I have a gas furnace (forced hot air) that was working fine a week ago. This week I noticed it wasn't blowing any hot air, in fact no air at all when the heat is on. I can hear the furnace fire up and run when I switch the thermostat to "heat on" but no warm air comes out. I thought the fan was not working - if the thermostat fan switch is moved to "on" from "auto", still no warm air. To be sure I turned the furnace off via the HVAC switch and turned the fan on, to my surprise the fan was working. I changed the batteries on the programmable Honeywell thermostat, then finally replaced it with a new Honeywell programmable. Same symptoms, furnace on but no warm air. Fan turns on when heat is in off position. I am hoping this set of symptoms will identify a failing part in the furnace. The unit is a Carrier Crusader, gas from the 90's, heat only. I cannot find a model number markings. Any help would be appreciated!