Boiler Thermostat Not Working? Failure Symptoms and How to Check

What Are the Symptoms of a Faulty Boiler Thermostat?

The usual symptoms are heating that will not come on when you turn the thermostat up, heating that will not switch off, rooms that never reach the set temperature, or a boiler that fires and stops far more often than it should. Any of these can be a thermostat fault, but they can also be a boiler fault, so the useful question is which thermostat, and whether it is really the cause.

• The heating does not come on even with the thermostat turned right up.
• The heating runs constantly and ignores the thermostat being turned down.
• The display is blank, frozen or showing a temperature that is clearly wrong.
• You no longer hear the faint click from a dial thermostat when you turn it past room temperature.
• Radiators or hot water get far hotter than the boiler setting, or the boiler cuts out on overheat.

Which Thermostat Has Failed?

A heating system has more than one thermostat, and people mean different things by 'boiler thermostat'. The room thermostat on the wall tells the boiler when the house needs heat. The temperature dials or buttons on the boiler set how hot the water in the radiators and taps should be. Inside the boiler, small temperature sensors (usually called thermistors or NTC sensors) and an overheat thermostat measure the water and shut the burner down if it gets too hot.

The first two you can look at yourself, and between them they cause most of the problems people describe as a broken boiler thermostat. The sensors inside the boiler you cannot, and when they fail the boiler usually tells you with a fault code rather than simply refusing to heat.

How to Check If Your Room Thermostat Is Working

Start with the room thermostat, because it is the most common culprit and the easiest to rule out. None of these checks involves opening anything wired to the mains.

1. Make sure the programmer or timer is calling for heating now. A thermostat cannot switch the heating on outside the programmed times.
2. Replace the batteries if it takes them. A weak battery is the single most common cause of a thermostat that looks fine but does nothing.
3. Turn it at least 3 degrees above the current room temperature. On a dial thermostat you should hear a click; on most boilers you will hear the burner and pump start within a minute or two.
4. Turn it well below room temperature and check the boiler stops heating.
5. On a wireless thermostat, check the receiver unit near the boiler has power and its light shows it is paired. Losing the pairing after a power cut is common.

If the heating comes on and goes off when you move the thermostat, the thermostat is working. If the temperature it shows does not match the room, that is a different problem, often its position rather than a fault, and is covered in why a thermostat does not match the room temperature.

Thermostat or Boiler? A Simple Way to Tell

If turning the thermostat up does nothing, check the boiler itself. If it shows a fault code, is flashing a warning light, or the pressure gauge is below its normal range, the boiler is refusing to fire and the thermostat is probably innocent. If the boiler shows no fault and still gives hot water normally, but never responds to the thermostat, the fault is more likely in the thermostat, its receiver, its wiring or the programmer.

Smart thermostats add another layer, because a Wi-Fi drop, an app schedule or a firmware update can look exactly like a broken boiler. Our guide to smart thermostats and boiler faults goes through those checks, and if the timer rather than the thermostat looks like the problem, see why a central heating programmer stops working.

Signs the Fault Is Inside the Boiler

When a temperature sensor inside the boiler fails, the symptoms tend to be about temperature being wrong rather than heat not being asked for. Hot water on a combi that swings between hot and cold, radiators much hotter than the boiler setting, the boiler cutting out on overheat, or a boiler that fires and stops every few minutes can all come from a sensor reading the wrong temperature.

Most modern boilers show a fault code when a sensor reads out of range, and the code narrows it down quickly. Replacing an internal sensor is a routine repair, but it means working inside the boiler casing, so it must be done by a Gas Safe registered engineer. Do not try to reset an overheat thermostat inside the boiler yourself: it has tripped for a reason, and the reason needs finding.

Repair or Replace the Thermostat?

A room thermostat that has failed outright is normally replaced rather than repaired. They are sealed units with no user-serviceable parts, and a like-for-like swap is a short job. Many older wired thermostats run on mains voltage, so changing one is electrical work, not a battery swap, and should be done by someone competent to do it safely.

A replacement is also the wrong answer if the thermostat was never the problem. If you have already changed the batteries and checked the programmer and the heating still does not respond, it is worth having the whole control circuit and the boiler checked before buying a new thermostat. If you have no heating at all, our no heating and no hot water repair service covers that diagnosis in London.