Heating not working after a smart thermostat was fitted? Here's what to check
Don't open the receiver, wiring centre or boiler. Smart thermostat receivers are wired to the mains. The checks below only use the app, the buttons and the lights. If the wiring needs changing, that's a job for the installer or a qualified electrician or heating engineer.
You've had a new smart thermostat fitted, such as Hive, Google Nest or tado, and now the heating won't come on. Or perhaps the hot water works but the radiators stay cold. The boiler is almost certainly fine. The problem is usually one of two things: a setting in the app, or something about how the new thermostat was wired in. You can check the first yourself. The second needs the installer.
- Check the app: mode, schedule and temperature.
- Check the receiver's lights.
- Check the boiler itself: power, pressure and its own dial.
- Test: does hot water work but heating doesn't?
- Get the installer back, and tell them what you found.
Each step has important details below. Tap a step to jump to it.
Why it happens
A smart thermostat usually has two parts: the thermostat you see on the wall or in the app, and a receiver (Hive calls it a receiver, Nest a Heat Link, tado a wireless receiver or extension kit). The receiver is wired to your boiler, and it switches the heating on and off.
When the heating doesn't work after installation, the common causes are:
- A setting: the thermostat is in "off", "frost" or "away" mode, the schedule is set low, or a hot-water-only mode is on.
- Pairing: the thermostat and receiver aren't talking to each other.
- Old wiring not finished: the old thermostat or timer was removed, but the wires that connected it were not joined or linked properly, so the boiler never gets the signal.
- The wrong model: for example, a heating-only version fitted where the home needs one that controls hot water as well, or the other way round.
- A setting on the boiler, like its heating dial turned right down.
Step by step
1Check the app
- Make sure the heating is in schedule, manual or "on" mode, not "off", "away" or "frost protection".
- Set the temperature a few degrees above the current room temperature. If you're asking for 18°C and the room is already 19°C, the heating won't come on.
- Try a boost for 30 minutes.
- Look for a warning or a "receiver offline" message. Check the thermostat's batteries if it uses them.
2Check the receiver's lights
The receiver is usually a small box on the wall near the boiler, or where your old timer was. Its lights tell you a lot. Look in your thermostat's manual or help pages for what each light means for your model. In general:
- No lights at all: the receiver has no power. Check the switch or fuse for the boiler and controls.
- Flashing lights often mean it's lost connection with the thermostat or hub. Your manual will explain how to pair them again.
- A light showing heating is "on" while the boiler stays off is an important clue. It means the receiver is asking for heat, but the signal isn't reaching the boiler. Write this down for step 5.
3Check the boiler itself
- Is the boiler's own display or light on? Is it showing a fault code?
- If it has a pressure gauge, is it about 1 to 1.5 bar when cold? If it's low, see our guide on topping up the pressure.
- Many combi boilers have their own heating temperature dial. If it's been turned to minimum or off, the boiler won't heat the radiators, whatever the thermostat says.
4Test: does hot water work but heating doesn't?
If you have a hot water tank, turn the hot water on in the app with the heating off, and see if the boiler fires. Then do the opposite.
- Hot water works, heating doesn't: the boiler and the power are fine. The problem is on the heating side: the wiring, the heating output on the receiver, or a valve. This is a very common sign of a wiring issue after installation.
- Neither works: more likely power, pairing, or a boiler fault.
On a combi boiler with no tank, run a hot tap. If you get hot water, the boiler is working, and the issue is just the heating signal.
5Get the installer back
If the app settings are right and it's still not working, ask whoever installed it to come back. If it was installed by the energy company or thermostat brand, contact their support. A properly installed thermostat should be tested before the installer leaves. Tell them:
- "The heating hasn't worked since the thermostat was fitted on [date]."
- What the receiver lights show when the heating is on.
- The result of step 4: whether the hot water works and the heating doesn't.
- Your boiler's make and model, and whether you have a hot water tank.
Ask them specifically to check the wiring where the old thermostat or timer was removed, and that the right model of thermostat was fitted for your system. These are the two most common installation mistakes.
Questions people ask
Can I just put my old thermostat back?
Only if you're qualified to do the wiring, as it involves mains electricity. If the installer refitted your old controls and the heating worked again straight away, that confirms the problem was with the new installation, not the boiler.
The heating comes on, but never switches off. Is that the same problem?
It's usually a wiring or setting issue too. Check the app isn't on a constant "on" or manual mode, then ask the installer to check the wiring.
Does a smart thermostat work with any boiler?
Most work with most UK boilers, but the right model and setup depends on your boiler and whether you have a hot water tank. Each brand has a compatibility checker on its website. Check yours before installing, or ask your installer to.