# Bath Fan Duct



## T2P (Nov 25, 2016)

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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## nealtw (Nov 25, 2016)

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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## Snoonyb (Nov 25, 2016)

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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## T2P (Nov 25, 2016)

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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## nealtw (Nov 25, 2016)

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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## nealtw (Nov 25, 2016)

https://resaveguide.lbl.gov/step-6-duct-design


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## T2P (Nov 25, 2016)

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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## nealtw (Nov 25, 2016)

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....ct-residential-4-inchx10-foot.1000449710.html


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## KULTULZ (Nov 25, 2016)

nealtw said:


> 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.



:banana:

I never thought of that... scribble...scribble... :thbup:


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## nealtw (Nov 25, 2016)

KULTULZ said:


> :banana:
> 
> I never thought of that... scribble...scribble... :thbup:



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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## Snoonyb (Nov 25, 2016)

The reason I use smooth wall vent is the noise reduction, noted in the link in post #6, because any ribbed vent will affect the smooth air flow, as well as the efficiency.

Before you compose the "jig", and start cutting, what kind of roof material do you have and what size is the existing roof jack? Also mentioned in the link in post #6.

Because warm air has a greater ability to contain moisture in suspension, having that "after run" feature on the fan will allow a greater volume of moist air to be evacuated and lower the potential for condensation.


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## T2P (Nov 26, 2016)

Alright got a chance to do some digging after getting home from the holiday. Found a few more sticking points.

1) Roof vent appeared sized large enough from on top of the roof. When I removed the existing duct to check it out from underneath, however, the jack has a metal base with a 4inch round hole cut in. Meaning 6in would not fit through the hole. Would this require a replacement jack to get to 6? My main concern doing that is the roof only has about 2-3 years left on it so shingles are probably brittle. For what it is worth the vent flap gets stuck in the open position when I open it (was closed when I got up there though)

2) The exiting hole in the roof is directly above the base units exhaust. Meaning from the unit to make the exsting roof hole there would be no horizontal run and a pretty sharp bend. Both of which I understand to be no-nos.


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## Snoonyb (Nov 26, 2016)

You have some alternatives as eventualities; You can budget for the eventuality of the eventual roof being replaced, or the asphalt shingles softening in warmer wether and using 6" material, as you had planned, and several "L's", with a 6" too 4" reducer and with the understanding that the volume will only be that of a 4", or with the 4" and several "L's".


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## nealtw (Nov 26, 2016)

I agree, run the 6" to the jack and adapt to 4", change the jack or not when the roof is replaced.


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## T2P (Nov 28, 2016)

Got the fan installed, wired, and ducts placed to try them out. I ended up doing a 6in rigid run reduced from 4 at the ceiling.  There is an L from the ceiling at a 45 tilt, into a 2 foot run into a 45 elbow and then a 90 after to come back to the fan. Meant to take pictures but ran out of time.

So took a long hot shower after and while the fan seemed to be working at first, condensation heavily built up on the ceiling to the point that it dripped on the bath mat multiple times. 
Couple of potentials. I did not add insulation yet while trying it out to avoid potential double work.

Also while working up there I moved a lot of the bat insulation out of the way and haven't been able to get that back.

I do live in MN and it is 29 degrees out.


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## Snoonyb (Nov 28, 2016)

When showering with a partner you tend to ignore the condensation.


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## T2P (Nov 29, 2016)

Haha nice Snoonyb.

So I got a chance to do a couple of dry runs and not sure if either of them are actually acceptable.  I also have a couple of 4in elbows I was thinking could come off the roof too.  Thoughts?







This would require a short length of straight cut to size


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## nealtw (Nov 29, 2016)

Go with the shorter route, every 90* bend is like adding another 5 ft to the length.


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## Snoonyb (Nov 29, 2016)

Anything I can do for the social wellbeing.

Yep, shorter is better, and more in keeping with the "KISS" principal.


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## Jeffh (Dec 5, 2016)

Yeah that definetly looks like a 1st year installed that. Its cute. Instead, Install a 90 right off the bath fan outlet aimed at the jack. Then aim another 90 from the jack to the bath fan 90. Add another elbow (<90) to make up any difference needed. Foil tape or duct seal pipe joints. Wrap with 1-2 layers of 1" fibreglass foilback insulation.


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## Jeffh (Dec 5, 2016)

Oh and every 90 is equivalent to 10' of straight


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## T2P (Dec 5, 2016)

Thanks. This is basically what I ended up doing except I used 2 45s for each 90 (top and bottom) and then connected the two with a straight. Used #8zip screws to secure, metal tape, mastic and then covered with foil backed fiberglass. I used R-38 12in batts in the cavity and on top.

Future project, fix the crap insulation job in the attic. Wood shaving insulation under the floorboards and fairly thin batts on TOP of the boards *sigh*. When we get the roof redone in a couple years I plan to bring the finished attic into the thermal envelope proper.


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## Jeffh (Dec 5, 2016)

Sounds like a good install to me


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