HVAC Engineering question for Bathroom Exhaust

[Michael1985]

Hi everyone. hoping to get some input regarding my bathroom exhaust fan Panasonic 90CFM. I live in a condo and currently renovating the bathroom. The existing 5" duct runs vertical from the fan to the outside 35 feet with (2) 90 degree elbows. I will attempt to reposition the fan to remove (1) 90 degree elbow. That said, my question is.

1. Do I require a booster for a bathroom exhaust fan that has a 35 foot run? Seeing that the condo developer never installed one, I assume it was not required to code.
FYI, the Dryer fan has a booster and it also is 35 feet run.

2. I would like to reduce the 5" duct down to 4" duct upon entering the bathroom to the actual fan outlet. The existing 5" duct from outside will remain until it reaches the bathroom at which point I would like to reduce it to 4". Will there be a significant impact loss by reducing at the fan?

Greatly appreciate any input.

Thanks,

M

 

[electrofelon]

I would probably just install the slightly larger Panasonic that does 135 and call it good.

 

[Michael1985]

Thanks for your input, that's a great suggestion. In my line of work, it's not a requirement as per code in the province of Ontario but I have installed them in the past on certain projects that had multiple 90 degree elbows in the path. That said, this situation is mainly a straight vertical path so a larger fan will also do the trick.
Thanks.

 

[electrofelon]

Thanks for your input, that's a great suggestion. In my line of work, it's not a requirement as per code in the province of Ontario but I have installed them in the past on certain projects that had multiple 90 degree elbows in the path. That said, this situation is mainly a straight vertical path so a larger fan will also do the trick.
Thanks.

I'm sure there is a scientific way to calculate the flow reduction from the ducting, but the amount of flow actually required for the bathroom is probably not really an exact science and I prefer more flow anyway.

 

[retirede]

I would probably just install the slightly larger Panasonic that does 135 and call it good.

I’ve seen oversized fans feeding undersized ducting generate some awful noise due to the excessive back pressure. May not be the case here, but something to keep in mind.

 

[LarryFine]

2. I would like to reduce the 5" duct down to 4" duct upon entering the bathroom to the actual fan outlet. The existing 5" duct from outside will remain until it reaches the bathroom at which point I would like to reduce it to 4". Will there be a significant impact loss by reducing at the fan?

I the fan outlet is 4", you're good; if it's 5", then no.

 

[wwhitney]

You can use a duct pressure loss calculator, such as this one:

Air Ducts - Major Friction Head Loss, Online Calculator

Head loss or major loss in air ducts - equations and online calculator for rectangular and circular ducts - Imperial and SI units.

www.engineeringtoolbox.com

For 100 CFM through a smooth (rigid, not flex) 5" duct, the pressure loss will be 0.213" wc per 100ft; for 4" duct, it will be be 0.654" w.c. per 100 ft. So each foot of 4" duct would be equivalent to 3.07 feet of 5" duct in terms of the pressure loss.

You can check your fan installation manual; it should have information on either the maximum allowable pressure drop for proper operation (I'm thinking around 0.1" wc) or the maximum allowable length. You also need to know the equivalent length to use for each elbow; IMC Table 504.9.4.1 says to use 5' for a mitered 4" 90" elbow on a clothes dryer exhaust; that seems like a reasonable starting point, or the fan manual may specify. A 4" to 5" transition will also have an equivalent length, but if it's somewhat tapered, that shouldn't be more than a foot or two, I would think.

Cheers, Wayne

 

[Michael1985]

You can use a duct pressure loss calculator, such as this one:

Air Ducts - Major Friction Head Loss, Online Calculator

Head loss or major loss in air ducts - equations and online calculator for rectangular and circular ducts - Imperial and SI units.

www.engineeringtoolbox.com

For 100 CFM through a smooth (rigid, not flex) 5" duct, the pressure loss will be 0.213" wc per 100ft; for 4" duct, it will be be 0.654" w.c. per 100 ft. So each foot of 4" duct would be equivalent to 3.07 feet of 5" duct in terms of the pressure loss.

You can check your fan installation manual; it should have information on either the maximum allowable pressure drop for proper operation (I'm thinking around 0.1" wc) or the maximum allowable length. You also need to know the equivalent length to use for each elbow; IMC Table 504.9.4.1 says to use 5' for a mitered 4" 90" elbow on a clothes dryer exhaust; that seems like a reasonable starting point, or the fan manual may specify. A 4" to 5" transition will also have an equivalent length, but if it's somewhat tapered, that shouldn't be more than a foot or two, I would think.

Cheers, Wayne

Thanks for the suggestion Wayne. I will certainly use the loss calculator to determine the pressure loss.

 

[Michael1985]

I the fan outlet is 4", you're good; if it's 5", then no.

agreed. Thanks Larry!

 

[Michael1985]

I’ve seen oversized fans feeding undersized ducting generate some awful noise due to the excessive back pressure. May not be the case here, but something to keep in mind.

For sure! ill keep that in mind. Thank you!