Exhaust fan .. 6 inch duct redcucing to 4 ?

[Beachfront71]

Hi and thanks for any feedback.

We are installing a new/upgraded exhaust fan in our remodeled bathroom. Current fan used 4" duct all the way out through the wall...

The new fan (panasonic 150 whisper cieling) calls for 6" duct ..

I am wondering if it would be OK to use the 6" duct with a 4" reducer for the last 8 inches in order to use the existing wall penetration cap?

Is there anythign wrong with doing this outside of increased noise? I know the directions do not call for this but it turns a tedious project into a rather simple one .. I am tryingto weigh the positives and negatives as I can always go back and add a 6" later if there is something wrong..

The bathroom does have a window that is typically left cracked most of the year .. we are in Newport Beach where the temp's are fairly stable year round..
Thankgs for any help.

 

[CarlH]

You would really be much better off using a 6" wall cap. Using the 4" wall cap will definitely result in a reduced air flow and more noise than using a 6" wall cap. The short 8" section of 4" round duct isn't really that much of a problem, but the 4" wall cap itself will be a major factor. What is the added effort of trying the 4" wall cap and using the results to determine whether or not to change out the 4" wall cap for the 6"? If it isn't too much effort, I say go for it. It may prove to be educational.

According to the Panasonic site, a 4" wall cap is equivalent to 30' duct length and a 6" wall cap is equivalent to 40' duct length. Using one of the calculators at http://efficientcomfort.net/jsp/DuctCalculator_Web.jsp yields:
30' of 4" round duct @ 150CFM = 0.9732" wc friction loss
40' of 6" round duct @ 150CFM = 0.146934" wc friction loss
Assuming you have a FV15VQ, it is rated for 150 CFM at 0.1" WC static pressure and 123 CFM at 0.25" WC static pressure. What is the fan going to do at 0.9732" WC+ of static pressure?
This is the loss for the wall cap itself. You'll need to factor in the rest of the ducting too.

Now here is strange thing. The 110 CFM version of that fan has a 4" outlet. Using a 4" wall cap with the 110 CFM gives a friction loss of 0.525033 "wc. It seems that you may want to use a 6" wall cap with that fan too even though it has 4" outlet.

 

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[Beachfront71]

thanks for the info ...

For the record, the total length of the duct run is 6 feet ...Does that change anything?

Why would I want to use the existing 4" wall cap? Really the reason is laziness and time .. the cap is 30 feet off of the ground and is already there as oppossed to finding a ladder and cutting out the new space for a 6" cap... not to mention having to figure out how to make it fit on our siding of the house as it has a much larger footprint......

The 8" section of 4" duct is actually just the length of the reducer I purchased ....

Thanks again Carl.

N

 

[hj]

ANY reduction in size creates back pressure and/or increased velocity. One will reduce the amount of discharge and the other will increase the sound of air movement. A 6" duct has over twice the area of a 4" one.

 

[CarlH]

For the record, the total length of the duct run is 6 feet ...Does that change anything?

It does not change anything. The good news is that it does not make things worse.
For your short duct run and assuming no elbows, the wall cap will be the largest restriction in the system by far. This will hold true for the 6" as well wall cap even though it should be much less restrictive than the 4" wall cap.

You can certainly go with the 4" wall cap for now and change it out later. If you do that, I would recommend that you try the fan with just the 6" duct connected and notice how it performs and then connect it to the 4" wall cap and then note the difference. With the 4" wall cap you might get something like 100 CFM or maybe less out of that 150 CFM fan.

 

[Chad Schloss]

The 6" panasonic fans that I have looked at are the super quiet ones. when you choke the outlet, it will speed up the fan and make more noise, so expect that as well.