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I have a 3 zone system controlled by arezel stksb-e 8x24 dampers. There is also a bypass module mod-bypas1 in a 10" duct running from the cold air return column to the air handler.

Zone 1 controls the 1st floor. Zone 2 controls the 2nd floor. Zone 3 was added later to control a 1st floor master at one end of the floor.

Problem: Zone 2 calls for AC but air is flowing from vents in Zones 1 and 3 (to the point of becoming uncomfortably cold).

For example, AC is set at 74 on all three thermostats. Zone 2 (2nd floor) temp is 75 and is calling for AC. Cold air is coming out of registers in Zone 2 but it's taking time to reach temp. Meanwhile Zone 1 is at 69, Zone 3 at 71 and registers in both zones (1 & 3) have cold air pouring out.

My inexpert guess in faulty dampers? At zones 1 & 3? Is it possible for 2 to fail? Is there other potential causes? Or is it working as it should?

How do these dampers even work? Are they normally closed, opening only when called upon to provide heat or cool?

As I think of it, I don't seem to notice any issues during heating season. But could any issues with dampers be covered by heat rising to satisfy needs of 2nd floor?

Any insight appreciated.
 
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