Question about extraction efficiency

**Arron** · 3rd July 2021, 05:01 PM

Hi. This is not a question about dust extraction specifically. It’s a question about air extraction from domestic extractor fans which I am posting here because I know there are people here with a sound knowledge of extraction principles and I have been unable to get reliable answers elsewhere. I hope people don’t mind.

I am building a house and I’m wanting to put some ducting in our very narrow roof space before the gyprock goes on. The problems are a w/c and an ensuite. We want these ducted to the rear of the house (for aesthetic reasons) and it will involve long runs - in each case about 5 metres. However the tiny space between our flat roof and the proposed ceiling has reduced my ducting options to really just flexible ducting of 100 and 150mm. I was originally hoping to get some low profile solid pvc ducting up there but I can’t see how and the suppliers seem to be unable to give any guidance.

So the w/c has just a toilet. Looking at charts, it looks like I need a fan with 100mm ducting for the room size.

The ensuite has a toilet, shower and basin and the reference chart suggests a fan with 125 or 150mm ducting.

So I have run this ducting (photos attached) but my gut feel is that due to the crinkly ducting walls, the long runs, and the slight flattening effect of the roof blanket, the airflow attenuation will be too great for the fans.

Fortunately it’s all straight, no bends or joins.

How to fix? This is very cheap ducting. My thinking is to duplicate the ducting - for the w/c there will be two of the 100mm lengths sitting side by side, joined to a ‘Y’ shaped fitting attached to the fan outlet. Two external vents as well.

For the ensuite, maybe duplicate the 150mm ducting. Or maybe not - it isn’t much constricted by the roof blanket and the 150mm is actually a bit overkill for the space, plus the ensuite has a very exposed window.

So my question is - does duplicating the ducting increase the air flow, bearing in mind it is coming from a standard opening on the fan and has a y join immediately after?

And also
Would it be better to overkill the size of the fan in the w/c to one with a 150mm outlet, but have two full lengths of the 100mm ducting. I suspect a pair of 150 to 100mm reducers will be required for this strategy.

All very confusing - the behaviour of air in pipes.

Cheers
Arron

**Arron** · 3rd July 2021, 05:08 PM

Oops, forgot the photos

100mm

53704FAE-94EB-4896-B1FB-519A380132F4.jpeg

And the 150mm, same just less constriction.

C25B648B-5BF3-4FA9-9500-407A8E7A6BC4.jpeg

**BobL** · 3rd July 2021, 06:39 PM

Can you please post a photo of the fan.
Also any fan specifications.
Thanks

**Arron** · 3rd July 2021, 08:38 PM

Originally Posted by BobL

Can you please post a photo of the fan.
Also any fan specifications.
Thanks

We haven’t chosen a fan yet.

For the downstairs bathroom, we just let the electrician supply and install a fan, it had only a short duct run so it wasn’t critical. He appears to have fit one of these Rapid Response 250 - Fantech Trade .

Unless we have any specific objections we were just going to let him install one of these in the ensuite, and perhaps a smaller unit in the w/c.

It is no problem for us to choose our own fan and provide it for the electrician to install.

**BobL** · 3rd July 2021, 09:09 PM

Thanks,

Question about extraction efficiency-screen-shot-2021-07-03-6-04-58-pm-jpg

The small narrow entry way slots and the fact that they have a non-return backdraft damper means those types off fans move relatively small amounts of air even without any ducting.

This turns out to be fortunate because it means those factor rather than the length of ducting is the bottle neck so I don't think it will make much difference whether you use 4"., or 6", or 2 x 4" ducting.
If you had to make a choice I would use 6"

**Arron** · 3rd July 2021, 09:41 PM

Unfortunately we can use the 150mm ducting only for the ensuite. The space is too low near the w/c so we are limited to 100mm there.

But does your comment mean that two runs 100mm ducting side by side and joined at a y junction will work as well as 150mm ducting ?

If so then I would use that fan in the w/c and 2 runs of 100mm ducting and problem solved.

**BobL** · 4th July 2021, 10:27 AM

Originally Posted by Arron

Unfortunately we can use the 150mm ducting only for the ensuite. The space is too low near the w/c so we are limited to 100mm there.

But does your comment mean that two runs 100mm ducting side by side and joined at a y junction will work as well as 150mm ducting ?

If so then I would use that fan in the w/c and 2 runs of 100mm ducting and problem solved.

Under high flow conditions (ie not your setup) 3 runs of 100 mm are needed to equate to one of 150mm.
Under your conditions is will be somewhere between 2 and 3.

In your case 2 runs of 100 mm would produce slightly more than 1 but it won't double the flow because the fan flow itself is the bottle neck.
However it will probably still be worth doing.

**Arron** · 4th July 2021, 02:12 PM

Ok, thanks.

I expected the improvement would be marginal.

I will put in two runs of 100mm, mainly because I can buy 6mtrs for $9 so why not. I will also look for a more efficient fan with better designed entry ports.

Cheers
Arron

**Lappa** · 8th July 2021, 05:04 PM

I’d be worried about condensation in any long run when exhausting from bathrooms or on suites.

**BobL** · 8th July 2021, 06:00 PM

Originally Posted by Lappa

I’d be worried about condensation in any long run when exhausting from bathrooms or on suites.

Thats a good point but it can be minimised if air is kept running through the duct for some time after steam making processes have stopped.

**Arron** · 8th July 2021, 06:10 PM

If the condensation is inside an aluminium tube what harm does it do ?

Is it just an issue of mould growing in the tube ?

Thread: Question about extraction efficiency

Thread Tools

Search Thread

Question about extraction efficiency

Similar Threads

Wet sanding question - re extraction

High Dust extraction efficiency...... a fools erand?

Dust Extraction setup question

Dust extraction question

TS Undertable Dust Extraction Question

Posting Permissions