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Using 4" duct instead of 6"

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TBFGhost

· Finish Carpenter
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Discussion starter · #1 · (Edited)
On a recent bath remodel I did, the bath fan was put in using 4" flex duct. About a 20 foot run of it. The fan that was installed was a NuTone ILF250. After getting a call about moisture in the bathroom I came over to see what was up. I found some of the connections were crappy so I re did them with clamps and the foil tape. But I still wasn't get much air flow.

I called the sparky, he said he would install a new fan and return the old as defective.

When I got home I pulled up the spec sheets and see NuTone asks for a 6" duct, not 4". The bath is 1000 cu/ft....so a 250 cfm fan should be overkill? Is the 4" duct restricting the flow that much?

Thoughts Comments?

I also should add that there are dual inlets in the bathroom, so its a dual 4" line for about half the run, then it y's into a single 4" for the other half.
 
Discussion starter · #4 · (Edited)
There is about a 3/4" gap at the bottom of the door, and I did a few tests with the door closed, door open, door open with window open. Nothing changed. I could change it all out to 6" duct, it all just up there in the attic, easy to access.

I hired this guy b/c he was reccomended to me, and he has been nothing but problems....

http://www.nutone.com/product-detail.asp?ProductID=10332
 
Discussion starter · #5 · (Edited)
I am on the phone with NuTone now to see what they have to say.

They say 4" flex is a BIG no no. 6" Ridgid is what they want.


EDIT:Called the Sparky...said he will swap the 4" flex out for the 6"

EDIT AGAIN: It is 4" foil flex that up there.
 
Discussion starter · #9 ·
Where would you guys place the remote fan? Or doesn't it not matter? Right now its installed near the gable end of the home to keep it further away from the bathroom for sound reasons, but should it be mounted closer to the bathroom?
 
#10 · (Edited)
TBFGhost said:
Or doesn't it not matter?
Click to expand...
I'd think it does matter, and to find out all the options with their downsides and upsides I'd get a manometer and air flow meter in addition to asking the manufacturer for the "fan curve".

The 'current' is CFM instead of amps, the 'voltage' is pressure, and the 'circuit diagram' is probably
house air, bathroom air inlet under the door, ductwork, fan, ductwork, outlet to outside air and, finally, outside air through infiltration going into house air, depending on how you lay this out.
And, a fan may push air easier than pulling air. . .?

IIRC, fan noise depends on several factors.
 
#11 ·
Did you say there were two 4" inlets, but was run in a single 4-4-4 Wye run? And it was vented with a single 4" outlet?

If so, it wouldn't be venting as well as it could, but it would be venting none-the-less. It's not easy on the motor, and may have a shorter life span, but you are still venting. Should that be addressed.... well, yes of course. But, the issue could be more than the ducting is too small.

This post does make me question why bath fans don't have a humidistat.
 
Discussion starter · #12 ·
flashheatingand said:
Did you say there were two 4" inlets, but was run in a single 4-4-4 Wye run? And it was vented with a single 4" outlet?

If so, it wouldn't be venting as well as it could, but it would be venting none-the-less. It's not easy on the motor, and may have a shorter life span, but you are still venting. Should that be addressed.... well, yes of course. But, the issue could be more than the ducting is too small.

This post does make me question why bath fans don't have a humidistat.
Click to expand...

Yes, the fan itself has a 4" intake and exhaust port. The two intake grills are 4" as well, so is the exhaust outlet on the home itself.

The two 4" grills "Y" into a single 4" duct that leads to the fan itself, which then exhausts through about 2 feet of 4" duct to the exterior of the home.
 
#15 ·
A quick spin of the Ductulator says bad things will happen with the 4-inch duct.
I’ve installed the Fantech remote fans using 6-inch insulated flex duct in runs before of about 20-feet or so without issue. The higher CFM fans generally have the velocity to move air fast enough to prevent condensation accumulation in the duct work. Don’t forget to install a back draft damper.
A few feet of flex duct connected between the remote fan and the inlet grille also helps keep the noise and vibration down.
 
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