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Topic Title: Extraction ducting
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Created On: 02 February 2012 09:04 PM
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 02 February 2012 09:04 PM
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SKElectrical

Posts: 1146
Joined: 01 February 2009

Im installing centrifugal in-line fan (situated in loft space), with ducting running: from bathroom, to fan (1 meter), to outside (3 meters) all in loft space.

As regards the ducting, should i use the standard pvc flexi duct or the insulated flexiduct?

Also, does it matter where i position the fan? (cant see why it might)
 02 February 2012 09:26 PM
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aligarjon

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Joined: 09 September 2005

i have similar lengths in my loft. i used solid pipe from the fan to the outside wall with a slight fall away from the fan. i just used normal pvc flexi to the internal vent, nicely wrapped in fiberglass. i don't know how good insulated ducting is but i would think you might still get condensation inside it with that length. i don't know how feasible a condensation trap would be in your situation ? Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 02 February 2012 11:34 PM
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SKElectrical

Posts: 1146
Joined: 01 February 2009

solid pipe? clever
 03 February 2012 09:41 AM
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OMS

Posts: 22864
Joined: 23 March 2004

I guess the answer is it depends on the loft temperature - if you have a warm void then you need less insulation (or none at all) - it's all about the dew point of the exhaust air and how much moisture its carrying

If it's cold you need insulation (or a condensate drain to get rid of the water) - as teh warm humid exaust air cools, it dumps water vapour as liquid condensate

Personally, I hate flexi duct - it's crap.

Use 100mm (or bigger) diameter PVC duct, laid to falls so any condenstae runs away to the outlet - you can get "insulated" tube (primarily for acoustic reasons) or you can but rigid glass fibre shells to go over 100mm tube. Keep the lengths of flexi to a minimum - it has a high resistance to airflow and you don't need much to seriously degrade the fan performance - it's also inefficient so you can easily fall foul of Part F and Part L if you use a "standard" fan.

Regards

OMS

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 12 June 2012 11:56 AM
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leckie

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Hi all

I went to look at a job yesterday as the client said that a fan was leaking water!

There was a centrifugal bathroom fan vented via solid into the roof space above. The duct was under the insulation and went horizontal for about 5 metres. It then went into an angle connection and into pvc flexi ducting. This was vertical for about 2.5 metres and entered a roof cowl.

There seemed to be condensation running through the duct system and into the fan.

I assume that the uninsulated vertical flexi duct is causing the problem due to condensation.

The obvious solution would be to fit a condensation trap but the flat is three floors up with no roof eve vents, so the overflow would need drilling through a gable end, a bit awkward at this height!

If I were to replace the flexiduct with insulated flexiduct would this solve the problem? Polypipes site says condensation trap OR insulated ducting to solve the problem. Any thoughts appreciated
 12 June 2012 12:13 PM
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OMS

Posts: 22864
Joined: 23 March 2004

Insulating the duct will usually cure the problem - as long as the dirty extract air is kept above the dew point temperature it won't condense.

regards

OMS

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Let the wind blow you, across a big floor.
 12 June 2012 01:21 PM
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AJJewsbury

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If I were to replace the flexiduct with insulated flexiduct would this solve the problem

Or just wrap insulation around the existing duct perhaps? - might be easier.

There can be problems with fans that operate intermittently - the duct will inevitably cool down when the fan is off, so the first blast of warm moist air will likely produce some condensation in the duct. Some manufacturers suggest a run-on time - i.e. keep the fan running for 10 or 15 mins longer than needed to clear the room of moist air, so that the warm but now dry(ish) air has a chance to dry out the condensation - so that it doesn't accumulate to the quantity where it starts to cause trouble.

- Andy.
 12 June 2012 05:31 PM
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mikejumper

Posts: 2810
Joined: 14 December 2006

Originally posted by: SKElectrical

Im installing centrifugal in-line fan (situated in loft space), with ducting running: from bathroom, to fan (1 meter), to outside (3 meters) all in loft space.

Worth installing a decent fan with that length of run.
I've always found Soler/Palau up to the job, although pricey.
TLC do a complete kit including fan, vents, ducting etc..
 12 June 2012 06:05 PM
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peteTLM

Posts: 3755
Joined: 31 March 2005

Originally posted by: leckie

Hi all



I went to look at a job yesterday as the client said that a fan was leaking water!



There was a centrifugal bathroom fan vented via solid into the roof space above. The duct was under the insulation and went horizontal for about 5 metres. It then went into an angle connection and into pvc flexi ducting. This was vertical for about 2.5 metres and entered a roof cowl.



There seemed to be condensation running through the duct system and into the fan.



I assume that the uninsulated vertical flexi duct is causing the problem due to condensation.



The obvious solution would be to fit a condensation trap but the flat is three floors up with no roof eve vents, so the overflow would need drilling through a gable end, a bit awkward at this height!



If I were to replace the flexiduct with insulated flexiduct would this solve the problem? Polypipes site says condensation trap OR insulated ducting to solve the problem. Any thoughts appreciated


Also dont forget that most fans are about as powerfull as waving a lolly stick around in wembley stadium and expecting something to happen.
Off the shelf 4'' fans are good for pushing 200mm through a wall and thats about it. Once you get past a metre or so you have the wembley scenario.
You need either a boxed wall mounted centrifugal fan, or inline centrifugal fan.
All in a box shower kits ok for 1m straight out a wall, your 5m run needs a £70 fan and use soil pipe for ducting.

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