I bought a house last fall; it was constructed in 2000, in Mass. It's a forced air system (heat and AC). There's a furnance in the basement, and one in the attic. The house is 3000 sq ft.
Curiously, the basement unit (and attached 4 ton AC), has two zones - one for the entire first floor, one for the upstairs master bedroom (only). Each zone has a damperer to shut down the ducts, I assume that's normal (I've never had a forced air system).
My problem is that the master bedroom thermostat is 3-wires only; heat/fan but no AC hookup. An HVAC person who was in doing some work on the house took a look and said the ductwork to the master bedroom is too small (I'd guess it's about 12"), and guessed that's why AC wasn't hooked up to the master bedroom. Obviously during the heating season air gets pushed through these vents.
As an experiment, I swapped the 'heat' wire on both the thermostat and the controller at the furnance to the 'cool' terminal. As expected, the upstairs thermostat now can turn on the AC and be cooled. If the downstairs zone is on at the same time obviously there's less airflow, but even with just the master bedroom on the air seems to be flowing OK. Maybe a bit brisker than usual, but nothing that seems crazy.
I suppose I'm putting some amount of extra load on the blower motor, but it doesn't seem unreasonable to me.
Am I likely to be doing something extremely bad? I can't understand why this was ever configured this way in the first place, the units were built as part of the house, it wasn't an issue of having to fit ventwork in after the fact. I'm tempted to fix a proper 4-wire cable and hook both heat and AC up permanently, but I hate mucking with things I'm not certain about.
And while I'm here -- a second question. When the AC was first used this year, one of the outdoor compressors was dead. When opened up, the power line to is was simply melted away. I feared the worst, but with the wire connected everything worked fine. There's no sign of any other damage, certainly no sign of a lightning strike or anything that could explain. The HVAC guy didn't have a guess, said it was very unusually. This is a reasonably heavy gauge (10 gauge maybe) copper wire that was simply melted, without blowing a circuit breaker and without any other damage as far as I can tell (and the unit has been running for 8 weeks since replacing the wire).
any time you design a zoned system. the smallest duct (zone) calling for cooling must be able to handle the cfm delivered from the blower.
lets say you have 4 tons cooling (1600 cfm) and the master bedroom is getting 600 cfm,main room 1000 cfm.with both zones calling for cool system working 100% designed. now main floor not cooling with just the master calling. the blower will still move 1600 cfm to a 600 cfm area.lots of noise and strain on blower.adding a BYPASS damper will solve the problem.