exhaust ducts six inch and four inch performance

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Greg1707

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Alexandria, VA
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Business owner Electrical contractor
Here is an engineering question?

I have an exhaust fan designed for a six inch duct. The length of the run is 10 feet. At the wall opening I have a four inch vent cap. I have two choices:

One, reduce at the fan and run 10 feet of four inch duct.
Two, run 10 feet of six inch duct and reduce to four inches at the wall cap.

Is there any difference in performance between these two options?
 
Here is an engineering question?

I have an exhaust fan designed for a six inch duct. The length of the run is 10 feet. At the wall opening I have a four inch vent cap. I have two choices:

One, reduce at the fan and run 10 feet of four inch duct.
Two, run 10 feet of six inch duct and reduce to four inches at the wall cap.

Is there any difference in performance between these two options?

The 4 inch outlet probably does reduce CFM, how much IDK, maybe about as much as having an elbow in the six inch run might?

10 feet of 4 inch duct will reduce CFM compared to using 6 inch duct. Either way will have reduced CFM with every elbow (if any) installed. Smooth walled rigid duct will have less air flow resistance then corrugated wall flex duct as well.
 
This is exactly why they do not allow electricians to do duct work in my area.:D

Do it right, make the hole bigger.

Im doing a house now where the pro vented the two 110 CFM fans into a T fitting then ran 35' to the roof cap, all in 4 inch. Each fan had 6 90's before the T then it had 4 after the T.
This is one of those jobs I should have done myself.
 
Im doing a house now where the pro vented the two 110 CFM fans into a T fitting then ran 35' to the roof cap, all in 4 inch. Each fan had 6 90's before the T then it had 4 after the T.
This is one of those jobs I should have done myself.

1/2 inch tubing and one of those air mattress pumps may actually move more air:cool:
 
The "quiet" fans are rated using the largest duct the fan itself will accept. Smaller duct will cause more noise as well as reduced airflow.
Best is to replace the vent too, but running larger duct go the old vent will still be better than using 4" the whole way.
 
Here is an engineering question?

I have an exhaust fan designed for a six inch duct. The length of the run is 10 feet. At the wall opening I have a four inch vent cap. I have two choices:

One, reduce at the fan and run 10 feet of four inch duct.
Two, run 10 feet of six inch duct and reduce to four inches at the wall cap.

Is there any difference in performance between these two options?

The larger 6" duct with restriction at the end would hypothetically flow more air than the restriction at the fan and 4" ductwork.

It seems about once every two weeks we get a call to install a new fancy high $$$ heated lit bathroom fan in some McMansion, all of which seem to be vented with the same roll of 3" flex. :slaphead:. Get a mechanical (HVAC) guy to do the ducting if it's not a straightforward connection to the existing. My HVAC skills end with a roll of ductwork tape (not duct-tape).
 
Interesting topic. The HVAC and plumbing giuys probably ask similar questions regarding electrical work.
Regarding larger ducting, yes, the CFM I would agree would increase and allow more air to be moved but one would agree that the velocity would decrease at the same time. The purpose of a vent fan is to remove oders it also is to remove humid air. The reduced velocity may allow the moisture in the air to condense in the duct causing a water buildup issue. Also, in freezing climates because of the reduced velocity the moisture may condense and freeze at the outlet which may eventually build up enough to restrict and close the outlet. I have seen this as being as problem with those HE condensing furnices where 2-1/2" may be specified and larger is commonly used.
 
Fans have maximum developed length for the duct work. Each 90 adds 5' of developed length.
 
The larger 6" duct with restriction at the end would hypothetically flow more air than the restriction at the fan and 4" ductwork.

Sure about that? When you first start the fan the air may accelerate easier in the larger duct but once static pressure is stable it really shouldn't matter where the restriction is (I think).
 
Probably about the same
total press drop = duct + orifice

drop across the orifice ~ (cfm/(4001 x area x orifice factor))^2 "wc
obviously the drop for the 6 < the 4
but since the drop is less Q at orifice will be greater
that means the drop thru the orifice will be greater for the 6

lower dp due to 6" + greater orifice dp due to more Q
~= greater dp due to 4" + lower orifice dp due to less Q

tilts in 6's favor the longer the run

you want ~ 600 fpm for good performance and low noise
assume 100 cfm
4 area 0.087 sq ft
6 area 0.2 sq ft 2.3 as much area and much less drop, 1/5th or so
4 vel 100/0.087 ~ 1150 fpm
6 vel 100/0.2 ~ 500 fpm
 
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Sure about that? When you first start the fan the air may accelerate easier in the larger duct but once static pressure is stable it really shouldn't matter where the restriction is (I think).

True, however the restriction (reducer) from 6" to 4" duct is not the only restriction. Option #1 is 10' of 4" duct. Option #2 is 10' of 6" duct. There will be less backpressure with the latter as it has a larger duct, regardless of where the restriction is located (provided it's on the discharge side). Centrifugal pumps - and fans - will always flow more media with less backpressure. However, with such a short duct run (10') and few 90s, the difference in the two designs Greg mentioned may not be significant. The orientation of the ductwork would make much more difference, i.e., moving that air straight up 10' to a roof would require more fan that moving the air horizontally 10' to an eave or soffit.

I've been researching fans and ductwork, since, as I mentioned earlier, we run into problems with existing ductwork vs new fan outlet size. On longer runs, or those with many 90s, flex, or high rises, say from 1st floor bathroom to the roofline, it may be beneficial for the EC and the HO to install an in-line booster fan, or simply buy a larger bathroom fan (say, 150CFM vs 110CFM) to achieve the desired ventilation rather than changing out all of the existing ductwork.
 
True, however the restriction (reducer) from 6" to 4" duct is not the only restriction. Option #1 is 10' of 4" duct. Option #2 is 10' of 6" duct. There will be less backpressure with the latter as it has a larger duct, regardless of where the restriction is located (provided it's on the discharge side). Centrifugal pumps - and fans - will always flow more media with less backpressure. However, with such a short duct run (10') and few 90s, the difference in the two designs Greg mentioned may not be significant. The orientation of the ductwork would make much more difference, i.e., moving that air straight up 10' to a roof would require more fan that moving the air horizontally 10' to an eave or soffit.

I've been researching fans and ductwork, as I mentioned earlier, we run into problems with existing ductwork vs new fan outlet size. On longer runs, or those with many 90s, flex, or high rises, say from 1st floor bathroom to the roofline, it may be beneficial for the EC and the HO to install an in-line booster fan, or simply buy a larger bathroom fan (say, 150CFM vs 110CFM) to achieve the desired ventilation rather than changing out all of the existing ductwork.
Is there less back pressure? The outlet is same size either way. Might take more time at start to build the pressure, but once pressure is there it is more uniform everywhere in the duct isn't it?

If your fan is rated 110 CFM it is rated at a certain static pressure. You put enough restriction in the duct to increase static pressure and that CFM is going to drop if nothing is done at the fan to increase speed or something of that nature. On the other end of the spectrum if you placed a "booster fan" on the other side of the outlet or had positive pressure already at the fan inlet you actually decrease static pressure in the duct and the CFM should increase.
 
Has anyone run into a problem with fans 400 CFM and over requiring fresh air intake?

Depends on the size/volume of the space
1500 sq ft house with 8' clgs 12000 cu ft
400/12000 = 1/30 air change per minute or 2/hour

exh fans work better if they have make-up air
otherwise they pull thru doors/windows/etc with high press drop resulting in low volume

the issue is complicated by exh conditioned air so should the make-up be conditioned?
 
Is there less back pressure? The outlet is same size either way. Might take more time at start to build the pressure, but once pressure is there it is more uniform everywhere in the duct isn't it?

If your fan is rated 110 CFM it is rated at a certain static pressure. You put enough restriction in the duct to increase static pressure and that CFM is going to drop if nothing is done at the fan to increase speed or something of that nature. On the other end of the spectrum if you placed a "booster fan" on the other side of the outlet or had positive pressure already at the fan inlet you actually decrease static pressure in the duct and the CFM should increase.


Yes, the 6" duct with 4" reducer at the end will have less backpressure than the reducer at the fan to 4" ductwork. When or if the CFM approaches zero, that would be shut off head pressure, and you're correct, you'd have to upsize the fan since changing maximum speed generally isnt possible with a bathroom fan

An electrical correlation would be having 100' of 10ga wire feeding a luminaire with a 14ga whip vs having 5' of 10ga wire coming to a j-box then 95' more of 14ga wire to the whip. The 2nd circuit will have more resistance. The 6" duct is the 10ga wire; the 4" would be the 14ga. The numbers arent specific by any means, just picked as an example.

You are correct that fans are rated at a certain backpressure; the 110 CFM fan may flow that with, say, 0.1in WC (water column), and might drop to 90CFM with .25in WC and 65CFM with 0.5in WC. You are also correct in that the booster fan would reduce the static pressure in the ductwork to allow the first fan to move more air.

I was thinking booster fans may be a better option with convoluted (high backpressure) duct setups where standard bathroom fans of any CFM would have a hard time moving air, or where the HO wanted the quietest possible operation - high CFM fans typically have higher SONE ratings than lower, and more restriction generally produces more noise.

Getting a switchleg to the booster fan so that it runs when the bathroom fan is switched on is the only technical/physical aspect that may be a bit difficult, but would be a lot easier than replacing ductwork (which an EC really shouldnt be doing, and in some areas cannot) or vents located 30' in the air on the side of a house.

Then again, in some cases an HVAC guy should replace the ductwork to current code and properly for the sized fan. A higher end bathroom fan installed can run upward of $800 for parts and labor; if the HO can afford that, s/he can pony up another few hundred for proper ductwork.
 
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