I recently replaced the 20-year-old, noisy, inefficient exhaust fan in my master bathroom with a new, quiet, powerful Panasonic 130 cfm unit with 6" ducting. The fan works great. However, I have noticed a significant draft going out of the fan when it is not on. If I hold a piece of tissue up to the grille it gets immediately sucked upwards. So clearly there is an updraft.

Doing some internet searching on this issue, I have found almost all of the problems people report with bath exhaust fans is with BACKdrafts -- they have cold air coming into the bathroom. I seem to have the exact opposite problem. Heated air is rising straight up and out of my bath fan.

The fan has a built in damper, but in this case it does no good - it doesn't prevent air from escaping out, only the other way.

Is there anything I can do to prevent air from escaping out when the fan isn't on? For now, I have a piece of plastic film up there, which I pull off every time I run the fan. But that's not a very elegant solution.

 

Six inch is a huge duct for only 130 CFM. Does this exit through the roof, a sidewall or downward through a soffit? Does the exit terminal also have a backdraft damper? You could install a six-inch motorized damper (normally closed version) in the six inch duct and then wire the damper operator to the fan motor. Downside of this is (a) it is another potential failure point, and (b) the damper assembly will set you back in the neighborhood of eighty smackers.

 

The natural air pressures in a house during cold weather are positive in the upper areas and negative in the lower areas. Being on the ceiling and especially if this is a second floor the stack effect pressure will be forcing air out and you are correct the internal damper and the one at the vent are not designed to stop outflow.

The irony is, modern well sealed homes will run that bath fan 24/7 just to provide the needed fresh air no longer provided by all of the normal leaks. All you have to do is seal a lot of uncontrolled leaks and then your natural air leaking bath fan would be perfect .

Bud

 

Thanks for the replies. Yes, 6" duct may be large, but that's what the fan is designed to accommodate (Panasonic WhisperGreen FV-13VKS3), and from the research I did before making a purchase, the larger the ductwork, the less airflow resistance. So that's what I went with. Our bathroom is about 13' x 10', so when I used Panasonic's bathroom fan size calculator, it actually told me to go with a 150cfm fan. That seemed to me to be overkill so I picked the 130cfm fan (which was the largest size available in the WhisperGreen model line) and is already a big step up from the rinky-dink piece of junk that was in there previously. It had been hooked into a 3" flexible duct, uninsulated, and ran down into to the soffit where it just ended (i.e. not connected to a soffit vent). When I pulled the duct from the soffit, it was full of ice.

So I replaced it with 6 inch insulated flexible duct, which I have running up into an existing roof vent. I know that terminating the duct at a roof vent is not ideal, but it is better than dumping moisture into the soffit and is only temporary until we reroof in the near future. At that time I'll have a proper hooded exhaust vent installed with a damper and will replace the flex duct with rigid. So currently no damper at the terminal end.

Bud9051, yes, that is kind of what I suspected with the upward pressure. Tracking down all the small leaks is a challenge with a larger house but I have been working on it. I suppose I should not worry too much about the updraft but I hate to think of all that expensively heated air just wafting away.

 

The natural pressures are very small when compared to the 130 cfm fan, so is it possible to add a gravity closure somewhere in the duct to the outside. I know they make ball type vent hoods for dryers and perhaps the light styrofoam ball approach would be able to block the natural pressures yet open under fan pressure. I think the dryer ones are only 4" so you may have to fabricate something for yourself.

If you wait, it gives you time to find something that will close naturally both ways.

Bud