I recently finished a 400 sq ft third floor attic and had an existing 2.5 ton which controlled 2nd floor replaced with a 3.5 ton unit and honeywell zoning, with single stage furnace and blower. I have now had the chance to test it during the summer. the 2nd floor (around 1500 square feet) cools great, the 3rd floor is the problem, when cooling 3rd floor alone, after a couple minutes, the DATS flashes green and the blower stays on, but the compressor cycles on and off and repeats for less than every two minutes continuously until temperature is reached. I am sure I will have the same problem only reversed in the winter with heat with the DATS overheating.
After researching, I think I understand the issue, but I do not know what my options are. My unit is trying to push 3.5 ton of AC through 1 ton of duct work when only the 400 sq ft third floor is cooling, so the temperature drops low and quick causing the DATS to shut off the compressor until its temperature rises to safe range, then repeats. This all happens in less then 2 minutes to shutoff and restart and repeats, which has to be terrible for the unit and my electric bills.
What options do I have for this now? I thought zoning was meant for different rooms, but this cant be normal. The 3.5 is great when both floors are cooling, but it is obviously oversized when just 3rd floor is going. How is this delt with in zoning applications? I realize the DATS is keeping my coil from icing up, but how can I cool my 3rd floor without this problem? I spent $ on this whole setup, and have contacted the contractor and waiting for a response. Would retrofitting a variable speed blower help? Or possibly adjusting the dampers so some of the cool air goes to 2nd floor when 3rd floor is cooling? Please help with some suggestions, i want to have some ideas for when the contractor calls me back monday. Thanks for any help.