Why do boilers switch themselves off?
A modern boiler monitors its own pressure, temperatures, flame and flue. When any of these goes outside safe limits it stops and, in most cases, shows a fault or lockout code. So a boiler that keeps switching off is rarely random: it is responding to a specific condition, and the code points to it.
What are the common causes?
- Low water pressure. Below roughly 0.5 to 0.8 bar many boilers stop. See boiler losing pressure.
- Frozen or blocked condensate pipe. In freezing weather this is the top cause. The boiler may gurgle before it locks out. See frozen condensate pipe.
- Overheating. If water cannot circulate fast enough, the boiler’s overheat protection trips. A sticking pump, sludge, trapped air or closed radiator valves are typical causes. On a Worcester Bosch this can show as E9.
- Ignition or flame faults. The boiler tries to light, cannot prove a flame, and locks out. Gas supply problems, a worn electrode or a blocked condensate trap can all cause it.
- Pump or pressure sensor faults. The boiler checks that the pump is moving water. If it sees no change, as with a Vaillant F75, it stops.
- Thermostat in the wrong place. A room thermostat next to a radiator, oven or sunny window can switch the heating off early. That is the controls, not the boiler.
What can you safely check yourself?
- Write down the fault code exactly as shown.
- Check the pressure gauge and compare with your user manual.
- In freezing weather, check the condensate pipe where it runs outside. Thaw it with warm, never boiling, water.
- Make sure radiator valves are open and bleed any radiators that are cold at the top.
- Check the gas supply. Does the hob light? Is a prepayment meter in credit?
- Reset once using the reset button. If the same code returns, stop.
When do you need an engineer?
If the pressure is right, the condensate pipe is clear and a single reset does not keep the boiler running, the fault is inside the boiler or system. Overheat and ignition lockouts in particular should be diagnosed rather than reset repeatedly. Send an enquiry with the make, model and code, and the engineer will find the cause and price the repair before starting.