Could be mis-wired and not openng and closig properly ,or at all.Does it maintain the temp.,and respond when there's a call to heat/cool?
Max air flow is worth a try,given what you have stated. You have only two zones,one is kida small,I'd try max.,not default,to allow more air tothe small zone,so the system doesn't have as much excess air to try and deal with.
Oh it responds fine and heats or cools the zone in question. Actually over heats or cools. I think you have nailed it - this zone is too small for the system. There are new errors in the log today citing execess static pressure. So - time for my installer to re-think his duct work.
Was a load calculation done on the house? Was the ducting completely redone? Sounds like you are saying that there was a ducting issue with the original 5 ton single system, so may we assume that the ducting has had a major overhaul with the increase in capacity along with adding zoning?
By the way, a properly sized system does run more and uses less energy to heat and cool our homes. Oversized systems that short cycle or staged systems that only run in the lower stages are real energy killers.
Load calculation - I didn't see evidence of a 'calculation' from anyone who bid. I got lots of explanations of what is wrong, but they all seemed to me to be experience based, rather than a calculation.
Duct work was .. overhauled .. is the best word I can think of. For the down stairs we only did a limited cut in, so we found the ducts for my office and reworked them. Up in the attic the old system was 2 zoned; upstairs, downstairs. There was a duct board distribution box that we cut out and reworked with a sheet metal one and all the downstairs ducts were right there. My installer was dilligent on re-routing, shortening ducts and making the whole airflow equation make sense (to him). I think he may however have been unprepared for the evolution system and the minimum airflow needed for a single zone.