# Using 4" duct instead of 6"



## TBFGhost

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.


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## ScipioAfricanus

Is there make-up air, or is the room so tightly sealed so that no make-up air is infiltrating into the bathroom?

Andy.


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## CookeCarpentry

When you say flex duct - is it flex aluminum, plastic or insulated?

Difference between 4" and 6" is significant. Any way to replace with all 6"?


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## TBFGhost

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


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## TBFGhost

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.


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## HVAC Girl

Your 4" is running about 3.5" SP, way too high. Still a little uncomfortable with 6" for a 20' run at 250 CFM but it will be much better than 4". Try to get the t-fin run straight as possible to reduce friction.


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## GettingBy

Cross sectional area for 4" is (4^2)/(6^2) = 44% that of 6", so you'd have to go shorter in length for the same airflow resistance.


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## beenthere

For the same CFM at the same length, a 4" is 5 times more restrictive then a 6".


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## TBFGhost

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?


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## GettingBy

TBFGhost said:


> Or doesn't it not matter?


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.


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## flashheatingand

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.


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## TBFGhost

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.



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.


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## flashheatingand

I looked at that chart, according what I read, you would be pulling about 125CFM with the 4" pipe. Not what it's setup for, but moving air nonetheless

I think it would meet requirement for that space, but don't know for certain. Is the fan operating on a regular basis /operating at the right time?


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## ApgarNJ

I hate the flex vent, I always try to use the rigid alum vent as the air is a lot less restricted with the inside being so smooth.

sometimes there are jobs where you have to use the rigid flex alum, still better than that flimsy fake foil plastic vent.

hope this works out for you.


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## DuMass

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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