Bathroom fan: duct routing question
kevinjm4
| Posted in General Questions on
is it advisable or necessary to begin your duct run out of the fan straight for 18” with a slight down-slope before adding any elbows? As opposed to coming out of the fan and immediately having an elbow or two making the 90° turn toward the gable wall.
I might be answering my own question by mentioning this, but want to get the GBA opinion too. Two different manufacturers of bath fans suggest starting out the run with about 18” of straight duct before anything else.
I’ve seen diagrams and pictures of installs without this detail in them so thought i’d ask.
thanks.
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Replies
Kevin,
Duct details will depend on the fan location and the location of the termination -- whether a gable wall termination or a roof termination. Many successful cold-climate installations include an elbow off the fan, aiming straight up, to allow the duct to reach a height suitable for the sloped duct that leads from the top of the vertical duct to the gable wall termination. (This long duct should slope to the exterior to handle condensation.)
For more information on this topic, see "Bathroom Exhaust Fans."
You can have an elbow right at the fan outlet, but it should be smooth. Many installers use flex duct and choke the flow right at this 90 degree turn. Also check for the location of the damper in the fan. Some bathroom fans have a flapper installed in a collar right at the outlet of the fan. An elbow might interfere with operation of the flapper.
The way I did it was I have three smooth 6” elbows right at the outset. The three elbows together form my 90°, which also has some elevation gain to it. So it sort of swoops up and over the ceiling joist, clears it by about 4”, then for the next 10’ or so I have smooth 6” sloping downarr to my termination point out the gable wall.
That recommendation is to reduce turbulence in the airflow before it has to make a turn, effectively reducing the pressure drop through the elbow. It's good advice and if you've got space for it, go for it.
The more gradual a bend, the less it restricts the airflow. Two 45s will introduce less backpressure than a single 90, for example. Smooth (rigid) duct is also much better than any type of flex duct.
Bill
The more tortured and the longer the path, the greater the resistance and the lower the air flow. Pretty amazing how so many installers run duct pretty much like wiring, even though air molecules are quite different than electrons in terms of how they run around...
Peter