I have a carrier forced air dual zone heating and cooling system with programmable t-stats at the first and second floor of my house. The unit is in the attic. The problem is at the first floor. The heat does not work at this level. When the user set temperature goes below 3 degrees than the room temperature (per carrier manual), the heat is supposed to kick-in. But instead the thermostat resets and goes into initial set-up mode (like when your power it on the first time). This happens only with heating. Cooling and fan control works fine. Also I don't have the same problem at the second level. Heat, AC and fan all three work at this level and the heating unit starts up as the temperature goes below 3 degrees. So I tried switching the t-stats to see if the t-stat at the first floor might be the problem. The answer is no, still there was not heat even with the t-stat from the second level. Please help. Thanks in advance.

Sounds like you have a low voltage short in the thermostat wires. The thermostat will reset(blank out) and try again if the amp draw is to high.