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Why are bath fan dampers so lousy??

I just installed a couple of Panasonic WhisperGreen bath fans on a job, and before nailing them in I took a close look at the internal backdraft damper. I had two different types on the job... one had a curved plastic damper like a potato chip, the other had a flat piece of thin aluminum, which I am used to seeing. Both types had a gap all the way around the perimeter... not a large gap, but there was no foam seal there, and I'm sure these will let in a lot of outside air during the blower door test, as well as during some windy conditions. I have taken to installing inline butterfly dampers in some cases, and generally use Seiho wall caps to try to reduce infiltration. I'm not sure why the fans themselves don't have better dampers. Maybe I should be using a different brand of fan? What are the ultra-low-blower-door builders doing in this regard?

Asked by David Meiland
Posted Jun 17, 2011 1:16 PM ET


7 Answers

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David, first let me thank you for using our fans. I'm a Product Manager for Panasonic and can give you a reasonable, if slightly unsatifactory, answer. It's good you have two styles of damper, we are changing from the aluminum one to the plastic one for two reasons. The aluminum one can become 'dinged' in shipment or installation and never quite fit back into the aperture appropriately. The plastic ones are also slightly lighter and I believe block the damper aperture better.

The gaps around the damper are there due to physics. Without air flow around the damper to initiate the movement to open the damper, the force of air would be insuffecient to open the damper to exhaust the air. As far as I know, all exhaust fans are designed the same way. If you prefer a butterfly damper it would need to be wired to open as the exhaust fan comes on.

I have heard from some Canadian Energy Raters that are working on a true Net Zero Home that they would like for us to completely seal the damper as well. I offered the same advice on the butterfly damper. I would also be curious to know how much air is being 'backdrafted' through our exhaust fans - looks like I have an experiment next time I go on a site with a flow hood.

Thanks for the comments; I'm always happy to hear about issues that we may be able to improve and offer better quality products to the market. Please feel free to contact me anytime.

Answered by Ted Cater
Posted Jun 17, 2011 2:34 PM ET


Ted, thanks for the response. I've installed a couple of types of butterfly dampers, and they are not powered, and in truth they are not a complete seal either. I put them in with the hope that they will reduce the reverse airflow through the fan, but so far there hasn't been a perfect outcome. Regionally the brand seems to be Columbia, and I have ordered Fantech from online sources. The Seiho wall caps are of very good quality but they are can be visually obtrusive if the owner is sensitive to mechanical warts on their house.

So far I have not actually measured backdrafted air volume through one of your fans, or any other for that matter. Recently I took some pressure pan readings at fans and can lights, with a blower door at -50pa, and was getting approx. 9pa. This does not differentiate between leakage through the damper(s) and leakage through and around the housing. This is in a case where I had already retrofit an inline damper, and there was a wall cap in place. I took the reading after noticing continued air flowing in. It's detectable using a biometric airflow sensor (i.e.back of the hand) but it isn't huge.

Here's what it looks like in infrared. Pretty much every bath fan in every house looks like this after the blower door has been running for a short while. Some are worse (sometimes much worse) than others. You can see that this one has a combination of leakage through and around. Actually, a flange on these for gluing the drywall would be a nice addition.

Answered by David Meiland
Posted Jun 17, 2011 6:02 PM ET


I had the same question and think I found a solution: http://www.larsonfan.com/index.php?app=cms&ns=display&ref=Why Unfortuantely they don't have the small sizes available yet for me :(

Answered by Alexander Mohr
Posted Jan 6, 2013 4:35 AM ET


David, I don't have any useful solutions for the damper problems but since incorporating the methods suggested in the Building Envelope Guide For Houses (A BC Government publication) my fan housings are a lot more airtight. I provide a piece of plywood flush with the underside of the joists with a hole just large enough for the housing. Once the fan is mounted to the joists above it is easy to seal between the housing and the plywood, and there is a large area to apply gaskets for the drywall below. Depending on how well sealed the housing itself is I either caulk or line the outside with peel and stick.

Answered by Malcolm Taylor
Posted Jan 6, 2013 6:49 PM ET


If you don't mind the looks, check out the ECOVENT from BROAN
i paid something like 18$ locally for it, there is a foam ball cover that falls by gravity on a curved track to "clap" the exit hole ...the design is very simple but super ingenious and it seems to work very well with my panasonic fan. ( so i have the damper and the outside vent damper on the same line )

Answered by Jin Kazama
Posted Jan 13, 2013 1:31 AM ET


Unless your bathroom has a large gap between the bottom of the door and the floor or some other way for air to get into the bathroom, the bath fan will quickly be pulling against a pressure that makes it difficult to move much air.

Answered by Jenny Belman
Posted Jan 20, 2013 8:01 AM ET


I would be interested in an explanation of the physics of how a leaky damper opens better than a sealed one.

A pressure differential between the two sides of the closed damper would create the force required to open the damper. If it is leaky, this pressure differential and thus the force to open would be less. In fluid mechanics, pressure decreases as fluid flow increases. The largest pressure and force on the damper would occur as the fan was on but before the seal of the damper gave way and opened, releasing the static pressure. When the air is flowing, the same holds true: there is a pressure from the flowing fluid on one side of the damper that is greater than the pressure on the other, holding the damper open. How far the damper opens (the force) depends on the pressure balance.

Could there be another reason the dampers are leaky? To ensure that a small pressure differential where the pressure outside the house is greater than inside doesn't "seal" the damper shut so that the fan cannot overcome the seal?

Answered by Bill Costain
Posted Jan 21, 2013 11:49 AM ET

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