Bath fan venting: when to transition from 4″ to 6″ duct
I wanted to get some advice on when to transition from 4″ to 6″ duct from a bath fan that will be serving a small bathroom with a shower. The output of the new fan I want to install is 70CFM with a 4″ output. I had my roofer install a 6″ vent cap on the roof so at some point I need to transition to 6″ ducting. Is it better to make the transition as close to the fan as possible or very close to the vent cap so that the air flows at a higher pressure through the 4″ duct? Unfortunately I have to route the ducting out through the roof – venting it through a gable or wall would be better but the house has a hipped roof.
Replies
It's best to follow manufacturer specifications in order to maintain proper performance. Call the manufacturer of the bath fan to get the best advice for your specific application. My guess is that a 4" duct should be vented with 4" roof duct boot. Along with that guess is the guess that adapting the duct size at the very end of the run would have the least impact on performance. I say that becasue I imagine that your roof boot has a flap that needs a cerain amount of positive air pressure to operate properly.
Length of the run
Thanks for the info Dan. The length of the run is about 8 feet. I live in the midwest so condensation in winter is a serious issue. Both the ductwork and fan housing will be very heavily insulated. The fan housing will be fairly close to the roof sheathing (probably about 5" on one side). The attic has about 8" of blown in cellulose. Any suggestion on what sort of insulation I should use over the fan housing so it stays a little warmer? Additional cellulose on top of it or is expanding foam better?
In that case I'd stick with the 4" to as close as possible to the vent. And the vent MUST have a flapper to close it off when not operating -- if not, replace it.
In terms of insulation, it doesn't matter much what you use over the fan -- insulation is insulation. Do tape closed (with FOIL duct tape -- not the Red Green stuff) any unused screw holes, etc, before putting insulation over the fan, so that air (and insulation) from the attic is not drawn into the fan.
Room Size
Way out here in the West, inspectors are getting very meticulous with calculating the volume of air in the room to size the fan and related ductwork. Even if this is not being inspected, you can research the code in your state/area and find the info easily. Also, I think that the mechanical code requires a timer for the fan, so that it runs a specific amount of time, rather that be shut down when you leave the room.
All good ideas, worth checking out.