Curbing heights for HVAC 3

[msquared48]

I have had a couple of projects recently where I had to design the roof structure to support HVAC equipment loads. I noticed that the HVAC unit suppliers supplied two heights of metal curbing (14" & 16") in addition to an option of the unit just sitting on 4X4 wood rails.

My question is, are there any particular mechanical factors that would prompt one to choose one curbing height over another, or is it just the sheer size of the unit involved? Just curious. Seems like the 14" height is pretty standard as are the 4X4's.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MechEngNCPE]

Cooling coil condensate drain depths are usually a factor, when you route it to the roof drain. The depth of the p-trap can get pretty deep, depending on the static pressure after the coil.

 

[msquared48]

So then it sounds like the curbing may be unit specific?

If so, then why don't the manufacturers list the size of the curbing that is standard to use with the equipment? I have not seen this done. I had to go thru all sorts of gyrations to get the information.

Again, just curious...

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[MechEngNCPE]

Well when I spec a RTU, I give it the shortest roof curb that will give acceptable depth for the condensate drain. I usually go no less that 12", however. But usually no more than 24" is required unless you have a large industrial AHU on the roof.

 

[msquared48]

OK. Thanks for the info.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Drazen]

When ducting is designed, it may appear that higher curb is needed to enable more room for part of duct reduction peace or bend height, enabling larger clear height on toop storey.

This can be issue in buildings with relatively low storey heights.

So you can say curb is system-specific rather than AHU-specific, and that is why it is not tied to some models - it is assembled during design and having more options is allways useful for designer.

 

[imok2]

Have you tried looking at theae people
if you have the madel # of the unit

http://seacoastcurbadapters.com/advancedSearchExec.asp

 

[cry22]

SMACNA Has some little known recommendations on height above finished roof depending on unit "WIDTH". Somewhere in the middle of that book, can't get a hold on a copy right now.

Beats me why, but it does.