Here is my situation: I have a Ruud gas furnace in the attic and a 2 zone house (upstairs/downstairs). I have two honeywell thermostats. The downstairs heat works fine. If I turn the set temperature up, warm air starts to blow out of the vents. Upstairs is a different story. When I turn the heat up, it sounds like the furnace turns on (for about 5-10 seconds), and air begins to blow out of the vent. The air, however, never gets hot. It stays pretty cool, and because of this the temperature slowly drops throughout the night. I have swapped the thermostats and still experience the same problem.