Exhaust fan with 6 inch duct.

[Greg1707]

I purchased a Nutone exhaust fan today that specifies a six inch duct. I noticed the square hole in the side of the metal box was the same size as fans with four inch ducts. The only difference was the plastic adapter that connects between the square hole and the round duct was six inches in diameter instead of the usual four inches. So instead of the four inch square to four inch round there is a four inch square to six inch round.

I wonder how much more air can flow through a four inch square hole if there is a six inch pipe on the other side rather than a four inch pipe?
Can anyone do the math?

 

[infinity]

I would guess it would be a fairly measurable amount unless the distance were extremely short. A 6" round duct will have a cross-sectional area more than twice that of a 4" round duct.

 

[drcampbell]

Here's a crude chart for estimating duct friction: http://engineersedge.com/hvac-systems/duct_flow_resistance.htm

It can be quite significant for long runs, and "long" is a lot shorter than you might expect when the fan is capable of developing only a little bit or pressure. Using a 6-inch diameter duct reduces the velocity by more than half that in a 4-inch, and reduces the duct friction by a factor of six.

The actual calculations can be found in any good fluid mechanics textbook, the standard handbooks for mechanical & chemical engineers, ASHRAE Fundamentals. and Crane's Technical Paper No. 410 - Flow of Fluids. (1942)

 

[jumper]

IIRC, one reason for upsizing to the 6" duct is more for noise reduction that air flow volume, the fan motor and cage/impellor will be pushing the same volume I believe.

 

[drcampbell]

IIRC, one reason for upsizing to the 6" duct is more for noise reduction that air flow volume, the fan motor and cage/impellor will be pushing the same volume I believe.

Yes, the velocity will be reduced by more than half and noise is a function of velocity.
The volume won't be the same unless the duct is very short; increasing backpressure will reduce the flowrate. (much like a transformer or feeder with a high impedance has a significant voltage drop with increasing load)

 

[JFletcher]

I purchased a Nutone exhaust fan today that specifies a six inch duct. I noticed the square hole in the side of the metal box was the same size as fans with four inch ducts. The only difference was the plastic adapter that connects between the square hole and the round duct was six inches in diameter instead of the usual four inches. So instead of the four inch square to four inch round there is a four inch square to six inch round.

I wonder how much more air can flow through a four inch square hole if there is a six inch pipe on the other side rather than a four inch pipe?
Can anyone do the math?

The math depends on duct length, number of turns, corrigated or smooth pipe, and elevation changes. Having 5' of 4" smooth pipe on the fan exhaust til it goes outside will result in a lot less backpressure than 55' of 6" that has 3 90s and 15' of elevation rise.

The fan outlet being 4" vs 6" will create some obstruction and backpressure, tho if it's the only obstruction, the fan is designed for it.

If you have unacceptable performance, the only options really are redoing the ductwork or installing a booster fan in the run. A larger bathroom fan will likely just make more noise if the ductwork is overly restrictive.

 

[jumper]

Yes, the velocity will be reduced by more than half and noise is a function of velocity.
The volume won't be the same unless the duct is very short; increasing backpressure will reduce the flowrate. (much like a transformer or feeder with a high impedance has a significant voltage drop with increasing load)

Yeah, you are correct. I was just trying to more explain that upsizing the duct with the same motor and cage will not increase volume. It was a simple answer to this"

I wonder how much more air can flow through a four inch square hole if there is a six inch pipe on the other side rather than a four inch pipe?

 

[drcampbell]

Actually, upsizing the duct with the same motor and cage will increase volume flowrate if the duct is long.
(ditto for making the duct smooth instead of corrugated and reducing the number/severity of bends)

Elevation change has a significant effect on pressure drop, pumping power & flowrate when pumping heavy fluids such as water. It has very little effect when pumping lightweight fluids such as air.
(unless there's a significant temperature difference and no fan, as in a chimney or a gravity furnace)

 

[Fulthrotl]

I purchased a Nutone exhaust fan today that specifies a six inch duct. I noticed the square hole in the side of the metal box was the same size as fans with four inch ducts. The only difference was the plastic adapter that connects between the square hole and the round duct was six inches in diameter instead of the usual four inches. So instead of the four inch square to four inch round there is a four inch square to six inch round.

I wonder how much more air can flow through a four inch square hole if there is a six inch pipe on the other side rather than a four inch pipe?
Can anyone do the math?

math would require a pencil, and they don't let me have
anything sharp here.

but it'll make a huge difference.
when i vented my dryer to the outside, it's about 35'.
4" discharge on the dryer, i bumped it to 6".
used straight lock seam instead of spiral. taped all the
seams and 90's.

dryer screams thru a load of levis. i have a grate on the
side of the garage, and made a u shaped tube into a
manometer, which i stuck into the duct next to the dryer.
when i hit a half inch of water pressure, the grate is
plugged and needs cleaning. a half inch of water column
in discharge pressure doubles the drying time.

 

[1793]

I replaced a bath exhaust fan for a customer about a week ago, I installed an AirKing 280 that has a 6" output. I put a small piece of 6" duct that was smooth to get it out of the Attic insulation until it could be vented out. It ran great and actually quite quiet.

My customer called and said that the fan is now quite noisy. I stopped by the other day and noticed that the fan has been vented outside to an eve vent. I asked when the noise started and he told me after it was vented.

I turned the fan on and got up on a ladder I noticed air was being forced OUT of the ceiling grate.

I'm going back in the morning to get up in the Attic to see what they did when they ran the duct. I suspect when the fan was ducted(?) they choked it down from 6" to 4" or maybe even 3".

Hopefully the problem is in the ducting.

 

[JFletcher]

I replaced a bath exhaust fan for a customer about a week ago, I installed an AirKing 280 that has a 6" output. I put a small piece of 6" duct that was smooth to get it out of the Attic insulation until it could be vented out. It ran great and actually quite quiet.

My customer called and said that the fan is now quite noisy. I stopped by the other day and noticed that the fan has been vented outside to an eve vent. I asked when the noise started and he told me after it was vented.

I turned the fan on and got up on a ladder I noticed air was being forced OUT of the ceiling grate.

I'm going back in the morning to get up in the Attic to see what they did when they ran the duct. I suspect when the fan was ducted(?) they choked it down from 6" to 4" or maybe even 3".

Hopefully the problem is in the ducting.

way too much backpressure. They probably ran metal flex and crushed it some getting over the top plate and below the roof sheathing. By the time you cut a bird's eye notch out of a 2x8 or larger, you lose almost half of your dimension where the truss meets the top plate. I too seriously doubt a 6" duct was installed.

 

[1793]

This what I found today.

I disconnected this mess and reinstalled the ducting I had originally installed, 6" duct, smooth, an articulating metal elbow with a damper on the end and the noise went away.

HO is going to get the appropriate folks in to duct this correctly.

The draw of the fan is excellent.

 

[Daniel malack]

I replaced a bath exhaust fan for a customer about a week ago, I installed an AirKing 280 that has a 6" output. I put a small piece of 6" duct that was smooth to get it out of the Attic insulation until it could be vented out. It ran great and actually quite quiet.

My customer called and said that the fan is now quite noisy. I stopped by the other day and noticed that the fan has been vented outside to an eve vent. I asked when the noise started and he told me after it was vented.

I turned the fan on and got up on a ladder I noticed air was being forced OUT of the ceiling grate.

I'm going back in the morning to get up in the Attic to see what they did when they ran the duct. I suspect when the fan was ducted(?) they choked it down from 6" to 4" or maybe even 3".

Hopefully the problem is in the ducting.

I have a question.? In the picture you show the 6" duct, why is it so Long? Is that the shortest Home Depot had?

Sent from my SM-T350 using Tapatalk

 

[JFletcher]

This what I found today.

I disconnected this mess and reinstalled the ducting I had originally installed, 6" duct, smooth, an articulating metal elbow with a damper on the end and the noise went away.

HO is going to get the appropriate folks in to duct this correctly.

The draw of the fan is excellent.


View attachment 18140

View attachment 18141

Yep, there is not enough clearance between the roof sheathing and top plate to get 6" thru there w/o crushing it.

The extra flex will be taken care of the the duct guys. They may just rip it all out and go with hard (smooth) duct, and probably (hopefully) run it thru the roof straight up.

 

[topgone]

Yep, there is not enough clearance between the roof sheathing and top plate to get 6" thru there w/o crushing it.

The extra flex will be taken care of the the duct guys. They may just rip it all out and go with hard (smooth) duct, and probably (hopefully) run it thru the roof straight up.

You should have installed a round to rectagular transition duct. Maybe a 4 X 7 inch duct will do.

 

[GoldDigger]

I have a question.? In the picture you show the 6" duct, why is it so Long? Is that the shortest Home Depot had?

Sent from my SM-T350 using Tapatalk

The pictured installation is not 1793's work, it was whoever came after him. So he may not actually know why it is so long.
I will say from experience that it takes more than a few seconds to properly cut that duct so that the ends are usable. Maybe the handyman or pseudo-roofer could not take the time.