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Exhaust Fan Runs, but Bathroom Stays Humid: Repair or Replace?
I’m trying to budget for a bathroom ventilation fix without replacing the fan unnecessarily. The fan runs, but the mirror and ceiling stay damp for roughly 30 to 45 minutes after a shower.
Before getting quotes, I’m planning to check the basics:
- Whether the fan’s rated CFM is appropriate for the bathroom size
- Whether the duct actually vents outdoors
- Long, crushed, poorly insulated, or sharply bent ductwork
- The exterior flap and backdraft damper
- Whether opening the door improves airflow enough to point to a makeup-air issue
For anyone who has dealt with this during a remodel, what did the diagnosis and fix end up costing? Was it usually a duct or damper repair, a new fan, or a larger ventilation project? At what point would you bring in an HVAC contractor versus an electrician?
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5 replies
The 30–45 minute damp period is a good reason to measure actual airflow before pricing a replacement.
- Hold a tissue at the grille. It should be pulled firmly against it, not merely flutter.
- An inexpensive vane anemometer can give a more useful comparison, though grille readings won’t exactly equal the fan’s rated CFM.
- Weak airflow points toward a clogged grille, failing fan, crushed or undersized duct, stuck exterior flap, or excessive bends.
- If airflow is reasonably strong, the fan may simply need to run longer after the shower, or the room may not be getting enough makeup air. Testing with the door cracked open can help distinguish that.
For the trade, duct and exterior-termination problems generally belong with an HVAC or ventilation contractor. An electrician makes more sense if the fan has power, switching, or wiring faults. A new fan won’t solve a restricted duct or blocked damper.
Before pricing a larger fan or duct repair, is there a clear door undercut or transfer grille for replacement air? It’d be useful to repeat the tissue test with the door cracked open: if the pull becomes noticeably stronger, inadequate makeup air may be the main restriction rather than the fan or duct. In that case, improving the undercut or transfer path could be a much smaller fix.
I’ll inspect the exterior termination while doing the door test. A screen, snow buildup, a flap that does not open fully, or a nearby wall or roof obstruction could restrict discharge even if the grille pulls well and the indoor duct appears clear.
For budgeting, I’d separate the exterior flap or damper service call from any concealed duct rerouting. A termination repair may be relatively contained, while rerouting duct through a finished ceiling or wall can add access and restoration work that costs more than the fan itself.
Since the door test and exterior inspection are already on the list, the key cost question is: can the duct and termination be reached without opening finished surfaces? If not, ask quotes to show ventilation hardware, access, and patching as separate line items.
Since the exterior termination is already on the inspection list, could each bidder also document the duct route and where it ends? A fan can pull well at the grille and still discharge into an attic or another unsuitable space, so that detail matters when comparing a duct repair with a fan replacement.