Extractor Fan Question\Problem

[cronas]

Hi Everyone, this is my first post so be gentle. I have recently bought a 3 story house and I am in the process of doing it up. The bathroom is on the middle floor with a bedroom above it.

The bathroom has an old style box extractor fan and the ducting from this is a piece of stack pipe that runs up through the cupboard in the bedroom; which is annoying because it is not nicely in the corner of the cupboard. I have attached a rough drawing of the layout.

Ideally I would like to move the ducting so it is more in the corner or to the other cupboard which has a bit of extract space to one side but this would mean unnecessary bends or the extractor would be next to the bathroom door.

so my questions are:
1. Does any one have a solution to my problem
2. I haven been looking at the inline extractors but the only place I can see it going is in the loft which is 3.4m above the bathroom. is this two far to provide enough suction? I can't find any information on the maximum distance one can be placed.

Extractor Fan Problem

• cronas
• 3 Nov 2010
• 3

 

[ban-all-sheds]

Is the wall on the LHS an external one?

 

[cronas]

no sorry no external walls it is a terraced house

 

[ban-all-sheds]

So no external walls in the bathroom or the bedroom?

 

[OwainDIYer]

You can get rectangular duct that will run between floor joists, and rectangular-to-round adapters, if that helps.

 

[cronas]

the joist in the bedroom run in the wrong direction to reach the outside wall. The otherside of the bathroom is a stairwell so it can't go that way either the only way is up!!

 

[stem]

Most domestic fans have 'Axial' blades, so they do not create much pressure and are best suited to ducts less than 3 metres long, after that they don't really have enough force to overcome the pressure losses in the ductwork, or the losses are so great the extraction rate is very low.

Find a fan with 'centrifugal' blades. These create more pressure and can easily cope with extended duct runs. However in a length of duct, more resistance to the airflow is created by bends than straight sections, so bends should be kept to a minimum.

As a guide, for 110mm diameter duct, one 90 degree bend is equivalent to about 1.5 metres of straight duct (depending on the radius of the bend)

So, a duct of 2 metres with 2 bends presents the same resistance to the fan as one straight length of 5 metres.

Also, if the fan passes through an unheated loft, you should insulate the duct and consider fitting a condensation trap to prevent warm moist air from the bathroom condensing in cold sections of duct and running back down it.

 

[cronas]

Most domestic fans have 'Axial' blades, so they do not create much pressure and are best suited to ducts less than 3 metres long, after that they don't really have enough force to overcome the pressure losses in the ductwork, or the losses are so great the extraction rate is very low.

Find a fan with 'centrifugal' blades. These create more pressure and can easily cope with extended duct runs. However in a length of duct, more resistance to the airflow is created by bends than straight sections, so bends should be kept to a minimum.

As a guide, for 110mm diameter duct, one 90 degree bend is equivalent to about 1.5 metres of straight duct (depending on the radius of the bend)

So, a duct of 2 metres with 2 bends presents the same resistance to the fan as one straight length of 5 metres.

Also, if the fan passes through an unheated loft, you should insulate the duct and consider fitting a condensation trap to prevent warm moist air from the bathroom condensing in cold sections of duct and running back down it.

thanks. Do you know the max distance that a centrifugal fan can duct to? i must be working with about 5.5 - 6 meters

alternatively do you know how far away i could put an inline extractor from the bathroom before it will lose suction?

 

[stem]

There seems to be a dearth of information from fan manufacturers about the performance of their units, although I have seen one or two performance graphs for some recently.

I have used a domestic centrifugal fan over a 6 metre distance and it worked fine. If you're worried, I would suggest that you select a fan you like, and then search the internet to see if you can find performance data for it.

In-line fans are subject to the same duct resistance to airflow, the only difference is that they have a negative pressure before the fan and positive pressure afterwards. Most in-line fans seem to have axial blades, so they're not good for long distances.

There is what's known as a "mixed flow" fan available now as an in-line unit, I believe the blade design is a combination of axial and centrifugal. Unfortunately I don't have any further knowledge or experience about them to be able to advise you.

 

[cronas]

I have purchased an Xpelair Premier DX200T
http://www.qvsdirect.com/Xpelair-Pr...al-Wall-Ceiling-Fan-Timer-Model-pr-20277.html

I have been assured by the guy in the shop who rang the manufacture that these will happily duct to 10m (depending on number of bends). Not sure how accurate these details are as there is nothing stating this in the manual but I will keep you posted.

I was hoping that I could get the round to flat converter and run flat ducting against the back wall of the cupboard hopefully minimizing space loss does anyone think this is going to cause me problems with air flow?

Is anyone able to help with adding a condensing trap?
1. where should it be put in the ducting? i.e how far up ect
2. can anyone provide links to ones I could use?

Many thanks for all the help so far this has really been a pain.

 

[stem]

I have purchased an Xpelair Premier DX200T

Good choice

I have been assured by the guy in the shop who rang the manufacture that these will happily duct to 10m (depending on number of bends).

Sounds reasonable.

I was hoping that I could get the round to flat converter and run flat ducting against the back wall of the cupboard hopefully minimizing space loss does anyone think this is going to cause me problems with air flow?

Find the Pa (pressure drop) per metre of the duct you are considering, and then multiply it by the effective length of the duct. Look at the graph for your fan on the link you supplied and you will see what extraction rate you will get. 100mm Flat duct has a much higher pressure drop than 100mm diameter duct. Having said that, you could convert it back to circular duct after the cupboard. Beware too, that the Pa changes according to the air flow, so the calculation will change according to the fan speed settings.

Is anyone able to help with adding a condensing trap?
1. where should it be put in the ducting? i.e how far up ect

The easiest place is the lowest point in the loft so that any duct below it is inside heated rooms where condensation won't form. The condensate drain pipe can then poke out below the roof tiles. Alternatively site it just before the fan, but this may make the drain difficult in your case.

2. can anyone provide links to ones I could use?

They are all pretty much the same really. Click here for an example. I've not seen one for rectangular duct though, so you would have to convert it back to circular duct, depending on where you site it.

 

[cronas]

That's brilliant thanks for the help