Why Do Boilers Have a Fan?

If you've ever taken the time to look carefully at your boiler system, you may have noticed that there is a fan inside. While you may assume that the fan in your boiler is there to help cool the system, this is not the case. The role of a fan in a boiler system is to blow any toxic fumes up and out of the flue.

The functionality of the fan is registered by the boiler through the use of an air pressure switch and is a vital component of the boiler system.

What Is a Boiler and How Does It Work?

A boiler is a type of central heating system that supplies heat to all areas of your home. It has replaced the indoor fires of old and takes much less maintenance and work. It also has the added benefit of providing hot water to the home.

It is often found in the kitchen and is a white metal box with a small digital display panel on the front. Some homes have the boiler located in the garage or the bathroom - they can be located anywhere, as long as their ability to perform their task sufficiently is not impeded by their location.

Depending on the type of boiler you have, you may also have a tank in your attic for cold water, as well as a hot water cylinder. Modern boilers are much more compact.

Inside the boiler, you'll find a heat exchanger and fuel jets. When the boiler ignites, you should see a blue flame inside. If the flame isn't blue, your boiler likely needs more oxygen. This is dangerous, as a cool flame will release carbon monoxide. It is therefore important to install a carbon monoxide detector near to your boiler in case of a fault.

The Different Types of Boilers

There are 3 different types of boilers typically available on the market: combi, regular and system boilers. They can be fuelled by either gas, oil or LPG. The main difference is that gas is supplied from the national gas network, whereas the oil or LPG comes in separate tanks that need to be installed on the property.

Combi Boiler

Combi boilers offer both a heating system and hot water from one wall-mounted unit through an instant heating system. They supply hot water to both systems by having two heat exchangers and two hot water pipes. They are supplied directly from the mains, so don't require a tank in the attic like other older boilers.

Regular Boiler

Regular boilers have a separate tank for hot water storage and another for cold water storage. They take up more space and are not always compatible with newer heating systems. They can't provide water for the household directly but can supply it from their hot water tank. These boilers are popular in homes that have multiple bathrooms.

System Boiler

System boilers are an upgraded version of the regular boiler. It still requires a separate hot water tank to supply your home with hot water but is supplied directly from the mains, eliminating the need for a cold water tank in the attic.

You may also like: Combi Boiler vs Regular Boiler vs System Boiler, What's the difference?

What Does the Fan on My Boiler Do?

What does a fan on a boiler do

If you've ever taken a close look at your boiler or been nearby when it started up, you may have noticed the whirring of the fan and asked yourself, "why do boilers have a fan?" Keep reading to find out exactly a fan is one of the most important components of a boiler system.

A boiler works by burning various types of fuel in the combustion chamber. This process causes the release of harmful gasses, which need to be vented somehow. You'll notice that your boiler has a flue - this is where the gasses exit the system.

The flue is a big white pipe that leads out the boiler. It provides an exit for the gasses but cannot push them out itself. The fan on your boiler assists this process by blowing the gasses along the flue to ensure that they are released safely.

The fan also helps to prevent the flue gases from condensing on the intake pipe of the heat exchanger, which can lead to serious corrosion, among other issues.

Boiler Fan Safety Mechanisms

The boiler fan is a critical safety feature of the boiler system and will start up before any other components of the system. A functional fan is vital for your boiler to work.

In order for your boiler to work, the boiler fan needs to be registered as functional by the printed circuit board. The PCB registers the fan by the use of an air pressure switch. If it doesn't register that your fan is operational, it won't start the boiler. The unit will display a fault code on the display panel.

The entire boiler system will lock if any fault is detected to prevent any chance of malfunction or fire and to prevent further damage to the machine. If you're locked out of your boiler system, you'll need to contact a Gas Safe engineer to come and assist in fixing the issue.

Is a Faulty Boiler Fan Dangerous?

If your boiler fan is faulty, and the fault is not picked up by the printed circuit board, then it definitely can be dangerous. There is the potential for a house fire or carbon monoxide poisoning. Make sure to install a carbon monoxide detector near the unit to prevent the latter.

However, the fan is started before the rest of the machine, and if the start is unsuccessful, the boiler will shut off, lock, and display an error code. This makes it unlikely that there will be any major dangers associated with boiler fan faults.

Common Causes of Boiler Fan Faults & How to Fix Them

Common causes of boiler fan faults and how to fix them

Low fan speed

The cause: A low fan speed can give you issues, causing the pilot light to go out shortly after ignition or can cause a full lockout.

The fix: A low-speed fan can be fixed by having its speed increased. You'll need to have this done by a Gas Safe Engineer.

Condensate pipe blockages

The cause: Another cause can be condensate pipe blockages, which can happen in winter, as they may freeze. These pipes vent harmful vapours from the system, and when they're blocked, there will be fluctuations in air pressure, which can result in the boiler registering various fan faults.

The fix: Because the condensate pipe will likely be blocked due to frozen vapours, it will need to be thawed out. Once it has thawed, it is recommended that you reinsulate it. If you live in a particularly cold area, it is recommended to chat with a professional for advice on the best type of lagging to use.

Air pressure switch malfunctions

The cause: If the air pressure switch malfunctions, then the boiler won't start, as it won't be able to register that the fan is working.

The fix: Provided that there are no blockages in the system and the switch is the only issue, you can just replace the switch to fix the problem.

Loose connections

The cause: The fan produces small vibrations, and this can cause wires and connections to become loose over time. If there is a loose connection between the PCB and the fan, you may get locked out, as the boiler won't register that the fan is working.

The fix: Get a Gas Safe engineer in to check for any loose wiring and connections, and ask them to give the whole machine a service while the casing is off. They'll be able to secure the problematic connections and fix any other small issues that he notices.


The cause: Unfortunately, it is common for boilers to develop various leaks. These leaks will cause damage over time and could damage the fan, causing the boiler to stop working.

The fix: If corrosion from a leak is the cause of the fault, then the engineer will need to find said leaks, repair them, and replace any damaged parts, such as the heat exchanger or pump seal. The fan may also need to be replaced.

Circuit board faults

The cause: While unlikely, if you've checked all the potential causes, the PCB could be at fault and may be sending the wrong messages to the various parts of the boiler, including the fan.

The fix: First, ask your Gas Safe Engineer to check the PCB with a multi-metre, as it could just be that there are some loose connections. If the connections are all secure, then you'll need them to replace the unit.

Unfortunately, this can be costly - make sure to get a quote first, as it might be more cost-effective to replace the boiler.

All of the issues mentioned above can be identified and fixed by a Gas Safe engineer.


How long should a boiler fan last?

Boiler fans should last for between 5 and 8 years. If your fan is over 5 years old and your boiler is giving issues, it could be that you need to replace the fan. If it's younger than 5 years, then it could just need a clean, or the fault could be elsewhere in the system.

Be sure to have a Gas Safe engineer come in to check your system, and see what the fault is. You made need a replacement pump seal, or there may just be a few loose connections.

Can you repair a boiler fan?

Depending on the fan's operation fault, a boiler fan can be repaired, and there are repair kits available. However, it is often best to opt for a replacement fan rather than trying to fix a broken one. Remember to be responsible and bring in a Gas Safe engineer to assist with the necessary repairs and replacements.

What happens if the boiler fan stops working?

When a boiler fan stops working, the entire system will shut off, lock you out, and there will be an error code displayed on the digital display. Before the fan breaks entirely, you will likely hear a low humming sound, which can indicate that the bearings of the fan need to be replaced.

It is best to get a qualified Gas Safe engineer in to check your boiler fan's operation and do any necessary repairs or replacements.