If you had a client ask you questions about this duct arrangement could you answer or dance. A rather strange system here 3 zone dampers and one zone stat with a duct between the supply and return as illustrated in the pics the supply side is to the top of the pic and the return is toward the bottom with the cross over in between
from what I see is coming off the back of the supply plenum is 1 zone,comeing off the front of the plenum is the 2nd zone. and comeing off the top of the supply plenum going into the return is a bypass with a barametric damper installed.the bypass is used when one of the zones is to small to handle the full cfm required for the a/c to function properly.(example A 2 ton system = 800 cfm) zone 1 requires 400 cfm and zone 2 need 400 cfm. when both zones are calling for cooling you have a perfect and quite system, But what happens when any 1 zone shuts off ? The remaining zone still operating is now TRYING to push 800 cfm in a 400 cfm duct system.very noisey and will distroy the system early.This is why a barametric bypass damper is installed. the extra 400 cfm will be put back into the return system to release preasur and keep the volocity down.
Some sort of flow balancer? Hard to comprehend with available info.
I don’t let my clients accompany me in attics or crawl spaces for this reason: I can’t dance.
Well Mr. Charles Jones you described very well for what you could see except all 3 zones where gathered at one end of the plenum nice job:D
I would say that it has the 2 control dampers installed for future zoneing since you say you have only 1 t-stat controling that furnace.