usually has two channels, one for heating and one for hot water. A combi boiler heats water as you use it, so the programmer on a combi normally only controls the heating. Our page on how a combi boiler works explains why.
Programmer, thermostat and radiator valves
These controls are often confused. Each does a different job.
| Control | What it does | Decides |
|---|---|---|
| Programmer | Switches the heating and hot water on and off at set times | When the system may run |
| Room thermostat | Measures the temperature in one room and switches the heating off when it reaches the set level | How warm the main room gets |
| Thermostatic radiator valve | Limits the heat from one radiator | How warm each other room gets |
| Programmable room thermostat | Combines the timer and the thermostat in one unit | Both of the above |
| Smart thermostat | Does the same through an app schedule | Both of the above |
Approved Document L says that when a boiler is replaced in an existing home, each room should have thermostatic room controls. That can be met by a room thermostat plus a thermostatic radiator valve on each radiator outside the room with the thermostat, or by a thermostat for each room or heating zone.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments It also says that if hot water and heating share one time controller on an existing system they may stay that way. Otherwise each should have its own time control. When work is done on a system with a boiler, a boiler interlock should be fitted, and the boiler controls are treated as part of the boiler installation.[1]Ministry of Housing, Communities and Local Government, Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments
A new 2026 edition of Approved Document L takes effect on 24 March 2027 for most building work in England. It keeps the same points on separate time controls and the boiler interlock.[2]Ministry of Housing, Communities and Local Government, Approved Document L: Energy and greenhouse gas emissions, Volume 1: Dwellings (2026 edition)
Example
Priya lives in a semi in Shipley with a regular boiler and a hot water cylinder. Her old programmer runs heating and hot water on the same times, so the cylinder reheats during the day when nobody is home. When her boiler is replaced, her installer fits controls with a separate hot water channel. She now sets the heating for mornings and evenings and the hot water for a short slot before breakfast, and the thermostat stops the boiler once the lounge reaches the temperature she chose.
What people get wrong
The most common mistake is thinking the programmer controls the temperature. It does not. It only decides the hours. The room thermostat sets the temperature, and the programmer gives the thermostat permission to ask for heat during the times you chose.
The second mistake is treating the programmer as a do-it-yourself part. Setting the times is covered in the user guide. Anyone employed to work on gas appliances in a home must be a Gas Safe registered engineer.[3]Health and Safety Executive, Domestic gas: frequently asked questions If the programmer or any control wired to your boiler needs replacing, ask a Gas Safe registered engineer.
If you want app control, a smart thermostat takes over the timer’s job, so you would not normally keep a separate programmer. BASI describes the Hive thermostat as letting you set a schedule and control the heating from your phone.[4]BASI Heating, Hive smart heating, supplied and fitted
The bottom line
A central heating programmer is the timer that decides when your heating and hot water run. It works alongside a thermostat and radiator valves, which decide how warm the rooms get. Building Regulations guidance expects separate time control for hot water and heating on most systems and treats boiler controls as part of a boiler installation. A smart thermostat does the same job through an app.
Common questions
What is the difference between a programmer and a thermostat?
A programmer sets the times the heating is allowed to run. A thermostat measures the room temperature and switches the heating off when the room reaches the temperature you set. Most homes need both.
Does a combi boiler need a programmer?
A combi heats hot water as you use it, so its programmer normally only controls the central heating times. You still need some form of time control for the heating, which can be a programmer or a programmable thermostat.
Can I replace a programmer myself?
Setting the times is a normal user task and is covered in the user guide. Replacing or rewiring controls connected to a gas boiler is work to leave to a Gas Safe registered engineer.
Is a smart thermostat a programmer?
In effect, yes. A smart thermostat such as Hive lets you set a schedule from an app and controls the room temperature, so it takes over both jobs.
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. ↩
- Health and Safety Executive. Domestic gas: frequently asked questions. Checked 11 November 2026. ↩
- BASI Heating. Hive smart heating, supplied and fitted. Checked 11 November 2026. ↩