Extractor fans for 9m duct length

[fanbob]

Hi,

I'm having an extractor fan fitted for an internal bathroom (with shower). The duct length to reach the outside wall is 9m (ducting is under floor) and is 100mm wide.
I've found a fan - the Airflow Icon 30 - which has a flow rate of 33l/second and which teh manufacturer says can work over up to 9m of ducting. So was going to install that. But then, looking at reviews, I found the Vent Axia Quadra. Which is a centrifugal fan that can be set to a flow rate of 60 l/s. So I'm thinking - even better, that will definitely keep the bathroom clear. However, I'm reading that it is only recommnded for a ducting length up to 6m.

Has anybody any idea why it would be that a fan with nearly twice as powerful a flow rate would apparently be less effective over longer ducting than the one half as powerful? Is it just manufacturers' having different, somewhat subjective standards in terms of what they are prepared to say their fans can do? Or is there some objective reason why the stronger fan would be less effective in practice?

Basically I'm wondering should I trust the manufacturer recommendations, and go with the Airflow? Or trust common sense and go with the more powerful, centrifugal fan?

Thanks for any help.

 

[JohnD]

No, get a better ducted fan

Example:

4"/100mm In-Line Duct Extractor Fan - 250m3/hr c/w Timer | Soler_&_Palau (5211364500)

 

[fanbob]

Thanks John. That fan has basically the same flow rate as the Vent Axia. By better ducted, do you mean that it's better cos it's an inline fan? Or just better manufactured? (I can't find a maximum ducting length in their technical specs.)

 

[JohnD]

it is a very quiet fan, of high quality. You can get more powerful ones but will need bigger duct.

There are graphs somewhere of airflow losses according to duct length and diameter. There may be some on the TLC site. Might be in the data sheets.

Soler & Palau TD Silent In Line Fans

goes up to 9 metres but the manufacturers may have a help desk

Ducting Applications

www.tlc-direct.co.uk

Rigid duct is much better than convoluted hose.

 

[flameport]

a fan with nearly twice as powerful a flow rate

Airflow is a poor comparison for fans.
What really matters is the outlet pressure, higher pressure being better for longer lengths of duct.

Airflow is usually quoted as the maximum with no ducting attached, so is mostly unrelated to real world applications.
For all types, airflow reduces significantly as the duct length increases. Convoluted and flexible ducting makes it even worse.

 

[fanbob]

Thanks both.

Flameport, am I right in thinking that outlet pressure is not something manufacturers will be able to tell you? It will depend on the specific installation and you'd have to measure it after the install is done?

 

[Murdochcat]

Are you installing the vent piping?

 

[fanbob]

Are you installing the vent piping?

No it's already installed

 

[fanbob]

The Vent Axia has a constant trickle, plus a pullcord that turns on the boost which takes it up to 60 L/s. So the Vent Axia just needs a permanent live connection. My existing fan is wired to a light switch (but with no lights on the circuit). Would it be ok to just wire the existing live from the light switch into the Vent Axia's permanent live conection? (Obviously the light switch would need to be left on all the time for the trickle to be permanently on.) Or would I need to get a proper isolation switch fitted?

 

[Murdochcat]

No it's already installed

Which type is it? Flexible or solid

 

[fanbob]

Which type is it? Flexible or solid

It's 1m of flexi then 8m of rigid

 

[Taylortwocities]

It's 1m of flexi then 8m of rigid

That’s not so bad. Flexi slows down air flow because of the ridges inside flexi-duct.

No, get a better ducted fan

Example:

4"/100mm In-Line Duct Extractor Fan - 250m3/hr c/w Timer | Soler_&_Palau (5211364500)

A powerful in-line fan may be a good solution (if you have space under the floor!). You'll get much better performance on that long duct length with one of these if you position the fan half way along the duct.

 

[ericmark]

One has to consider noise. All fans will make some noise, much of it due to the ducting vibrating, so the quoted dB noise level does not help that much.

Our aim, or at least mine, is to stop mould growth, to do that we want the room to dry reasonably fast, but also we don't want the room to get too wet to start with. So we don't want to drag water vapour out of the shower cubical into the room.

Two methods, one, extract directly from the shower cubical, and two don't extract until finished in the shower. Also ensure either top or bottom of the cubical is sealed, or don't use any cubical or curtain, so the chimney effect does not cause the water vapour from being pumped into the whole room.

I was forced by building control to fit a fan in mother's wet room, attempt one was an in-line fan, which failed before the completion certificate was issued, found could not access it to remove, so one with a light in it placed above the shower rose, it was set to run on for 15 minutes, and the noise was a problem so switched off on the isolator, and never used again, and no mould problem, but no shower cubical or curtain, so toilet area of wet room would stay dry.

This house fan has failed, again no problem, the cubical seals at bottom, so water vapour not circulated.

However, last house, the shower was in a special bath, designed to have a shower, and the glass doors left a gap over the bath, and a gap at the top, so air would circulate, so the whole bathroom would be damp after a shower, and we had a running battle with mould, maybe down to location, the house was in Mold!

But the point is not down to fan extraction rate, but down to how the shower area was constructed, it would have likely been better with no door over the bath void, but we did not realise this at the time, it was only when considering why one house had mould problems and the other two did not, that we realised what the problem was.

 

[JohnD]

The Soler & Palau fans are exceptionally quiet.

 

[ericmark]

The Soler & Palau fans are exceptionally quiet.

It depends on where the bathroom is, in this house it is under my bedroom, so would tend to wake me if the wife used the toilet after I had gone to bed, but the utility room is well away from the bedroom, so the washing machine and tumble drier on at night does not affect me. I would assume shower downstairs if the duct is under the floor?