I do some rating for my local Habitat Affiliate and recently have noticed an uncomfortable amount of air coming in from the exhaust fans when the house is de-pressurized. So I guess the first issue is whether the fans are designed poorly and the flapper doesn't seal well against leakage OR perhaps they were installed poorly and flapper is consequently hung up in a position where it can't seal?
Are there Energy Star fans out there that do a better job of sealing during de-pressurization than others?
Thanks for your input, Eric
In my experience the "best fan" is one that's installed properly.
The trouble with fans is that there are typically three trades that are involved in putting them in (electricians, ducting, typically done by the HVAC guys, and the siders). There may be some generalizations in that list but it means that there's many places where the fan performance can be screwed up, your issue of backflow being one of them.
Typically that backflow damper inside the fan is never air tight. The termination on the exterior is more important and those can vary considerably in both their resistance to airflow (reducing fan flow) and ability to seal up to prevent backflow when the fan isn't operating. The thing I've seen in my area with production builders is that when the siding package arrives it comes with standard fan terminations. These are terrible, from both standpoints. It's worth springing for a quality termination that seals with a single flap.
As per your specific issue of "air coming from the fan", are you sure it's coming from the fan itself or from around the unit? - if the cover is in place it can be hard to place the actual leakage. If the junction between the drywall and the unit is not air sealed, air can leak in from the attic. That junction needs to be air sealed.
I certainly agree with Mark regarding the quality of the installation. Leakage around the housing after installation is usually the major culprit in leakage. In my testing, I have found that the majority of backdraft dampers in fans work quite well. The interesting thing is that since they operate on airflow and gravity, they can't be completely tight. That small amount of leakage is designed into the product to allow a flow of air to start when the fan begins to operate. Without that flow, the damper won't open. To get a tight seal the damper has to be powered - motor or piston or something similar.
Hi Eric, Great observation and I have to agree with Paul and Mark that it is all about the installation. I recently received a call from a customer who just switched from another brand and they were concerned with the leakage during depressurization. The backdraft number was abnormally high, so we dug deeper to understand the situation. What we eventually found out was that the installing contractor had gotten into the habit of removing the backdraft damper at the roof termination because the previous fan could not pass the minimum required flow rate with the damper in place. Their solution .... remove the damper so the fan exhausts at least 50 CFM to pas code!
Is it possible that you may be seeing the same thing occurring?
Remember that if there is a decent amount of air back drafting, you may experience condensation problems in the duct - insulated or not.