Bathroom extractor fan not working as well as expected. Any advice?

Whiffle

Whiffle

Whiffle

Associate
Joined
27 Jul 2009
Posts
386
Hi there.

Any general pointers on getting my extractor fan working properly would be much appreciated.

I have a small bathroom (8.5 cubic metres) which, until recently, had a wooden slat on the wall with an airbrick on the other side and nothing else - as you would expect, this was pretty rubbish and the bathroom suffered with humidity problems and ultimately black mould.

I found the most powerful axial 4 inch fan I could find (97 cubic metres per hour, with a humdistat), which should easily have been enough to sort things out.

I had my electrician remove the wooden slat thing (which I am told is called a louvre), plaster over the square hole, and then whack in the extractor fan so it vents to the air brick.

This is where things started to go a bit iffy.

For about two weeks, now, the fan (despite being on the least sensitive setting at 90% humidity) seems to be on almost all of the time and I cannot quite work out why.

I am starting to think that it could be the installation itself - is venting the extractor to a square airbrick going to make it significantly less effective in comparison to the usual round tube with a regular vent on the end?

I did a bit of a test, with a candle, and the smoke only really seems to get sucked into the fan when it is about six inches away.

For a fan that powerful, in a bathroom that small, I really was hoping that things would be working better than they are.
 
Your first step is to actually measure the humidity in that room - have you got a sensor / gauge that is actually measuring it?

What design is your air brick on the other side? (the designs vary massively).

You say the fan is on "almost all of the time". Is it actually on all of the time, or it does stop / start during the day? You could have the live connections wired up wrong so it's on all of the time instead of switched by the humidistat
 
Thanks for that.

Air brick piccy:

https://ibb.co/pjsnNTk

Sorry - yes, the fan does eventually turn off for a half hour or so, every so often, so I am confident that it is wired up OK.

Good point about actually measuring the humidity - I think I shall buy one for testing purposes. I sort of guessed that humidity had to be below 90%, given that everything was completely dry, but it is possible that the air in the bathroom just is not dropping below the threshold.
 
The shape of that airbrick certainly doesn't lend itself well to good airflow. There will be a lot of air disruption from those rather thick grids in the airbrick.

Ideally you need something that actually allows the air to flow out (remember air bricks were designed to let ambient air in, not force inducted out) e.g. https://www.ebay.co.uk/itm/193868528203?hash=item2d2376fa4b:g:Bl4AAOSw3QldaQEu

(I know that's easier said than done with your walls though).

So first measure whether you actually have high humidity or not (fan could be faulty) and if the humidity is consistently high then look at the airflow problem.


These are great by the way, cheap in black friday at the moment and will store 30 days of data that you can transfer to your phone via bluetooth to track the history (and pretty accurate compared to the other thermometers and humidity sensors I've used)
 
Thanks so much for this, guys, extremely helpful.

It suddenly occurred to me that I have a Meaco 25L dehumidifier that has a humidity gauge on it - I have left it running on maximum speed and humidity is now down to 66% and still the fan is on, so something is up with it.

I had not realised that air bricks are designed to let air in, so I think you may be onto something here - i.e. as you say, it has been very damp for quite a while so I wonder if humid air is indeed coming in backwards.

I think I am still going to get the suggested Govee sensor, anyway (thank you for the recommendation) as I have a few sensors being used on my HomeAssistant server and it would be nice to check how accurate they really are.

When I bought the fan (I think it is safe to post on here, as not a competitor), in addition to this fan:

www.extractorfanworld.co.uk

Silent Tornado ST100HT Bathroom Extractor Fan + Humidistat Timer | Extractor Fan World

Shop for the Silent Tornado ST100HT Bathroom Extractor Fan + Humidistat Timer. Order online from Extractor Fan World.
www.extractorfanworld.co.uk www.extractorfanworld.co.uk

I also bought the recommended vent:

www.extractorfanworld.co.uk

4 Inch 'No Resist' Back Draught Energy Saver Grille - White

The Tornado No Resist Energy Saver back draught grille has been designed to both prevent back draughts and reduce resistance offered by standard mavity grilles.
www.extractorfanworld.co.uk www.extractorfanworld.co.uk

I think I am now tempted to knock out the airbrick, stick in a straight piece of ducting and then cement back around it to fill in the void.

With a bit of luck, that should prevent humid air from coming back in and shall increase the extraction rate of the fan.
 
Fans also need fresh air coming in. If your bathroom door and windows create a well sealed room when you close the door. The fan is then trying to work like a vacuum and will create negative pressure in the room. Really you need a passive air brick or trickle vent on your window as a minimum, or just open the window. And then the fan will work to it’s full potential. The fan will work best if fresh air can come into the room as it is pumping the moist air out. The best thing would have been to leave that air brick and install the fan on the opposite side of the room to the air brick.

The air brick shape will not be great for airflow either. Hole saw through it. Put in a straight piece of ducting, don’t use flexi, and get an external vent outlet. Then use masonry filler around all the remaining holes, paint, put your outlet on and seal around it.
 
Last edited:
Damn - I wish I had done a bit more research on this, now.

I have an 8.5mm gap under the bathroom door, which I was hoping was enough for airflow in.

Interestingly, I just checked on my dehumidifier again and it was showing 59% and the fan was off - encouraging that it is at least sort of working.

When sealing up the remaining holes, is it ok to leave a gap between the pipe and the inside of the void? I wasn't sure if I should stick some insulation (or even expanding foam) in there or just leave it completely empty.
 
Last edited:
Perhaps a bit obvious - but could you open the window a bit to support air flow? Our bathroom has no extractor fan, the air brick is plastered over (previous owner) but we don't have any humidity problems. Granted it is a bit nippier in winter..
 
Macky said:
Perhaps a bit obvious - but could you open the window a bit to support air flow? Our bathroom has no extractor fan, the air brick is plastered over (previous owner) but we don't have any humidity problems. Granted it is a bit nippier in winter..
Click to expand...
I have been experimenting, with the bathroom window open at various angles, and the air flow just does not seem to change.

Also, I did notice that some water was running down from under the fan this morning - suggesting that perhaps water vapour is collecting on the other side, rather than being pushed out.

I am convinced that the air brick is messing up the fan's ability to work properly.

I think I am going to knock out the air brick, put in proper straight duct, and then cement around it.
 
Air bricks are for small volumes of air to passively move, I'd imagine you're generating back pressure. You either need a low resistance outlet designed for mechanical ventilation, or replace the air brick with proper ducting.
 
Or an alternative POV. I suspect the vent you've got is perfectly fine for 3/4 the year. However winter, especially during an energy crisis when we're all keeping our houses colder than before, is a bit different.

I'd recommend a dessicant dehumidifier in the bathroom. Keep your valuable heat in, and just remove the moisture from the air.
 
You must log in or register to reply here.
Back
Top Bottom