Hello all! I have an issue, and I'm hoping someone can provide some insight as to what is wrong with my system...

First, here are the facts. I have a 3 zone system, with all 3 units being 20 seer American Standard Platinum ZM A/c units with matching variable speed air handlers. Two of the units are 2.5 ton units, and the other is a 4 ton unit for the main living space. In total, I have about 4500 sq ft of living space, with 40% of the home having vaulted ceilings (10+ feet).

Two of the zones are cooling fine, and maintain relative humidity levels around 40-45%. However, one of the units consistently runs at about 55-70% humidity, and that are of the house just feels wet. Everything was replaced when the new units were installed last year, including ducting, vents, returns, etc.

So, Since I've had the a/c company out twice, and they told me nothing was wrong, I bought a humidity gauge and started some testing of my own. On the two units that seem to be able to maintain a comfortable level of humidity, these are readings: supply temp of 10-16 degrees below return air temp and humidity level at supply of 30-40%. On the unit I feel is malfunctioning, I am getting a supply temp of 10-16 degrees below return air temp, but the humidity levels are 75-90%.

Any suggestions on what is going on, and what I can tell my a/c company to look at? Or, is there anything else I can test to narrow the scope of the issue???

Thanks in advance for your help!