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The Problem with Extractor Fans

26 Apr 2019 at 2:54 PM

How much attention do you pay when you fit an extract fan?

Problem with extractor fans

It’s fair to say that it’s not top of every contractor’s agenda.

However we are seeing an awful lot of intermittent fans failing when tested under Part F requirements. Building regs require that all intermittent fans are tested on completion of a build to ensure they perform as they should.

The overwhelming evidence from our experience is that the vast majority do not.

The problem with axial fans

Testing in this sense is only concerned with intermittent fans – those that come on and off with lights or other controls. So we are not talking about any form of active centralised or heat recovery system.

Most intermittent fans fitted by developers or contractors are axial fans – very often the type bought relatively cheaply from a builders merchant.

Bathroom fans for example must achieve an extract rate of at least 15 litres/second under testing. A fan may be rated at, say, 21 litres/second, but this manufacturers performance is based on a fan extracting air directly through a 300mm wall. So fan-wall-exhaust.

Problem with extractor fans

If any sort of longer duct run is installed, typically anything over 1.5 metres then performance will be SIGNIFICANTLY impaired. We would go as far as to say that unless you are ducting directly through a wall, then most co-axial fans are simply not powerful enough.

Other Issues

Before talking about fans, it is important to note that there may be other installation issues affecting fan performance. Any kinks or bends in ducting will severely affect airflow. We would always recommend rigid ducting for this reason especially for long runs.

In tight situations onsite it is oh so tempting to make a piece of flexible duct fit where it doesn’t! Any squashing or squeezing is bad news.

The length of the run itself is also crucial, and even with rigid ductwork will cause issues for axial fans.

Extract Fan Ducting Issues

Credit: Addvent

What if it fails?

Well lots do fail, and it hurts. In our experience around 70-80% of fans tested are not performing as they should. The option for the client is either to fix any ducting/installation issues, replace the fans, or both. Then you’ll need to pay for a retest.

Co-axial wont work for any sort of duct run, so there are two main options:

Centrifugal Fan

Centrifugal Fans

These fans work with a cylinder effect and ‘spin’ the air outwards at right angles to the fan intake. Centrifugal fans can create more pressure for a given air volume so are far more effective over long and even very long duct runs.

The down side is that they are often bulkier, noisier and more expensive.


Inline Fan


Inline fans

These are mounted inline – that is inside of, or replacing part of the ducting. The motor and moving parts are not in this case next to the intake grill, and can be some distance away mounted in a roof void or attic space.

The benefit of an inline is that because there is that separation from the room, they can be very quiet or near silent depending how far away the motor is. They also perform very well.

They do however need extra space above the ceiling line so may not be suitable for some builds.

We’ve found that clients replacing axial fans with either of the above, especially centrifugal types for longer runs, find a significant improvement in performance under retests.

Why should you care?

As a developer or contractor, there are two reasons why you should care.

One is that the testing regime has noticeably been stepped up, and whilst certainly not at 100%, we are finding that BCO’s are far more proactive with Part F regs now, especially when they suspect bad practice onsite.

The second and more important point is that these fans are absolutely crucial in naturally ventilated buildings. Our homes in particular are increasingly well insulated and air tight – often a simple intermittent fan is the last defence against moisture build up. Indoor air quality is especially poor in urban areas where windows may not be opened readily.

The associated damp and mould issues down the line are dangerous for occupants health, not to mention the complications we are saving up from a defects and warranty perspective.

Need help with Part F Testing?

Call us for a chat on 0330 055 34 05 or email







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