Fix the fan

By Hannah McKenzie
Fix the fan

To test your bathroom fan, place a two-ply tissue on the grille and see if the fan holds it.

Q: I have mold growing on my bathroom ceiling despite having a bath fan. I installed a new bath fan but that didn’t fix the problem. What should I do?

Often the problem is not with the fan but with the installation. Grab this checklist, some toilet paper, a ladder, flashlight and a screw driver so we can diagnose the problem. The fixes are often a one-man DIY project. In the case of my home, it was a one-woman endeavor.

  1. Start in the bathroom.
    Test: If the fan holds two stacked 2-ply squares of toilet paper at the fan grille, the fan is pulling approximately 50 cubic feet per minute (cfm), which is ideal. If the fan pulls less than 50 cfm, let’s start the investigation.
  2. Cut off the circuit breaker to the fan and leave a note that says you are working on the fan.
  3. Remove the fan cover.
    Rating: Look for a sticker or stamp that indicates the rating. Ratings of 50 cfm and 70 cfm are typical. Subtract 20 from the rating. This is the most air you should expect the fan to pull.
    Sealing: The fan housing (metal box) should be caulked to the ceiling drywall. This will ensure that air is being pulled from your bathroom rather than the attic.
  4. Remove the fan so you can reach the start of the duct. Take note of how everything is put together so that you can put it back properly.
    Damper: Bath fans have a back-draft damper to keep outside air from entering the bathroom. Tap the damper to make sure it swings freely. You may need to remove shipping tape or you’ll find that the ductwork is holding the damper shut. Use your hand to gently straighten the duct.
  5. Head up to the attic. If your fan is between floors, skip this step and move to the outside.
    Connection: Using a flashlight from a distance, make sure that a duct is attached to the fan and directed to an outside termination. A 4-inch insulated duct is ideal. Some homes will have the fan blowing moist air into the attic. Moist bathroom air in the attic will often cause rot and mold damage to your roof or ceiling. An easy fix is to have roofers add a termination to the roof when installing new shingles.
    Length: The duct needs to be as short and straight as possible. If the duct needs to be shortened, be careful not to crush the ceiling insulation or fall through the ceiling. Use a metal tie band (like you’d find on a dryer duct) to reattach the shortened end. If you want this repair to last, do not use duct tape.
  6. Go outside to the termination if you can safely reach it.
    Termination: Make sure the termination operates freely. Remove any remaining packing tape.
    Connection: Verify the duct is attached and open to the termination. Sometimes the duct can get bunched and prevent air from leaving. Fairly often, I find that the termination was never cut into the roof, wall or soffit. A handyman can easily solve this problem.
  7. Cut the fan power back on and head out to your local hardware store.
    Your fan should not sound like a jet engine. If the fan rattles, the fan box might not be securely attached to the ceiling joists. Refer to the installation manual and a handyman to get this fixed.

Another curve ball might be the duct diameter. My parents installed a 70 cfm-rated fan that won’t pull more than 20 cfm. Why only 20 cfm? When they replaced the fan, they used the old 3-inch duct even though the fan instructions called for a 4-inch duct. Don’t expect a more powerful fan to pull more air. The connections, ductwork and termination play key roles in the fan’s performance.

Operable bath fans are very important for all homes, because a lot of steam is produced from bathing. Getting the steam out of the house is the best solution for preventing mold or mildew growth.

About the Author

Hannah McKenzie is a residential building science consultant at Advanced Energy in Raleigh who specializes in working with nonprofit developers like Habitat for Humanity to make new affordable housing energy efficient.

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