Why does my boiler fire up randomly?
If you’re concerned that your boiler seems to have developed a mind of its own, particularly if it’s firing itself up all the time, it could mean there’s a problem – or it might be completely normal. Let’s have a look at what it might be.
What causes a boiler to fire up on its own?
If you have a combi boiler, it will regularly turn itself on and off, especially in the winter.
That’s because it provides hot water on demand, rather than overnight as with conventional boilers.
So if the central heating thermostat tells the boiler to start up, it will. And if someone turns on a hot tap, the boiler will start too.
To test it, turn your thermostat down and don’t use any hot taps for a while. If you set your main thermostat as low as it will go, the boiler should never need to heat up the radiators, so it won’t turn on.
Is it still firing up? There could be a problem.
Potential causes and solutions
Check your boiler’s documentation to see if it has a “pre-heat” cycle. This warms the water in the system so you’ll always have a fair amount of hot water when you turn on the hot tap.
It should do it once every hour or two, and each time will last a couple of minutes. Again, this is perfectly normal, so you don’t need to do anything.
If it’s turning on regularly then turning off, and doing this repeatedly, that’s more likely to signal a fault. It could be a component, such as the heat exchanger or an internal thermostat, in which case you should not attempt to fix it – call a gas engineer who will be able to assess the situation and recommend a repair or replacement. Making sure your boiler has its annual service should prevent this stop-start pattern.
Similarly, if your water pressure is too high, the boiler might be shutting down for safety reasons. Once everything cools and the boiler re-starts, it will fire up for a pre-heat cycle. Check your pressure on the gauge, and if it’s far above 1 bar, bleed a radiator until it reduces to around that level.
Your main thermostat in your home is basically a switch that turns the heating on when it detects that the temperature is below your setting, and off when the room warms up past that temperature.
If your thermostat is in a drafty place, or somewhere with regular temperature changes like a front door or near to a washing machine, it won’t be getting realistic measurements of the air temperature, and will switch the boiler on and off all day.
If you have a portable thermostat, make sure it’s in a living room or deep inside the hallway. If it’s fixed to the wall, you might have to look at having it upgraded or moved.
Finally, modern boilers have electronic circuits on them, and although they’re pretty basic, they do sometimes fail, and might need replacing or having new software installed. There’s nothing regular homeowners can do in this case – call us out.
Still no luck? Speak to a team of professionals
We can’t emphasise enough how important it is not to attempt to fix your boiler yourself. Gas, water and electricity can be a dangerous combination, so it’s vital that you get professional help if you’re concerned about how your boiler is performing.
Firing up is normal behaviour, but if it’s over short durations and without any specific input from you, you should get in touch so we can have a look.
Published: 10 December 2020
