Why does my boiler fire up and then turn off?

Why does my boiler fire up and then turn off?

Posted on April 13, 2021

This isn’t necessarily a problem – If you have a combi boiler and it fires up then turns off then it could just be ‘preheating’ so that there is always some hot water ready for when you turn on the hot tap.

Where it becomes a problem is if it keeps on firing up and then turns off again quickly. If this is happening, then your boiler may be short cycling. If this is the case, you may notice your heating bills becoming more expensive as well as it leading to the boiler getting damaged over time.


What is boiler short cycling and what causes it?

Your home will likely have thermostats set to a given temperature to keep your home at the warmth you require. When your thermostats read the specified temperature, your heating system doesn’t need to be on. This means that the water in the heat exchanger will naturally start to cool down.

The boiler will also have an internal thermostat to keep a track of the temperature inside the unit itself. When a boiler is short cycling, it is detecting the temperature of the water in the heat exchanger, and because it has cooled while the heating system has been off, it powers up to heat that water back to the ‘correct temperature’.

Due to the amount of water needing to be heated being small, this doesn’t take very long and so it quickly turns back off again after firing up.

Imagine this happening every few minutes! Every time it fires up, it will use gas but not to heat your home. Thus this is a waste of fuel leading to your heating bills going up (as well as carbon emissions) and potentially leading to a damaged heat exchanger causing repair bills or a replacement.

To replace a heat exchanger you would likely be looking at a repair bill of between £300 and £500 (boiler model dependent). Plus the labour costs of the engineer on top!


So what causes this boiler short cycling to happen?

An Oversized Boiler –

The most common cause of a short cycling boiler is having an oversized unit for the home you are trying to heat. If your boiler is too powerful for the demand your home puts on it, then you’ll likely notice it firing up and turning off frequently.

The reason an equality between power and demand can cause boiler short cycling is because the boiler will produce more steam than it can condense. Due to it cycling on and off too often, it will then eventually overheat.

Modern boilers are extremely efficient and so you need only get the size that suits your home.


What size boiler would be suitable for my home?

This is a really important issue and there isn’t really a simple answer that we can write in a single paragraph. It’s dependent on the size of your home, how many radiators are a part of your heating system and the pipes, among other things.

People often think that they should just get the biggest sized boiler they can afford, but this shouldn’t be the case. If it is too big it will short cycle like we’ve explained, making it less efficient and more expensive to run. Clients often think they need a bigger boiler than they actually do.

Keep your eyes peeled for our blog next week which will explore the issue of boiler size in more detail.

If you’re not sure what size boiler you should have and think you may have a problem with short cycling, then just give us a call. One of our local heating engineers will be pleased to look into it for you.


What if my boiler is the right size for my home but it’s still short cycling?

Occasionally there are other reasons that could cause a short cycling boiler.

Boiler pressure –

For your boiler to be able to circulate hot water effectively and efficiently around your home, it is essential that your boiler pressure is correct. If it’s too low then your boiler won’t be able to heat your home properly, and if it’s too high it can cause lots of boiler problems.

Your boiler pressure should be around 1.5 bar, but make sure you always check your boiler manual and follow the instructions. It’s easy to check what your boiler pressure it – Simply look at the pressure gauge on the boiler.

If the boiler pressure is too low, then it will turn off. The most common cause of low pressure in a boiler is a leak somewhere, so check all your pipes, radiators and the boiler itself for any signs of a leak or water. If you find evidence of a leak, then turn off the water and call a Gas Safe registered heating engineer.

If the boiler pressure is too high, your radiators will likely not feel as warm as they should be or may have cold spots. This is usually due to there being too much air in the system which causes a blockage, preventing hot water from circulating around to your radiators. To get rid of any excess air, bleed your radiators to release the trapped air and get your hot water flowing around effectively again.

A poorly functioning thermostat –

If you have a functioning thermostat, it will measure the temperature in a room and then prompt the heating system to come on when needed when it drops below a specified temperature. If a thermostat isn’t reading the temperature accurately, then it could cause the boiler to cycle on and off more than is needed or more than usual.

It could also have something to do with the location of the thermostat. For example, if it’s located by your front door and the temperature drops every time you open it, you’ll find your heating firing up when your hallway is cold, but your other rooms are still warm.

There are Smart Thermostats that can learn the specific heating habits of your home. Have a read of our previous blog to see if you and your home would be eligible for a free Nest Thermostat under a scheme currently being run by our local council.

A fuel supply issue –

As with all appliances that run on fuel – If the supply is restricted or stopped then the appliance will stop working. In order to keep your home cosy and warm, your boiler will need a reliable and consistent supply of gas, oil or electricity. If it doesn’t get it, it will just cut out.

If your boiler runs on gas, then there are a few simple ways you can check if there’s a problem with the gas supply:

  • Check to see if any other gas appliances are working
  • If you use a prepayment meter then make sure it is in credit
  • Ask a neighbour if they are also experiencing difficulties with their gas appliances (this also applies with electricity)

If none of these are the issue, then you will need to call a Gas Safe registered engineer to come and take a look.

A poorly functioning water pump –

Your heating system relies on the hot water being effectively pumped around your home. This means the water pump needs to be in excellent working condition. If water isn’t being pumped away from the boiler successfully, then it will start to overheat and thus cause short cycling.  You will need to call a Gas Safe registered engineer to determine if this is the problem. To replace the water pump, you could be looking at a bill of approximately £300.

Frozen condensate pipe –

Most modern boilers will have a condensate pipe to lead waste-water down a drain away from the boiler. During the Winter, if this pipe freezes and causes a blockage, the water will not be able to reach the drain and the boiler will stop working. A frozen condensate pipe is usually straight forward for you to resolve yourself by thawing it out with warm water. It is important to not use very hot or boiling water, otherwise you may crack the pipe.


A short cycling boiler is not something to be ignored

Whatever the reason for it happening, it needs to be looked at to make sure it stays safe and continues to heat your home. Continually ignoring a short cycling boiler could lead to:

  • Your heating bills increasing
  • Your boiler suffering with more frequent faults, ending with costly repairs
  • Needing to replace the boiler completely, sooner than necessary

If your boiler is short cycling then give us a call today and let’s find out the cause of the problem so we can get it fixed, hopefully before a more serious repair is needed!