I need some advice. I have to Carrier/Bryant units in my attic. Both of them have the same circuit board (HK42FZ009). I went to turn on the heat upstairs and downstairs last night, and the thermostats went dead. I checked them this morning, and I was getting a "24" error message, which is the their is an open short on the 24v connection. I have bipassed the Com, by using a thermostat with batteries, instead of a 24v connection. It does the same thing, it blows the 3ATC fuse each time. By bypassing the com, I feel that I have eliminated a short in the wiring. BOTH of my units started doing this only when the heat was turned on. We also had a surge back in September. The AC still worked, but hadn't checked the heat till now. Does this sound like a furnace problem, or circuit board problem? Any help would be appreciated.