Zone Control Dampers broken again - need advice

I have a 40 year old, 1700 square foot house, California. Gets to a low of 30 degrees in the winter and 110 in the summer. Five years ago had new ductwork installed, along with Trane furnace and 3 ton air conditioner. Honeywell motorized MARD dampers and zone controller used to regulate air flow for upstairs family room/home office and the rest of the house downstairs. We went with the zone control as the upstairs room was at the time used as a home office, and the rest of the house was unoccupied. And now most of the time the lower section of the house is used and the upstairs room is unoccupied most of the time. In the summer though the upstairs room is like an oven, it can easily be 25 degrees warmer than the downstairs and can get up to 100 in the room.
There are two problems with the system now. The most serious is that the damper that opens to allow air to go upstairs seems stuck in a halfway open position. The second problem is that on the very hot summer days and both upstairs and downstairs t-stats are calling for air conditioning, the system really can't keep up. The temperature of the upstairs continues to rise even though the compressor runs constantly. I specifically mentioned to the people who installed the system that I wanted ice cold air blowing out of the vent upstairs, and I was assured that 3 tons would be enough. Then a year later one of the a/c techs came by to service the damper, I mentioned that the system couldn't keep up, he mumbled something about how we should have gone with 4 tons.
Now I just don't know what to do. I don't want to call the people who installed the system, as they just can't seem to get things right. The tech who came by to fix the damper the first time (the motor slipped off the damper shaft) burned out a zone on the zone control board. Moved the wires to an unused zone on the board and things sort of work. And then he said that it's very difficult for a motorized damper to close completely when the blower is operating full blast, almost like he was hinting it's not going to be the last time we have trouble with the dampers. I've also never been completely convinced the system is fully charged, although their expert tech said the pressure is at the high end, he can't put in any more R-410A or the overpressure switch will kick in, it was doing that when they first charged the system. They keep saying the system will only drop the air 20 degrees from the outside temp, so if it's 100 outside, I should be happy to get 80 degrees coming out of the vent.
If I can get somebody to crawl under the house and fix/replace the bad damper, the system itself is still not going to work as I wanted as the compressor just doesn't seem to be able to cool down the entire house on the hottest summer days. If a larger compressor were installed outside, does that mean a larger coil has to be installed inside the house and replace the existing coil? Or maybe the whole zone control thing with dampers is going to be a money pit and never work, and should be scrubbed? And how do I know if 3 tons is enough to cool the house. 4 tons? 5 tons?
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you got taken to the cleaners. have you heard the not-so-good-news?
Ca WILL experience rolling brown-outs this summer. Forget about your ac woes and think about installing a backyard inground swimming pool.....
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From the information you listed above... get a new HVAC Company and get one that KNOWS what they're doing this time.
What size? Until an actual 'load calculation' is done... it's all a guess.
20 degree drop... it can be whatever you want it to be. It just means you need a larger system, which also requires larger ducting.
No more refrigerant or switch will trip... sounds like they never heard of Subcooling and Superheat. Which is a complete SHAME!!
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If you have to replace the system to get the cooling you want then bag the zone control and go with two systems. Either that or find an hvac company that knows how to work on zone systems. If the upstairs is really small see if the 3 ton is not overkill for the downstairs and if a 1 1/2 ton will do for the upstairs. If the 1 1/2 ton is too big ask about a mini-split for the upstairs. But you don't want to oversize it all.
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Five year old system won't cool the house even when it runs constantly. I'm sensing that your tech needs to go back to basics. Check the super heat, and sub cooling. With any luck, the condensor is in serious need of chemical cleaning.
As to cooling, the system should be pumping out 15 to 20 degrees cooler than the air coming in the return ducts.
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Christopher A. Young
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