I have a gas furnace and a daypart thermostat. The heat level never rises above or below 64 degrees, even though the temperature is set for 72 during the day. This started about 1 week ago. I have replaced batteries in the thermostat, replaced filters - and even replaced the whole thermostat. Still mainitaing 64 degrees. Any ideas.

Jump the thermostat out of the system at the furnace and run the unit. This should cause the unit to run all the time. Then see if the temperature goes up. Watch the furnace burner while you are doing this. If the burner cuts off then the problem is isloated to the furnace. Post back with results.