We have 2 HVAC systems. I believe the Zone 1 (1st floor) also feeds the basement (I could easily check, too lazy and not sure if that's important). There were 2 returns in the wall we cut down to install that barn style sliding glass door wife wanted. One return for the Zone 1 and one duct that went upstairs to return air to Zone 2.

So the return that went up to the 2nd floor was interrupted, but there is another return. We had to reconnect a room upstairs to another return to Zone 2.

back on the 1st floor where we install the door, between those 2 rooms we installed a fancy register in the floor to allow return. I realized that also feeds the 2nd floor unit. So to clarify:

1st floor return is not affected, just in the floor instead of in the wall.
2nd floor return is re-routed for one room, but it also takes air from the 1st floor.

If I really wanted, I could open up the HVAC ducts and seal the part for Zone 2 that such air from Zone 1. Should I do that? we usually keep both units ON at very similar temperatures.



Edited 1 time(s). Last edit at 01/12/2021 08:36PM by space-time.

TL/DR version: after installing the door, Zone 2 sucks some air from Zone 1.

Good luck. Going to have to ask an HVAC person.

I would have thought that you would want zone 1 and 2 segregated for their returns. Doesn't make sense to me because you need the return to be pulling air from the rooms it is pushing air into. That is how you get airflow in the room to warm them up.

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C(-)ris
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Input and return capacity should be roughly equal.