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If I reduce my bath fan duct from 6" down to 4" for a length of 5" at the exhaust point, will it have any meaningful effect on back pressure or CFM?

Is it even worth it to run 6" duct for the majority of the run if it will reduce down to 4" at the end?

I have a Panasonic exhaust fan that will compensate and run faster to maintain CFM if there is back-pressure on the duct. I would like to have the fan run as low as practical to save power and reduce noise. I cannot replace the 4" outlet that the fan uses to escape the attic without great expense, so I am trying to figure out if it really matters.

The duct run from the fan is insulated flex duct. The 4" duct to go out the louver is rigid.

References I have already found on the topic are not totally clear on this point.

https://www.energyvanguard.com/blog/what-happens-air-flow-ducts-when-size-changes/

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  • It might be easier to expand the duct exit than you think. You may want to ask a "how do I..." question about doing that.
    – FreeMan
    Commented Oct 11, 2022 at 14:50
  • It's 30' up in the air, with steep slope underneath, that is the main impediment.
    – derekdata
    Commented Oct 11, 2022 at 22:09

3 Answers 3

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You're cutting the area down by more than half (6" round is 2.25X the area of 4" round) so of course it will have an impact.

Sounds like you don't really have a choice, or are unwilling/unable to make the upgrade to 6". So you're going to get what you're going to get in terms of fan speed, which certainly would be lower with 6" all the way. If it's too much and too annoying you can always revisit the great expense of upgrading the hole in the wall.

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  • Thanks. Do you think it is worth it to run 6" duct if it will be reducing to 4" at the end or should I just leave it at 4" for the whole run?
    – derekdata
    Commented Oct 10, 2022 at 19:51
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    @derekdata 4 inches at the end would be much better than a 4 inch run all the way, if the fan is built for a 6 inch duct. A long taper at the end instead of a short taper.
    – crip659
    Commented Oct 10, 2022 at 20:22
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It will absolutely reduce the airflow. It will not reduce the airflow as much as having the whole run at the smaller size.

Since reducers are inexpensive and easy to source, you could install it as you describe. Then if you think the performance is not up to snuff, do the work to replace the exterior vent with 6 inch at that point in time.

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Yes it will reduce.

Let's do the CFM calculation to know.

How many CF is your bathroom,

Square feet x Height.

The key here is air exchanges.

Let's say the fan is 100 cfm and the bathroom is 300 cf.

It will take 3 minutes to exchange the air.

If the flow is reduced by 50%, you have 50 CFM, and it will take 6 minutes to exchange the air in the room.

In summary, you will have to let the fan run longer with the reduced flow.

Now it is your choice if that is worth extra work.

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