Why bathroom fan isn't working as good since range hood installation?

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Posted by: from Pierrefonds
6/12/2015 at 12:00:31 AM

I have had a bathroom fan and a range hood installed at the same time in the house. I was informed that both exhaust to the outside through one 10 inch by 3 and a half inch hole through the exterior wall. There is a contraption that makes sure that neither one mixes with the other when either fan is working. I find that the bathroom fan has diminished in it's capacity since the range hood has been installed. Any comments?

I find neither one is working at a decent capacity. Prior to their installation I was informed that each fan would have their own exterior exit and remain as 2 separate entities.

Any info welcome.


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Date/Time6/12/2015 at 2:51:39 AM

Often when I find a bathroom fan working inefficiently after a new installation the first suspect is the flap inside being prevented from opening when the fan is exhausting - some folks will put a screw in the connection from the hose to the fan and this can keep it from venting.

But based on the range also not venting but being connected on the same line, best guess would be too sharp of corners in the piping, or generally undersized piping. Additionally, depending on how far the pipe has to go from either exhaust point to where it gets to outside, if too small a pipe is used this could make for a dramatic reduction.

Another major problem is too low a CFM fan being installed, but this is not usually problematic on range exhaust, but certainly bathrooms. Often you see 80 CFM fans being installed because they are the cheapest. Unless a bathroom is tiny I always recommend going to 110 CFM. They are a bit more because you don't want the loudest one possible, but still a smart investment.

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Robert from Electrilight Ltd. in Oakville
Date/Time6/12/2015 at 7:05:01 AM

Installing a "Y" unit in a ducting line to save costs is a bad idea and poor workmanship showing an amateur installation. When both units run at once, which will happen, they will choke the other from doing it's job, (not force both capacities through tightly like some think). It's like trying to pour liquid through a funnel then doubling the contents through that same funnel while it is at capacity. Luckily it is only air it is trying to force out, and not CO2 gases from your furnace and hot water heater. I've seen too many of those jobs and just shake my head. Get them separated and your problem will go away.

Like the previous comment of the screw causing the damper to jamb, that is possible. Depending on how dusty your home is, it could be clogged.

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Date/Time6/12/2015 at 8:59:32 AM

Hi Mona,

If you have a contract that states 2 separate ducts and exits then you should insist that it be done that way. 2 separate fans blowing into the same duct are just going to cause turbulence and slow down the flow for both. It also sounds like the dampers have been damaged somehow, probably with poorly placed screws or installed backwards (seen that one a couple of times).

Good luck with it!

Jim Kuzma

Kettleby Handyman

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Date/Time6/13/2015 at 6:43:53 PM

There are a couple possible causes, but first they should each have there own exhaust pipeing. Under no circumstances, is using the same exit a good idea and most times is not allowed or recomended. Pipe length might be to long or the pipe size might be to small. Some of the higher end range fans call for 8 inch round ducting. Also both units just might be cheap units with a lower CFM rating than your previous fans. Besides the screw holding the damper, he might have forgotten to take the tape off the damper, used when they package and ship the fan???

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