I have a 3 zone system in my house. When one zone is working, air flow is really good. When the next two zones turn on, air flow drops dramatically and if all three zones are open, there isn't sufficient air flow to cool all zones.
Here's what I think. When only one zone is open, the damper in the bypass duct opens because of excessive air pressure. But, when the other two zones turn on, it may take a while for the bypass damper to close. Therefore, a lot of cool air is circulating in the AC unit rather than through my house.
When the zoning was originally installed, there was a counter weight on the bypass damper. However, the service tech removed it several days after installation saying that we didn't need it.
I believe that because there's no counter weight, it may take time for the bypass damper to close when all zones are running, therefore, air flow is deminished.
Does this make sense or am I full of s*?