How Do Thermostats Work? A Plain Guide to Room Thermostats
How a room thermostat switches your boiler on and off, how it differs from the boiler dial and your TRVs, where it should go, what to set it to, and what to check when it stops working.
A room thermostat measures the air temperature where it hangs and acts as a switch for your heating. When the room drops below the temperature you set, it tells the boiler to fire and the pump to send hot water to the radiators. When the room reaches that temperature, it switches the heating off again. It doesn't control how hot the radiators get (that's the boiler's flow temperature) and turning it up higher won't warm the house any faster. It only makes the heating run for longer.
Key Takeaways
- A room thermostat is an on/off switch for the heating, triggered by the air temperature around it. It has no effect on how hot the radiators get.
- Turning it up to 30°C doesn't heat the house faster. The boiler runs at the same rate and just keeps going for longer.
- The boiler dial sets the water temperature, TRVs trim each room, and the timer sets when heating is allowed. The room thermostat decides whether it's needed.
- Put it on an inside wall at roughly shoulder height, away from radiators, sunlight, draughts and the kitchen. A badly placed stat gives bad readings.
- To test one, turn it well above room temperature and listen for a click and the boiler firing. Turn it well below and the boiler should stop.
A room thermostat measures the air temperature where it hangs and acts as a switch for your heating. When the room drops below the temperature you've set, it tells the boiler to fire and the pump to send hot water round the radiators. When the room reaches that temperature, it switches the heating off. It doesn't control how hot the radiators get, and turning it up to 30°C won't warm the house any faster. It just keeps the heating running for longer. With the heating going back on for the season, this guide explains how the different controls fit together, where a thermostat should go, and what to check when it stops doing its job.
How a room thermostat works
Every room thermostat does the same basic job. It compares the air temperature around it with the temperature you've chosen, and closes or opens a switch depending on the answer.
- Room colder than the setting: the switch closes and sends a "call for heat" to the boiler. If the timer also allows heating at that moment, the boiler fires and the pump runs.
- Room reaches the setting: the switch opens, the call for heat stops and the boiler shuts down. Many boilers keep the pump running for a few minutes afterwards to move leftover heat out of the boiler, so a short hum after it stops is normal.
To stop the boiler clicking on and off every few seconds, thermostats switch with a small margin. The heating comes back on once the room has cooled a little below the setting, not the instant it dips. That's why the temperature on the display drifts slightly either side of the number you chose.
Older dial thermostats do this mechanically, with a bimetallic strip or a gas-filled bellows that moves as it warms and flips a contact. Digital thermostats use an electronic sensor and a relay. The result is the same: a switch that the boiler reads as "heat wanted" or "heat not wanted".
Types of thermostat
| Type | How it works | Worth knowing |
|---|---|---|
| Mechanical dial | Turn the dial to a temperature. A bimetallic strip or bellows makes and breaks the contact | Simple and long lasting, but less precise. Often wired to the boiler at 230V |
| Digital, wired | Electronic sensor and display, wired to the boiler or wiring centre | More accurate. Some need batteries for the display even though they're wired |
| Programmable | Thermostat and timer in one, with different temperatures at different times | Lets you have a cooler night setting without a separate timer |
| Wireless | Battery-powered unit you can place anywhere, talking to a receiver wired near the boiler | Flat batteries and lost pairing are the most common faults |
| Smart | Connects to your Wi-Fi and phone, and may learn your routine or react to when you leave home | Some can also turn the boiler's output up and down rather than just on and off, if the boiler supports it |
That last point matters. A standard thermostat can only tell the boiler "on" or "off". Some modern thermostats can ask the boiler to run at a lower output when the house is nearly warm, which is gentler on the boiler and usually more efficient. Whether yours can depends on both the thermostat and the boiler, so check the manufacturers' compatibility lists before buying one for that reason. Our smart thermostat installation page covers what fitting one involves.
Thermostat, boiler dial, TRVs and timer
Most confusion about heating comes from mixing up four controls that each do a different job. Building regulations guidance in England expects a heating system to have both time and temperature control, and in most homes that means all of these:
| Control | What it decides | What it doesn't do |
|---|---|---|
| Timer or programmer | When the heating is allowed to run | Doesn't know how warm the house is |
| Room thermostat | Whether the house needs heat right now | Doesn't set how hot the radiators get |
| Boiler heating dial | How hot the water sent to the radiators is | Doesn't switch the heating on for you |
| TRVs on the radiators | How warm each individual room gets | Can't make the boiler fire on their own |
For the heating to come on, the timer has to allow it and the thermostat has to call for heat. If either says no, nothing happens. That's the first thing to check when the heating won't start.
The room thermostat and the TRVs can also work against each other. If the radiator in the same room as the thermostat has its TRV turned down, that room never reaches the target, the thermostat keeps calling for heat and the boiler runs longer than it needs to. Leave that radiator's TRV fully open and use the others to trim individual rooms. Our guide to TRV settings explains what the numbers on each valve mean.
Where a thermostat should go
A thermostat only knows the temperature of the air right next to it. If that air isn't typical of the house, the heating will be wrong all day. Good positions have a few things in common:
- On an inside wall, not an outside wall that runs cold in winter.
- Roughly shoulder height, not near the floor or ceiling.
- In a room you use, usually the living room or a central hallway in a smaller home.
- Away from radiators, open fires, lamps, televisions and cookers.
- Out of direct sunlight and away from front doors, windows and draughts.
- Not behind curtains or furniture, and not in a cupboard.
A thermostat in a sunny room switches the heating off on bright afternoons while the rest of the house goes cold. One in a draughty hall keeps the boiler running while the living room overheats. If your heating feels wrong in a way the settings can't fix, the position is often the reason. A wireless thermostat solves this cheaply, because the sensor can go wherever reads best.
What to set it to
Most people are comfortable somewhere between 18°C and 21°C. Public health advice for cold weather is to heat the rooms you use to at least 18°C, and older people, babies and anyone who is unwell may need it warmer. A sensible way to find your number is to start at 19 or 20°C, live with it for a few days, and move it by a single degree at a time.
Two habits waste the most money:
- Turning it right up to warm up quickly. The boiler doesn't work any harder. It runs for longer, overshoots, and the setting usually gets left there.
- Using the thermostat as an on/off switch. Turning it down to 10°C every time you go out and back up to 25°C when you come in is what the timer is for. Set the timer for when you need heat and leave the thermostat alone.
If you're heading into the first cold spell of the year, our guide on when to turn the heating on has a simple pre-season check of the whole system.
When a thermostat stops working
You can test a room thermostat in a couple of minutes. Make sure the timer is set to allow heating, then turn the thermostat a few degrees above the room temperature. You should hear or see it switch (a click on most models, or a flame or heating icon on a digital one) and the boiler should fire within a minute or two. Turn it well below the room temperature and the boiler should stop. If that works, the thermostat is doing its job and the fault is elsewhere.
| What happens | Likely cause | What to try |
|---|---|---|
| Blank display | Flat batteries, or no power to a wired unit | Change the batteries. If it's wired and still blank, check the heating fused spur is on |
| Display works, boiler never fires | Timer set to off, wireless receiver lost pairing, or a boiler fault | Check the timer, then look at the receiver near the boiler for a fault light. Re-pair it using the maker's instructions |
| Heating won't switch off | Thermostat set too high or in a cold spot, failed switch, or a zone valve stuck open | Turn it right down. If the boiler keeps running for the heating, it needs testing |
| Reading doesn't match the room | Poor position, or a failing sensor | Compare with a separate thermometer before assuming it's broken |
| Boiler fires, radiators stay cold | Not a thermostat fault. Could be the pump, a valve, air or low pressure | See central heating not working |
Two words of caution. Most wired thermostats and wiring centres carry mains voltage, so don't open them up unless you're competent with electrical work. And anything inside the boiler casing is for a Gas Safe registered engineer only. If the boiler is showing a fault code as well, deal with that first, because a boiler in lockout won't respond to any thermostat.
The room thermostat and the temperature dial on the boiler do different jobs. For the boiler side, see what temperature your boiler should be set at.
When to call an engineer
Call us if the thermostat passes the test above but the boiler still won't respond, if the heating won't stop even with the thermostat turned right down, if a replacement thermostat needs wiring in, or if you want your old controls upgraded to a programmable or smart system. We handle heating controls, central heating repairs and no heating call-outs across London. Our labour is £95 an hour, the £60 call-out is waived when the job runs over 2 hours, and we confirm the price before any work starts.
Heating ignoring the thermostat?
Tell us what the display shows and what the boiler does when you turn it up. We'll confirm the price before starting.
📞 Call 020 7870 3200Or see our heating controls service and pricing.
Frequently Asked Questions
Does turning the thermostat up heat the house faster?
No. A standard room thermostat is a switch, so the boiler fires at the same rate whether you set 19°C or 30°C. A higher setting only means the heating keeps running after the room is already warm. Set the temperature you actually want and leave it there.
What temperature should I set my room thermostat to?
Most homes are comfortable somewhere between 18°C and 21°C. Public health advice for cold weather is to heat the rooms you use to at least 18°C, and more if you are older, unwell or have a baby at home. Start at 19 or 20°C and change it by one degree at a time.
Why does my heating stay on when the thermostat is turned down?
Check first that the timer isn't overriding it and that no second thermostat or frost stat is calling for heat. If the boiler still runs with the room thermostat turned right down, the thermostat's switch may have failed closed, a wireless receiver may be stuck on, or on a system boiler a motorised zone valve may be stuck open. Those need an engineer to test.
Why is my thermostat showing the wrong temperature?
Usually because of where it is. A thermostat above a radiator, in direct sun, next to a lamp or TV, or in a draughty hallway reads the air around it, not the room as a whole. Check the reading against a separate room thermometer. If it's still out by more than a degree or two in a sensible spot, the sensor or batteries may be failing.
Is a room thermostat the same as the thermostat on the boiler?
No. The dial or buttons on the boiler set the temperature of the water sent to the radiators. The room thermostat decides whether the heating needs to run at all, based on air temperature. They work together, and changing one doesn't replace the other.
Sources and further reading
Source links checked 27 September 2026. Scheme rules and manufacturer instructions can change; use the current official guidance for your account or exact appliance.