rooftop a/c units

[chicopee]

for argument sake, let's say that I have a 50 ton ac unit mounted on a roof. Center line Spacing between roof trusses(open web steel type) is 24". must the trusses be cut to form a hole for supply & return air plenums or can the plenums fit between the uncut trusses?Let me know what the industry does.thks!!

 

[lilliput1]

Generally it would be less expensive to split the duct in multiple sections to clear the load bearing truss. Mount the unit on dunnage (elevated steel platform with column attached to building columns or beams) so the plenums can be above the roof. Run duct branches to fit within the trusses. Provide curbs at roof opening to make it air & watertight. Have structural frame around roof openings to provide structural support for the roof edge at the opening. Also give equipment weights & sizes to structural for them to check if the roof structure can support the load.
Plenum & duct above the roof should be minimum 24" above the roof to allow future repair/replacement of the roof. It not possible to elevate, then the whole duct/plenum portion must be within a air & watertight curb structure.
See also detail for streamline section around obstructions, Figure 2-8 page 2-10 1995 edition of SMACNA HVAC Duct Construction Standards Metal & Flexible.

 

[DouginMB]

If this is a new structure, you might be able to convince the structural engineer to box-out openings in the structure for your duct openings or they might shift and/or double-up trusses to allow access to the openings.

Usually the supply and return connections to rooftop units are much larger than the duct they would connect to. You might be able to blank-off a section of the opening where the truss runs underneath. If the truss runs in the dead center of the opening (usually my luck), you might have to do what lilliput suggested.

 

[ChasBean1]

I'm guessing this is about a 20,000 cfm unit. If you can use a 1.5' x 8' duct wedged between the trusses, velocity is limited to about 1,667 fpm, which isn't too bad (shouldn't have too much loss at roof penetrations). For return a 1.5' x 6' duct would do.

 

[chicopee]

I like the idea of boxing the openings and doubling up on trusses eventho it will be more work. Thank you to the particpants.