Thought I'd get a second (or 3rd, etc) opinion on the new Trane 4-Zone system I just got put into my house. I'm in CA and I get sun on my house from sunup to sundown. We replaced the old (30 yr +) system with a 4 ton dual phase XL19i compressor and 4 ton XV80i variable speed furnance. It has the trane zone controller and 4 damper controllers.

3 of the zones are doing great so far. But the smallest zone (a 10 inch damper branching to one 5 inch and one 7 or 8 (?) inch duct) is pushing more CFM than I'd like, causing the furnace to spin up hard (well, harder than the other zones) in order to push the required CFM. I've set the controller card down to 20% (as low as it'll go) and I've even adjusted the furnace to only push equivalent to a 2.5 ton furnace to get the smallest amount of CFM to that zone and the fan is still too loud.

I'm having the contractor come out tomorrow to put in another register on the bigger duct run (of the small zone) to relieve some of the pressure on that zone. It'll still be the smallest zone, but hopefully the furnace will be able to push the CFM it needs to without straining.

So here's my question(s).

Is there an actual downside to having the Furnace set at 2.5 tons vs 4 tons?
In order to get the same airflow in the other zones I changed them from 40% to 65%. Will this mess things up when multiple zones are open (having a total of 215%)? Also, does anyone know how the Trane Zoning system adds up the percentages on the cards? My installer said I shouldn't go over 200% total for the 4 zones, does anyone know why? Is the 20%, 30%, 40%, etc that I select using the dip switches a percentage of the furnaces max CFM? Can the furnace push more than 100%? It seems odd that 20% is the smallest zone you can have if it's 20% of 100% CFM for the unit and you can have 8 zones...on avg each zone would be 12.5% of 100% of the ducts.

I guess basically what I'm looking for is a better understanding of how my system works. My installer has been good about getting it working and the quality of his work has been excellent, but hasn't really explained it in detail to me yet or given me the "whys" of it. I get the feeling that systems like this in residential applications are still a bit of a mystery...even to the installers