Three AHU's One Duct

[palmetto engineer]

I recently surveyed an old church that had three Trane TWE120 air handling units connected to one supply/return duct system. Two units have a 10 ton heat pump condensing unit and the third has a 7-1/2 ton. The units were tapped into a common plenum and the duct routed to the crawl space from there.

Has anyone encountered this? If so anyone have any comments?

[just_opinion]

seen too many of these. as long as they are all running and the duct can handle the CFM, then they are fine - not best.

[ralphtheplumber]

I've seen it done a few times. It used to be a popular design for multiple computer room units.

Do they have motorized dampers at the discharge of each unit?

[cxagent]

I've seen it done many times. There is a problem with the single phase blowers running backwards unless they all start and run at the same time. If one starts before the others it can start the other spinning backwards. Applying power to a backwards spinning motor may NOT cause it to turn the right direction. Using motorized dampers or backdraft dampers may not fix this problem unless the motor is spun up BEFORE the damper is opened.

Other than the motors running backwards it works fine.