Does Your Exhaust Fan Suck?

You likely don’t even think about it when using the bathroom. Flip the switch, hear the exhaust fan, and everything is working as it is intended…right? Far too often, the answer is NO and it is no fault of the user. Sure, homeowners should take a minute each year to vacuum the inside of the exhaust fan housing, but otherwise, these fans should just work. So why do so many of them not work as intended? Hint…it all depends on how it was sized and installed.

The purpose of exhaust ventilation is to remove contaminants (including moisture) that can compromise health, comfort, and durability. Exhaust fans are one of the simplest mechanical systems in your home, but decades of experience working in homes has shown us that even the easiest things can get screwed up. Far too often, exhaust fans rated for 50 or 80 cubic feet per minute (cfm) of air removal are actually exhausting less than 20 cfm. In theory, the exhaust fan should be installed in a suitable location and then ducted to the outside via the most direct path possible. However, the installation of an exhaust fan can involve up to three trades; an electrician typically installs and wires the unit, an HVAC contractor supplies the ductwork, and the builder/sider/roofer may install the end cap termination. What could go wrong?

As energy efficiency standards and construction techniques have improved over time, new and retrofitted buildings have become more and more air-tight. This air-tightness can lead to moisture issues if not properly addressed. Quickly removing moisture generated from showers is a key component of any moisture management strategy. While manufacturers have made significant advancements in the performance, durability, and controls of exhaust fans, it can all be side-stepped by a poor installation.

So how do you correct this issue? Proper installation, of course, but the only way to be certain is to measure the exhaust airflow after installation. In a recent project, I inspected what appeared to be a perfectly installed exhaust fan (exhaust fan collar facing the exterior wall and five feet of straight sheet metal duct to a side wall termination cap) and it sounded like it was pulling a lot of air. However, when I actually tested the 80 cfm rated fan, it was only pulling 5 cfm. After a little investigation, I found that the installer used screws to secure the duct to the exhaust fan collar and the screws impeded the opening of the built-in backdraft damper. So while eyes and ears can help, the only way to truly verify performance is by quantitative measurement.