Extractor fan ducting for toilet

[Adam-the-Kiwi]

Hi All,

We have a flat with a toilet (pretty much literally a WC - just a room with toilet and small handbasin - a little less than 2.5m3) bang in the middle of the building (it's a "colony upper" in Edinburgh for those familiar with them). That means that it is, give or take, 5-6m from the outside in any direction. The path that we are proposing for the extractor ducting involves going upwards from the (false) ceiling for about a metre into a shared loft space, then around 5m horizontally to a vent in the roof (already installed). Was planning to have an axial fan in line at the outside vent end - axial because the loft space there is right next to our neighbour's bedroom and I thought a centrifugal fan might be too noisy.

Questions:

- I was planning on using 4" circular section ducting for the 1m rise, then the box-section ducting for the horizontal run. Any problems with that plan?

- There are a couple of oblique angles - what is the best way to achieve these?

- We may have to navigate the ducting round some very old piping. Obviously, we'll try to minimise curvature, but is there an alternative to the bellows-type ducting for this?

- Am I being hopelessly optimistic in using a 4" axial fan to drive this?

Any other advice or experience will be gratefully received!

Thanks - Adam...

[Andy Hall]

Over this distance, that's going to add up to a lot of air resistance.

Yes. A better solution would be to get a length of air conditioning flexible ducting for the whole run. You can get it with an almost smooth internal surface. If you think that there might be condensation (shouldn't be from this room but possible others?) , then there is an insulated version as well.

Go for a 150mm duct rather than 100mm and that will reduce air resistance considerably. You can get size adaptors.

With what you were proposing, yes. The general recommendation is up to about 3m for a typical axial 100mm fan. A 100mm centrifugal fan would be OK on a reasonable duct, or you could go up to a 150mm axial fan with a speed controller and that would be very quiet.

Have a look on the Vent Axia site for good design information and good products.

[The Natural Philosopher]

Totally agree with the above, and will add one thing to the mix.

Mount whatever fan you use on some rubber or foam rubber blocks. And at least have flexible pipe on that section, and throw a bit of rockwool over it. All in the interest of noise. Wrapping some lead round it may help also, and boxing it in.

[Owain]

A centrifugal fan should not be any noisier than an axial, and if good quality may be quieter. In-line fans can be mounted at any reasonable point in the ducting, not just at the ends.

Ventaxia or Xpelair, don't use the cheapy ones in the sheds.

For a domestic toilet you can increase the run-on time to compensate for slower air movement.

Owain

[Adam-the-Kiwi]

Andy, thanks for the info. I haven't yet been able to find any on the Vent Axia site. I'll keep looking, but can you give me any specific products or suppliers?

Thanks for the help - Adam...

[Andy Hall]

Do a search in Google using keywords 'insulated ducting' and you will see loads of suppliers.

[Owain]

IME it's the backdraught shutter clicking that makes more irritating noise than the fan.

Owain

[Adam-the-Kiwi]

Hi Owain

Thanks for the info.

The positioning of the fan at that end was more for spacial convenience - further away from the toilet means less noise in the toilet (obviously) and nearer the vent puts the fan body nearer the eaves and so less in the way of storage. I assume I can put a backdraught shutter that that end too...

My understanding is that a centrifugal fan will generally produce a greater pressure drop for a given power - is that right?

Cheers - Adam...

[Andy Hall]

Have a look back at the Vent Axia site. They publish curves for all of their products.