Boiler Thermostat Not Working: Is It the Stat or the Boiler?

The two-minute test: stat or boiler?

Most people who search for this have a boiler that will not come on for heating and a thermostat they suspect. The quickest way to find out is to take the controls out of the question one at a time.

• Set the programmer or timer heating channel to on, constant or a one-hour boost
• Turn the room thermostat up to its maximum, well above the room temperature
• Wait a couple of minutes and listen for the boiler fan and burner, or watch for a flame symbol on the display
• If the boiler fires: the boiler is fine, the controls were not asking for heat
• If nothing happens: open a hot tap on a combi. If it fires for hot water, the boiler works but is not receiving the heating signal
• If it will not fire for hot water either: the boiler itself has a fault, and the thermostat is a red herring

Do not bridge or disconnect thermostat wires to test them. Many wired room thermostats and programmers in the UK switch mains voltage (230 volts), and the terminals can be live even when the heating is off.

Wireless and smart thermostat problems

Wireless thermostats have two parts: the dial or screen you carry around, and a receiver, usually a small box wired in next to the boiler. The receiver is the part that actually switches the boiler. When the two lose contact, many receivers switch the heating off or run a reduced failsafe cycle, so the house goes cold even though the thermostat on the wall looks perfectly happy.

Start with the batteries in the thermostat itself. A low battery symbol is easy to miss, and some models simply go blank. Then look at the receiver: most have a light that shows heating on and a manual override button. Pressing the override to force heating on is a useful test. If the boiler fires on the override but not from the thermostat, the problem is the wireless link, and re-pairing the two, following the maker instructions, is usually the fix.

Smart thermostats such as Hive, Nest, Drayton Wiser and Honeywell evohome connect to your broadband, some through a hub plugged into the router. After a power cut or a router change, the hub can lose its connection, and the app shows the heating as on while nothing happens at the boiler. Check the hub lights, then the receiver near the boiler. Most keep working on their schedule without the internet, but a receiver that has lost pairing will not.

Wired dial thermostats and old timers

A traditional wired room thermostat with a rotary dial has no batteries. Turn it slowly up past the room temperature and you should hear a faint click as it switches. No click at all, or a click with no response from the boiler, points to a failed thermostat or a loose wire. Older mechanical stats also drift with age and can end up several degrees out, so the heating switches off while the room still feels cold.

Position matters as much as the unit. A thermostat in a hallway next to a radiator, above a lamp or in direct sun turns the heating off early. One on an outside wall in a draught keeps it on too long. If the heating works but the house never feels right, move the thermostat before replacing it.

Mechanical timers with push-in segments round a dial are simple but fail in predictable ways. The clock motor stops, so the dial no longer turns and the heating comes on at the wrong time or not at all. Check the dial against the real time. Some digital programmers lose their clock after a power cut, and some return to off or miss the change to British Summer Time. Our guide to a boiler that keeps turning off covers timer settings that shut the heating off early.

When the thermostat is fine and the boiler is the fault

If the boiler will not fire on constant heating with the thermostat turned right up, and will not fire for hot water either, stop testing the controls. Check the display for a fault code and the pressure gauge for a reading of about 1 to 1.5 bar with the boiler cold. A boiler locked out with a code, or down on pressure, will ignore every thermostat in the house.

A boiler that fires for hot water but not for heating, even with the controls forcing it on, points to a fault in the boiler heating side, the wiring centre or a zone valve on a system with a cylinder. Our boiler not heating radiators guide covers those. For a boiler that will not light at all, start with the boiler not firing up.

If you need the heating back tonight and the test points at the boiler, our emergency boiler repair visit covers a boiler that ignores its controls at any hour.

What the engineer checks

An engineer measures the switched live coming back from the thermostat and programmer at the boiler or wiring centre. That one reading splits the job cleanly. No signal when the thermostat is calling means the control, receiver or wiring is at fault. A signal with no response means the boiler is at fault.

On a wireless setup, they check the receiver relay switches when commanded and re-pair or replace it. On a system with a cylinder and zone valves, they check that the heating valve opens and closes its end switch, because many systems only fire the boiler once the valve reports that it is open. Where a thermostat is badly placed, they can advise on moving it. Gas work is carried out by Gas Safe registered engineers.

What a thermostat or timer fault usually costs

The £60 call-out covers getting an engineer to you, and the repair price is agreed with you on site before any work starts. The rate card shows what a boiler call-out costs. These are typical UK market ranges for the parts behind a thermostat that is not working, fitted.

• Batteries or re-pairing a wireless thermostat: often sorted in the call-out time
• Basic wired dial thermostat replaced: typically about £80 to £180
• Programmable wireless thermostat and receiver: typically about £150 to £300
• Smart thermostat supplied and fitted: typically about £200 to £350 depending on the brand
• Digital programmer or timer replaced: typically about £100 to £250