How Does a Room Thermostat Work? Settings and Common Faults

Updated July 2026 · Emergency Plumber Chelsea

The thermostat is one of three controls that work together. The programmer or timer decides when the heating is allowed to run. The room thermostat decides whether it needs to run at that moment, based on the temperature. The thermostatic radiator valves on each radiator then trim the heat room by room. On a system with a hot water cylinder, a separate cylinder thermostat does the same job for the stored hot water. When the programmer is on, the thermostat is calling for heat and at least one radiator valve is open, the boiler fires. If any one of those says no, it does not. For the full set of controls a home should have, and what the rules require with a new boiler, see which heating controls you need.

Inside, the principle is simple. Older dial thermostats use a component that moves as it warms and cools, such as a bimetallic strip or a small sealed capsule, to open and close a switch; you can often hear a faint click as you turn the dial past the room temperature. Digital thermostats use an electronic sensor and a small circuit to do the same thing, and many of them switch the boiler on and off in short cycles to hold the temperature more steadily. Wireless thermostats send the signal to a receiver box, usually mounted near the boiler, which does the actual switching. Smart thermostats add an internet connection and an app, and some can talk to the boiler in more detail than a simple on and off.

Turning it up does not heat the house faster. This is the most common misunderstanding about thermostats. When the heating is calling, the boiler runs at the same output whether you set 20°C or 30°C. The higher setting just means it keeps going long after the house is comfortable, using more gas. Pick the temperature you want and leave it there. If the house takes too long to warm up, set the programmer to start the heating a little earlier rather than turning the thermostat up.

What temperature should you set? Around 18 to 21°C suits most homes. Lower than about 18°C for long periods is not recommended if someone in the home is older, very young or has a health condition, and a house that is kept very cold for days at a time is more likely to suffer condensation and damp. Each degree you turn the thermostat down reduces your heating use, so it is worth finding the lowest setting you are comfortable with and sticking to it. Our guide on when to turn the heating on covers the start-of-season checks.

Where it is fitted matters as much as the setting. A thermostat should sit on an internal wall about one and a half metres from the floor, in a room that is used and heated, usually the hall or the main living room. It should not be next to a radiator, above a lamp, in direct sunlight, in a draught by the front door, or in a kitchen where cooking warms the air. In a sunny Chelsea sitting room or a hallway next to a hot water cylinder cupboard, a thermostat can read two or three degrees warmer than the rest of the home and switch the heating off while bedrooms are still cold. Radiators in the same room as the thermostat should not have their TRVs turned down low, or the thermostat will never be satisfied and the boiler will keep running.

Thermostats and radiator valves are not the same thing. TRVs sense the temperature at each radiator and restrict the flow when that room is warm, but they cannot turn the boiler off. The room thermostat is what stops the boiler running when the whole house is warm enough. If you have TRVs but no room thermostat, the boiler may keep cycling all day with nothing to switch it off. Our guide on a thermostatic radiator valve not working covers the valve side.

How to tell if the thermostat has stopped working. Turn it several degrees above the current room temperature. On a dial type you should hear a click; on a digital one a flame or heat symbol should appear. The boiler should fire within a minute or two. If nothing happens, check that the programmer is set to on or timed, not off. Replace the batteries in a battery-powered or wireless thermostat, and look at the receiver box near the boiler; a flashing or red light usually means it has lost contact and needs re-pairing, which the manual explains. A thermostat that reads a temperature far from what the room feels like, or switches on and off every few minutes, may be faulty or badly placed. Our guide to a boiler that keeps turning on and off covers the other causes of short cycling.

Test the boiler without the thermostat. If the boiler fires on the programmer's boost or advance button, or on its own heating override, the boiler is fine and the fault lies with the thermostat, its receiver or the wiring between them. If the boiler does not fire even on boost, the problem is more likely with the boiler, and our guide on what to check when a boiler is not working goes through power, pressure and fault codes. If you have hot water but no heating at all, see our guide on no heating but hot water.

Leave the wiring to a professional. Many wired thermostats and receiver boxes carry mains voltage, so do not take the back off a wired thermostat or the receiver to check connections. Replacing or moving a thermostat, adding a wireless one, or upgrading to a smart thermostat is a small job for an engineer, and it is often worth doing at the same time as a boiler service. Our guide on smart thermostat installation explains the options, including what works with a system that has a hot water cylinder.

Emergency Plumber Chelsea diagnoses and repairs heating controls, thermostats and boilers across Chelsea, Knightsbridge, South Kensington, Sloane Square and Fulham. We check whether the fault is the thermostat, the wiring, a zone valve or the boiler itself, explain the options before any work starts, and there is no call-out fee on jobs over 3 hours. See our central heating repairs and boiler repair services, or call 020 3002 6667.

Quick tips

• Set the thermostat to around 18 to 21°C and leave it there; higher settings do not heat faster.
• Use the programmer to start the heating earlier if the house takes too long to warm up.
• Fit the thermostat on an internal wall away from radiators, sunlight, draughts and the kitchen.
• Keep the TRVs in the thermostat's room fully open so the thermostat can read the room properly.
• If the boiler fires on boost but not from the thermostat, check batteries and the receiver before calling.

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