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Old 11-25-2016, 11:16 AM  
T2P
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Default Bath Fan Duct

So I am going to install a new fan and duct in a relatively small bath (8x6.5x8.5). I am looking at an 80cfm Panasonic whisper. Currently there is very poorly installed 4inch flexible duct that I plan to replace. Run is about 3 feet horizontal, 90degree and about 5 feet to exhaust out roof vent.

Here is my plan and couple of questions. All input appreciated. My thinking is to use flexible metal from the fan to the elbows stretched tight with no bends. That will lead into 2x45* elbows to make the 90* to the roof. From the elbow to the roof, use rigid metal to the roof vent. Mastic and tape on all seams.

Questions
Fan supports 4in or 6in duct. To use 6 I would need to use a hole saw to expand the roof vent. Is this worth it or stick with 4 and just upgrade the vent?

For the horizontal run should I use a 1/4" slope downward, keep flat or slight slope upward? My thinking is the slight slope down to prevent runback of condensation into the bath.

Any other thoughts?



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Old 11-25-2016, 11:32 AM  
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They are all quiet in the display at the store, it is the duct work that provides resistance and the noise is from the fan chopping the air when there is too much resistance.
In new houses here we see mostly insulated flexible, not sure the size but it looks big. Not sure about slope, never had that problem.

If you are enlarging a hole with a hole saw cut a hole in a piece of plywood first tack that up for a guide to keep the hole saw in place.


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Old 11-25-2016, 12:19 PM  
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There are several thing to consider.

Will the fan be switched separately, or tied to the lighting.

Is the bathroom heated.

Sloping the duct down will prevent condensation from flowing back into the room, but so will running the fan for an additional time after the room is occupied. However, the down slope can also cause moisture to accumulate at the low point and eventually rote the ducting.

In a short run 4" or 6" may make little difference, however the loss of efficiency can be attributed to the size of the duct and the blower's ability to overcome it.

Using the two 45's is a step in the right direction. Around here, I only us rigid ducting for vented appliances.
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Old 11-25-2016, 02:56 PM  
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Thanks for the feedback. I noticed I made a mistake (corrected) and asked if I should keep 6. Meant should I keep 4 or upgrade to 6.
The suggestion for creating a guide for the hole is smart.

Answers to questions.
Fan and light already separated and will keep that way.

Bathroom is heated but old house with radiant heat and low insulation so stays cooler.

The manual for the fan suggested using flexible for the initial 2-3/feet to reduce noise.

The fan also has a setting to allow you to set a timer so after it stops detecting motion it stays on for X minutes after. Would likely set that for about 15 min.
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Old 11-25-2016, 03:09 PM  
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For a short run matching the size of the port on the fan sounds most reasonable. They call for flex to keep vibration under control. I don't see the advantage to change to hard pipe that would then need insulating for such a short run.
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Old 11-25-2016, 03:16 PM  
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https://resaveguide.lbl.gov/step-6-duct-design
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Old 11-25-2016, 03:17 PM  
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The fan can utilize 4 or 6. The current setup is uninsulated plastic duct that someone ran straight from the fan to the roof with no elbows and then tied bat insulation around with nylon string.
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Old 11-25-2016, 03:23 PM  
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This is the stuff we see here. I think 6" but not sure, you would loose some with the ribs inside but you gain with more sweeping bends.
https://www.homedepot.ca/en/home/p.f...000449710.html
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Old 11-25-2016, 03:58 PM  
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Quote:
Originally Posted by nealtw View Post

If you are enlarging a hole with a hole saw cut a hole in a piece of plywood first tack that up for a guide to keep the hole saw in place.


I never thought of that... scribble...scribble...
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Old 11-25-2016, 04:03 PM  
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Quote:
Originally Posted by KULTULZ View Post


I never thought of that... scribble...scribble...
The other trick is to cut a plug that fits the old hole and load it in the cutter with a spring or spacer behind it. You just need to get it started.

Things you do when you have to just figure it out.


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