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What causes a bathroom vent fan to drip water?

steve196 | Posted in Mechanicals on

A customer has a bathrrom exhaust fan which drips water. The pipe originally went up then turned horizontal and exited the bldg. I now goes straight up. He covered the aluminum duct- short run- maybe 18″ with spray foam to insulate. Do we need more insulation?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "What causes a bathroom vent fan to drip water?"

    A. Condensation (assuming that the phenomenon happens during cold weather). It's possible, but unlikely, that it's rain entry -- but that would only happen when it's raining.

    If the duct is well insulated, this may be tough to fix. Switching back to a horizontal duct (sloping slightly to the exterior) that exits at a gable is one possible solution. I would certainly go up into the attic and make sure that the insulation job was carefully done. I would also verify that there is a backdraft damper to limit exfiltration through the duct when the fan isn't operating.

  2. steve196 | | #2

    Thank you Martin.
    Puzzling is that it has been dripping even when the weather is not cold. Another idea I read about was to increase the CFM of the fan. I will check both the insulation and for a backdraft damper.

  3. GBA Editor
    Martin Holladay | | #3

    It's possible that the house (or specifically, the bathroom) is unusually humid. Running the fan more often might help lower the indoor relative humidity and reduce the condensation problem.

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