I hooked up a 75,000 BTU heater in my garage last weekend and at first everything worked fine. Later, I heard the heater running, even though the thermostat was in the off position. I called Honeywell and we concluded the thermostat was bad. Got a new one, installed it, and everything was fine for two days... the heater was off when the thermostat indicated it should be. This morning, the heater was running again, even though the thermostat was in the off position. Any ideas?

Your thermostat wires are probably touching somewhere. Shut it all down and check for continuity with a meter, if so replace t-stat wire.