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Why Does My Boiler Fire Up Randomly?


Have you recently discovered that your boiler has developed a mind of its own and is firing up randomly? Well, one thing is for sure. You are worried about the cause of this behaviour and wonder about its implications on your boiler’s lifespan.

There are several reasons why your boiler may be firing up on its own. Some of these reasons are harmless and are part of the boiler’s operation mechanism. Other’s however, are due to faults in your hot water or central heating system and require the immediate intervention of a certified heating engineer.

Here is a breakdown of the most common reasons why your boiler may be firing up randomly:

Spontaneous hot water demand

If you have a combi boiler, you must have noticed that it regularly turns itself on and off, especially during winter. This is simply because, unlike conventional boilers, combi boilers supply hot water on demand. As such, if someone turns on a hot water tap, the boiler will spring to life to meet the demand for hot water.

Also, if the central heating thermostat instructs the boiler to start up and supply hot water for central heating, the boiler will comply immediately.

To confirm whether this is the reason behind your boiler firing up randomly, turn your thermostat to the lowest reading and don’t use your hot taps for a while. With no demand for hot water, your boiler shouldn’t turn on by itself. And if it does and continues firing up randomly, then there could be a problem with your boiler.

Pre-heat cycle

If your boiler fires up for a few minutes every one or two hours, there is nothing to worry about as this isn’t necessarily a problem. This cyclic firing up and shutting down of your combi boiler is known as the pre-heat cycle. During the pre-heat process, the boiler heats the water in the system to the required temperatures so that there is always hot water available when you turn on the hot water tap.

To confirm whether the random firing up of your boiler is due to it performing a pre-heat cycle, check your owner’s manual as it mentions if your boiler has a pre-heat cycle.

If, however, your boiler keeps turning on more regularly, say six times in an hour and then turning off, it is a clear indication that there is a fault within the unit. In no circumstance should you attempt to fix the problem on your own. Instead, contact a certified heating engineer right away who will examine your boiler and resolve any underlying issues.

Frost protection feature

Almost all modern boilers have a frost protection feature that protects your pipes from freezing during the coldest of winters. And since the only way to prevent freezing is by warming up, the boiler fires up whenever temperatures are low, and your pipes are at risk of freezing.

Once the cold spell passes, the boiler will shut down and await the next blast of cold weather to self-ignite, hence the random firing up.

Short cycling

Short cycling refers to the cyclic switching on and off of a boiler to prevent overheating when the heat generated cannot be dissipated quickly enough. This is a common issue where you’ve installed a powerful boiler that generates heat at a faster rate than it can be dissipated by your hot water or central heating system.

Other causes of short cycling include:

  • Oversized boiler
  • Pump issues
  • Undersized pipes
  • Poor system design
  • Radiator sludge

High water pressure

When the water pressure in your central heating system is too high, it can cause damage to system components, and the boiler will shut down for safety reasons. Once the coast is clear and the water pressure has dropped, the boiler will fire up again and initiate the pre-heat cycle.

If you suspect that high water pressure is the culprit behind your boiler firing up at random, you should bleed your radiators to lower the water pressure and resolve this issue.

Poor location of the central heating thermostat

The central thermostat in your home acts as the on and off switch for your home’s central heating. When the thermostat detects interior temperatures below your settings, it instructs the boiler to turn on and supply hot water for central heating. Once the internal temperature rises past the set value, the thermostat instructs the boiler to turn off.

When this thermostat is located in a drafty location near the front door, a fan, or a washing machine, it will be affected by the draft coming from an opened door or operating fan or washing machine. For instance, every time someone opens the door, the sudden breeze will fool the thermostat into thinking the interior temperatures have dropped below the set temperature. The thermostat will then instruct the boiler to fire up and warm the room.

To prevent this issue, consider installing your thermostat in the living room or deep in the hallway, which won’t be affected by regular temperature changes.

PCB fault

Lastly, your boiler may be firing up randomly due to an electrical fault in the PCB unit.

We all know that the PCB is the heart and brain of your boiler and central heating system in extension. It coordinates the operation of the different components of the boiler to ensure they are working in sync. If the PCB is faulty, it might fail to recognise the timer settings and result in the boiler being on despite the timer settings saying otherwise.

A faulty PCB is a serious issue and should be left to professional heating engineers.

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