What is a boiler thermostat?
People use boiler thermostat to mean several different controls, and mixing them up is the most common reason settings go wrong. On the boiler itself there is usually a heating temperature control, marked with a radiator symbol, and on a combi a separate hot water control, marked with a tap symbol. Away from the boiler there is the room thermostat, and on a system or regular boiler there is also a cylinder thermostat on the hot water tank.
| Control | Where it is | What it controls | Typical guidance |
|---|---|---|---|
| Heating dial (radiator symbol) | On the boiler | Flow temperature: how hot the water sent to the radiators is | Around 60°C on a combi, per the Energy Saving Trust |
| Hot water dial (tap symbol) | On a combi boiler | Temperature of hot water at the taps | Hot enough for use without needing lots of cold mixed in |
| Room thermostat | On a wall in the house | When the boiler runs for heating, based on air temperature | 18°C to 21°C, per the Energy Saving Trust |
| Cylinder thermostat | On the hot water cylinder | When the cylinder stops heating | 60°C to 65°C, per the Energy Saving Trust |
| Thermostatic radiator valves | On each radiator | Heat output of that radiator | Lowest setting that keeps the room comfortable |
What is the difference between a boiler thermostat and a room thermostat?
The boiler thermostat controls how hot the water is, and the room thermostat controls whether the heating is on at all. Think of the boiler dial as the strength of the heating and the room thermostat as the switch. When the room drops below the temperature you set, the room thermostat tells the boiler to fire. When the room reaches it, the thermostat switches the heating off, whatever the boiler dial says.
That is why turning the boiler dial up does not make the house warmer in the long run, and turning the room thermostat up does not make the boiler heat faster. A higher boiler setting makes the radiators hotter so rooms warm more quickly, but it costs efficiency. A higher room thermostat setting just keeps the boiler running for longer. Building Regulations guidance expects both: a way to control the boiler temperature and a room thermostat or equivalent that switches the boiler off when the house is warm enough.
What temperature should my boiler thermostat be set to?
For the heating dial on a combi boiler, the Energy Saving Trust says around 60°C is a reasonable starting point, adjusted by trial and error. If the boiler also heats a hot water cylinder, it advises keeping the flow temperature at least 65°C so the stored water stays hot enough to control legionella. For the room thermostat, it suggests the lowest comfortable temperature, which for most people is 18°C to 21°C.
- Take a photo of the current settings before you change anything, so you can go back.
- Change one thing at a time and give it a few days of normal weather before judging.
- Turn it back up in cold snaps if rooms struggle to reach temperature. Our guide to heating on but the house still cold covers flow temperature in more detail.
- Take extra care for vulnerable people. The Energy Saving Trust notes that lower settings can leave some rooms too cool, which matters for older people, babies and anyone who is unwell.
For hot water on a combi, set the tap dial to the temperature you actually use, so you are not running very hot water and cooling it at the tap. If hot water is not reaching a useful temperature at all, see hot water not hot enough.
A lower heating dial works because a condensing boiler only condenses fully when the water coming back from the radiators is below about 55°C. Cooler flow means cooler return water and more heat recovered from the flue gases.
Choosing a thermostat for a boiler
Any modern gas boiler can work with a simple on and off room thermostat. The choice comes down to how much control you want and what your boiler supports.
| Type | How it works | Good for |
|---|---|---|
| Wired dial thermostat | Switches the boiler on and off through a cable | Simple systems with a separate timer |
| Programmable room thermostat | Combines the timer and thermostat in one unit | Replacing a separate timer and dial |
| Wireless thermostat | Sends a radio signal to a receiver wired to the boiler | Moving the thermostat without new cables |
| Smart thermostat | Wireless thermostat with app control and scheduling | Remote control, learning and zoning features |
| Modulating (OpenTherm or brand control) | Tells the boiler how much heat to produce, not just on or off | Steadier temperatures and better condensing |
In England, the Boiler Plus rules from April 2018 mean every new gas boiler must be fitted with time and temperature controls. A new combi also needs one extra energy saving measure, and a smart thermostat with automation and optimisation functions is one of the options, alongside weather compensation, load compensation and flue gas heat recovery.
Modulating controls and compatibility
A basic thermostat switches the boiler fully on or off. A modulating control tells the boiler how hard to run, so it can tick over at low output as the room approaches temperature. OpenTherm is an open standard that many boilers and thermostats use for this. Some manufacturers use their own systems, such as the EMS bus on Worcester Bosch boilers and eBUS on Vaillant boilers, and their own controls plug straight into these. Check the controls section of your boiler's installation manual, and use the thermostat maker's compatibility checker, before you buy.
Combi boiler thermostat options
A combi boiler heats hot water on demand, so it only needs a room thermostat and a timer for the heating. Many combis have a plug-in timer slot on the front, and many homes use a programmable room thermostat instead. A system or regular boiler with a cylinder needs separate time control for hot water and a cylinder thermostat too; see our system boiler guide for how those controls fit together.
Wireless thermostat for a combi boiler
A wireless thermostat for a combi boiler comes in two parts: the battery-powered thermostat you put in a living area, and a receiver fixed near the boiler. The receiver is wired into the boiler's room thermostat connections, or on some brands plugs into the boiler's control slot. Some kits are designed to replace an old timer on a standard wall plate, reusing its wiring. Whichever type you choose, keep the receiver within range and away from large metal objects that could block the signal.
How to connect a thermostat to a boiler
Connecting a thermostat to a boiler means wiring it, or its receiver, into the terminals the boiler provides for external controls. The exact terminals differ by boiler and are shown in the wiring diagram in the installation manual. In outline, a qualified installer will:
- Isolate the power at the fused spur and confirm it is dead.
- Identify the boiler's external control terminals, which on many boilers have a factory-fitted link that is removed when a thermostat is connected.
- Wire the receiver or thermostat to those terminals, or to the wiring centre on a system with zone valves. Modulating controls use separate low voltage terminals.
- Pair a wireless thermostat with its receiver, following the maker's steps.
- Set any timer on the boiler so it does not override the new control, then test that the boiler fires and stops as the thermostat is turned up and down.
How to connect a wireless thermostat to a boiler follows the same steps, with pairing at the end. If a new thermostat later stops talking to the boiler, our guide to a boiler not responding to the thermostat covers batteries, pairing and receivers.
Most boiler control terminals carry mains voltage, and they sit close to the boiler's internal wiring. Do not remove the boiler case or wire anything into it yourself. Have controls fitted by a Gas Safe registered engineer or a qualified electrician.
Do I need an electrician to fit a wireless thermostat?
You need a qualified person to wire the receiver, but it does not have to be an electrician. A Gas Safe registered engineer who fits heating controls is often the better choice, because they know the boiler's wiring and can check the settings at the same time. The thermostat itself only needs batteries and a spot on the wall. Some smart thermostat brands offer their own installation service, which is worth considering if you want the work done alongside set-up.
What to do next
Find your boiler's make and model on the front or the data plate, and look up the controls section of its manual to see which thermostats it supports. Try the Energy Saving Trust starting points on the dials you already have before spending money. If you want a new thermostat, ask your engineer to fit it at the annual boiler service, when they can also check the boiler is set up to suit it.