Published on December 5th, 2014 | by Henri0
Venting two bathroom fans through a gable wall
Q. I have two bathroom fans that now exhaust in the attic. The closest gable wall is blocked by a cathedral ceiling and skylight. The other gable wall is 28 and 21 feet away from each fan, respectively.
Is it acceptable to run the plastic drain pipe that distance? Can I run both fans into a single 4-inch pipe? What type of vent would I put on the exterior of the gable wall – something like a dryer vent?
A. The longer the run, the harder the fan works, but if you have no choice, keep the duct as straight as possible, avoiding bends. Do not run both fans into the same duct, as that often results in recirculation from one bathroom to the other.
I once investigated an apartment building where this had been done; one of the two bathrooms in an adjacent apartment was covered with mildew and mold even though it was not used. Crawling in the attic revealed the connection of four bath fans into one long duct going to a gable vent. The resistance to the airflow in the long duct caused the air to back up in the nearest bathroom, especially when more than one fan was on at the same time.
The best way to vent bathroom fans is with Schedule 20, bell-end drain pipe. These pipes come in 10 foot length, and the bell end must face the fan. If possible, place two small wood blocks of diminishing thickness under each section of pipe to encourage drainage of any condensate to the outside. Snug 4-inch thick fiberglass insulation to the pipe on each side and place another layer on top to reduce the risk of freezing of the condensate in a cold attic.
The best wall jacks are hooded aluminum or plastic types; louvered jack have a tendency to break, leaving places for rodents or birds to get in the pipe.