Can't understand air extract calculations

[bludger]

Hi,

I'm planning to fit an inline duct fan to ventilate my bathroom (with toilet). The volume of the room is 12.83 cu metres.

The recommended air changes seem to vary somewhat, anything from 6 to 15 times an hour. For ease of calculation I'd be happy with ten times an hour.

Now the fan I want will extract 58 cu metres an hour. If I divide that by the room volume of 12.83 cu metres the answer is 4.52. Would this be the minutes I set the overrun timer at? I have an uneasy feeling that it is not....

 

[flameport]

4.52 is the number of air changes per hour you will get using that fan and your room - less than half of what you want.
Are you sure the fan is only 58 cubic metres per hour - that is extremely feeble and will probably be useless.

This fan: http://www.screwfix.com/p/manrose-chrome-white-25w-in-line-centrifugal-shower-fan-kit/41379 would provide about 8 air changes per hour.

Or this one: http://www.screwfix.com/p/manrose-in-line-centrifugal-32w-bathroom-fan/98482 would be 14 air changes per hour.

The timer would normally be set to 15 minutes or so regardless of the fan capacity, this ensures the room is cleared after people turn the light off. With the 25w fan, this would in theory mean the air in the room was replaced twice during the timer period. In reality it is more complex, as the airflow in the room won't be equal in all areas.

Larger fans are available, however they cost more, make more noise and in the winter will remove warm air from your house far more quickly resulting in an increased heating bill.

 

[bludger]

flameport,

Glad you replied. I had no idea of the extract rate I needed to clear the room and would have been disappointed in my choice.

I checked my selected fan again, Domus TID 4", as sold by Fastlec.co.uk, and they do give the extract rate as 58m3/hr. However, on going to Domus itself, part of Polypipe, they say that a specification sheet is not availble for this model. Hhmm. So I'll now look at the fans you've linked to.

It seems a complex subject all round to me. You've given me exactly what I needed to know and I'm most grateful.

 

[bernardgreen]

Unfortunately there is another factor in the equation. Air pressures.

The fan's extraction rate is affected by the pressure difference between the air in the room and the air around the vent to the outside world. As air is drawn out of the room the pressure in the room drops, the pressure diffential across the fan increases and the extract rate drops. Very often the quoted extraction rate is that with a very low ( some times zero ) pressure differential and this rate is seldom achieved in real use.

A rule of thumb "solution" is to ensure there is a route for replacement air from the rest of the house to enter the room by a vent. ( most often a gap under the door ) and divide the extraction rate on the data sheet by at least two.

 

[bludger]

bernardgreen,

Yes, that's clear. Thanks. Perhaps, then, an inline duct fan, which I gather creates more output pressure, could actually extract more air than a similarly rated surface mounted fan?
I don't have a choice in my situation, anyway.

I'll make sure that the gap under the door isn't impeded by carpet for a start.