Question: I purchased a new house about a year ago with super quiet fans. While they were quiet for a while, now they're incredibly loud. What's up?
Answer: A bit difficult to diagnose without looking at your setup but I'll share a couple of the most common reasons for fans becoming louder after an initial installation period.
Almost all sound related issues are related to obstructions. Obstructions can occur in the blower wheel area, wall/ceiling cap, or the duct itself. Here are some examples of the most common obstructions that can occur after an initial time frame.
The builder used the bath fan to clear construction dust and to help dry sheet rock mud during construction. This dust builds up on the blower wheel and after showering with the fan on, the moisture mixes with the dust to become concrete like. This can alter the balance of the blower wheel and the air flow, which will add noise.
The builder didn't seal the fan to the ceiling. I've seen fans put in at all angles and with ceiling cutouts that don't match the "fan can" shape. When the fan is turned on, it draws attic insulation down from the top of the ceiling into the blower wheel. Insulation will upset the balance of the blower wheel, creating noise and reducing air flow. Both of these are easily checked, just remove the grill and look at the blower wheel. It should be free of dirt, dust, and any other material that might have been drawn in.
Wall cap obstructions are usually a result of someone painting the cap closed, bee/bird nests, or squirrel (or other pest) moving in to the cap area to enjoy the warm moist air being evacuated from your home when showering.
Duct installations. This is by far the most common "over time" problem I run into. The builder will use a flexible duct to run from the fan to a roof cap. The most common installation method is to use a "cloth like" hangar material to suspend the duct from joist to joist. This typically occurs outside the thermal envelope, meaning above the insulation, where the attic and the ducting are exposed to both hot and cold temperature extremes. On a hot day, flex ducting will do as advertised – flex, creating a belly in the middle of the duct. Once stretched the ducting does not return to its original position, it simply hangs lower, creating a "belly or P-trap". This belly, over time (a number of showers) can fill with water creating an air flow obstruction. This happens when the water vapor gets to the roof line where the outside temperature difference can cause the moisture to condensate back to the belly of the duct – pulling it down even further.
I've literally dumped a gallon of water out of duct bellies for homeowners who can't get their fans to exhaust steam from their bathrooms. The key words for me in this question were the time frame. About a year! It takes a hot summer to soften and sag the duct and then if fills full of water all winter.
You should be able to look for duct sag by poking your head into the attic and following the ducting path with a flash light. If you see flex ducting touching the top of your ceiling joist or bellying out before exhausting out of the house you know you've got a challenge.
If you don't see any obvious obstructions you should then test the fan for noise by removing the ducting and turning the fan back on briefly. If the fan makes an unusual amount of noise without the ducting, then it's time to review the fan warranty!
Ken Nelson is the Northwest Regional Sales Manager for the Panasonic Eco Products Division, specializing in ventilation solutions for residential and multi-family living environments. Over the past four years, Ken has spoken throughout the Northwest, teaching and training builders, building science advocates and professionals on the physics of moisture and air movement in homes of all sizes, types and age.