High Aspect Ratio Duct OK?

[nuuvox000]

Doing a remodel and trying to meet client desires for tall ceilings. Is an 80x12 duct okay to use? I know the aspect ratio should typically be less than four but is there an issue other than cost? I used a ductulator to make sure the static pressure drop per 100 ft was equal to the square duct size we're transitioning from. Seems to me that if the pressure drop is the same it shouldn't cause an issue but does anyone have some input on this? Thanks!

 

[3DDave]

I would be concerned with oil-canning and the noise that makes when the air or heat comes on. I don't foresee a problem with moving the air.

 

[immsk]

Other than cost - not really. SMACNA has guidance on duct reinforcement that you may want to look at and probably include in your duct construction specification.

 

[goutam_freelance]

Check the strength against negative pressure. It is expected to be more critical as unsupported span is higher.

Engineers, think what we have done to the environment !

https://www.linkedin.com/in/goutam-das-59743b30/

 

[dbill74]

Noise, specifically a whistling noise. At 80x12 you will need to pay close attention to the transition into and out of the narrow duct. Even then, you may or may not have an noise issue, air and noise can be funny that way.

 

[EnergyProfessional]

SMACNA technically limits the ratio to 4:1. the wider it gets, the more re-enforcement you need. That also adds in height. and the wider it is, the less efficient flow will be, so adding a lot of width, only reduces pressure drop very little.

 

[nuuvox000]

Thanks for the great info all!

 

[willard3]

Read also on hydraulic radius and its effect on flow.

 

[dbill74]

What kind of lighting is being used? Perhaps you can gain an extra couple of inches in height if you can sneak the duct between lights and other obstructions.

Situations like this is when you have to get real friendly with your architect. Depending on the ceiling height, 6-8 inches will hardly be missed and greatly improve HVAC performance.
Warn the architect about the potential for noise problems. Electric junction boxes or other building utility located above an 80 inch wide duct is going to be real sporty getting access to.

A soffit around the edge of the room(s) has worked for me in the past and helps the high ceiling look higher.

Sometimes you just have to get creative.

Piping for domestic water and fire sprinklers can be routed around duct work easier than you can move your ducts.

 

[nuuvox000]

willard3 - thanks, will do
dbill74 - Unfortunately this is the size we've come up with to run above ceiling and below perpendicular steel. Pipes will be offset around the ductwork. The owners want really tall ceilings and this building was definitely not made for that. I like the soffit idea but the new owners aren't a fan; ceilings seem to be their number one priority. We're sending them a price today and they may change their mind after seeing it.