Why does my boiler fire up randomly? Causes and reasons why

boiler fire up randomly

Did you just discover that your boiler is acting on its own? One thing is certain. This behavior is causing you concern and could have serious consequences for your boiler’s longevity.

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Your boiler could be starting up by itself for a variety of reasons. Some of these causes are normal and part of the boiler’s operation but it may be a good idea to consider checking out the latest new boiler costs along the way.

Others, however, can be caused by faults in the boiler’s central heating system or hot water supply and should be addressed immediately by a qualified heating engineer.

Here’s a list of common reasons your boiler might be randomly firing up:

Reasons why a boiler will randomly fire up

Spontaneous hot water demand causes the boiler to fire up

You may have noticed that your combi boiler turns on and off frequently, especially in winter. Combi boilers provide hot water on demand, which is unlike traditional boilers.

The boiler will respond to a demand for hot water by bursting into life when someone turns on the hot water tap.

Turning on your thermostat to warm up your home

If the boiler is instructed by the thermostat to heat water and startup, it will obey immediately. If this is the time your combi boiler fires up then this is normal.

If your boiler fires up randomly and turns on, you can confirm this by setting your thermostat to the lowest setting and not using your hot taps for a few minutes.

Your boiler should not turn on by itself if there is no hot water demand. If it continues to fire up randomly, your boiler could be in trouble.

Preheat cycle on a combi boiler

It is not a problem if your boiler only fires up once or twice a day. The pre-heat cycle is the cyclic firing of your combi boiler and its shutting down. The boiler heats water in the system to the appropriate temperatures during the pre-heat cycle. This ensures that hot water is always available when you turn on your hot tap.

Most of the best combi boilers have this pre-heat setting so it is one to be aware of.

How to check if your combi boiler has a preheat setting?

Check your owner’s manual to confirm if the boiler is performing a preheat cycle. Some boilers like the Ideal Logic have the option to turn it on or off on the front panel.

However, if your boiler is constantly on, for example, six times an hour, and then turns off, this is a sign that the boiler is having problems. You should not attempt to solve the problem yourself. Instead, call a Gas Safe registered engineer certified to inspect your boiler and fix any underlying problems.

Frost protection feature – common on modern boilers

Most modern boilers include a frost protection function that prevents pipes from freezing in the coldest winters. Since heating up is the only way to prevent freezing, your boiler will heat up when temperatures drop and your pipes are at high risk. You could always check for a frozen condensate pipe too.

The boiler will be shut down once the cold spell is over. It will then wait for the next cold snap to ignite, hence the random firing of the boiler.

Short cycling – a boiler protecting itself

A Short cycling boiler is a process of turning on and off a boiler in a cyclic fashion to avoid overheating. This prevents heat from being dissipated too quickly. This happens when a boiler generates heat faster than your hot water or central heating system can.

Short cycling can cause a boiler firing up randomly for the following reasons:

  • A boiler that is too large for the property
  • Internal boiler pump problems
  • Pipes that are too small for the central heating system
  • Poor central heating system design
  • Residues or blockages known as radiator sludge

Check your gauge for high water pressure

If the water pressure in your central heating system gets too high, it can damage system components and cause the boiler to shut down. The boiler will start the pre-heat cycle once the boiler pressure has fallen to the right.

You should check your radiators for high water pressure to reduce the possibility of your boiler starting up randomly.

Als check if you have a low boiler pressure gauge

If you have low boiler pressure this could also be an issue. As a boiler will not fire up if the pressure is low, but if the pressure rises above the right level it will start hot water flowing.

If the boiler doesn’t circulate hot water effectively, then it will shut down again. If this keeps happening you may need to check your hot water pump or you may have a damaged heat exchanger. Both of these will require a Gas Safe engineer.

Location of central heating thermostat

If a room gets too hot quickly…

Your central thermostat acts as an on/off switch for central heating in your home. If the thermostat senses that the interior temperature is below the set value, it will instruct the boiler to turn off and provide hot water for central heating. The internal thermostat will turn off the boiler if the internal temperature exceeds the set value.

If a room is too cold quickly…

If the thermostat is in a drafty area near a front door, fan, or washing machine, it can be affected by drafts from open doors, operating fans, washing machines, and other moving parts. The thermostat will be fooled by a sudden breeze if someone opens the front door. The thermostat will tell the boiler to heat the room and turn on the heating.

This will prevent you from having to deal with this problem. You can install your thermostat in the living area or in the hallway. It won’t be affected if the temperature drops regularly.

Printed circuit board (PCB) fault

Your boiler could be randomly starting up due to an electrical fault within the PCB unit. If this is the cause you may need to start to consider how much a new boiler cost

The PCB, as we all know, is the heart and brain of your boiler and central heating system. The PCB coordinates the operation and coordination of all the components of your boiler in order to make sure they work together. The PCB can fail to recognize the timer settings, which could cause the boiler to turn on even though it is set for the opposite.

Faulty PCBs are a serious problem and should not be taken lightly you should contact one of your local heating engineers right away.

Why does my boiler fire up randomly? do I need a new boiler?

If your boiler is old you may need a new boiler installed. If you think that it may be a cheaper option than continuously calling out a heating engineer and labour costs then you can get free boiler quotes now in under 20 seconds with one of our recommended installers.

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