Boiler Keeps Turning Off: 7 Reasons

Start by noticing when it turns off

The same symptom has several unrelated causes, and the fastest way to separate them is timing. Watch it through one heating cycle and answer three questions.

• Does it restart on its own after a few minutes, or stay off until you reset it?
• Is there a fault code or flashing light when it stops?
• Does it happen only with the heating, only with hot water, or both?

Restarts by itself with no code usually means short cycling or controls. Stays off with a code means the boiler has protected itself from a fault, and the code tells you which.

Reasons 1 and 2: it is being told to stop

Reason 1 is short cycling. The boiler fires, reaches its target temperature within a few minutes and switches off, then fires again shortly after. It does this because it cannot shed heat fast enough. The usual causes are most radiator valves closed or TRVs turned low, a boiler flow temperature set very high, or a boiler that is simply too powerful for the home. Open the radiator valves, turn the flow temperature down a little and see whether the cycles lengthen.

Reason 2 is the controls. A thermostat in a warm hallway, next to a radiator or in sunlight thinks the house is warm and switches the boiler off. Wireless thermostats with weak batteries drop their signal and the receiver treats that as no demand. Smart thermostats can drop to an eco or away setting on a schedule you forgot about. Check the thermostat reads what the room really feels like and replace the batteries. Our page on a boiler thermostat not working covers wireless receivers in more detail.

Reasons 3 and 4: water pressure and water flow

Reason 3 is low pressure. The boiler watches the pressure in the heating circuit and stops firing when it falls below its minimum, often around half a bar. It may run for a while, then cut out as the pressure drops. Look at the gauge. If it is under 1 bar cold, top it up and see boiler pressure keeps dropping if it falls again.

Reason 4 is overheating from poor flow. If water is not moving through the boiler fast enough, the heat exchanger gets hot very quickly and the boiler shuts down to protect itself, often with an overheat code. The pump may be failing, sludge may be narrowing pipes and the heat exchanger, or valves may be closed. Signs that point here are radiators hot near the boiler but lukewarm further away, cold patches at the bottom of radiators, and banging or kettling noises. On a Vaillant the overheat cut-out shows as F20.

Reasons 5 and 6: the flame goes out

Reason 5 is a frozen or blocked condensate pipe. In freezing weather the plastic pipe outside freezes, condensate backs up inside the boiler and it cuts out, often with a gurgle first. Thaw the outside section with warm water, not boiling, and reset once.

Reason 6 is flame loss, or on older boilers a pilot flame that will not stay lit. The boiler lights, then loses the flame and either retries or locks out. Causes include a dirty flame sensor, a worn electrode, a fan or flue problem and low gas pressure, which can show up only when the hob and boiler are running together. If you see a lockout code, read our guide to boiler ignition lockout before pressing reset.

Reason 7: the power drops

If the display goes completely dark and every light on the boiler goes out when it stops, it is losing power rather than choosing to stop. Check the fused spur next to the boiler for a loose switch, and the consumer unit for a circuit that has tripped. A trip that happens only when the boiler fires often means a component inside is leaking current to earth, such as a pump or a damp connection. That needs testing with the casing off, not another reset of the trip switch.

A boiler that switches itself off at the same time every day, with the display still lit, is almost never a power fault. Look at the programmer or the smart thermostat app first. Timers that have drifted after a power cut, a holiday mode left on, or two heating schedules set in different apps are all common, and they look exactly like a boiler that keeps turning off. Setting the heating to constant for an evening is a quick way to rule them out.

What to do tonight, and what the engineer does

If the house is still getting some heat, you can keep using the boiler while you arrange a visit, as long as there is no gas smell, no water from the casing and no repeated lockout. If it has locked out twice, leave it off. Smell gas? Open windows, turn the gas off at the meter if it is safe to reach, avoid switches, then get outside and call 0800 111 999.

An engineer on a boiler repair visit reads the stored fault history first, because it shows exactly what made the boiler stop and how often. They check pump speed and flow, the flow and return temperatures, gas pressure while running, the flame signal and the condensate route. Gas work is carried out by Gas Safe registered engineers.

As typical UK market ranges, a flame-sensing or ignition electrode is about £80 to £200 fitted and a gas valve about £200 to £500. Sludge problems may need a system clean rather than a part. The £60 call-out covers getting an engineer to you at any hour, and the repair price is agreed with you on site before work starts. See what an emergency boiler call-out costs, or book an emergency boiler repair if you have no heat at all.