I posted a few days ago and now can't find my post and don't know how to find it. Anyway, my problem is my furnace kicks on and heats up but then the fan doesn't want to kick off and it blows cold air. My hubby thought about the new filter he placed and wondered if that had anything to do with it. Well, it did. When we take the filter out, the unit is normal, BUT, it won't kick off because the thermostat is right below the return air duct in the hallway and it keeps it too cool and the rest of the house it's scorching. What would be wrong that taking the furnace filter out in the return air duct would make it heat up. Would the furnace not be getting enough air? For the gas to burn properly? One other thing, I was diagnosed with allergies and was told to get paper furnace filters. We did that in the summer and had no problem w/AC, but now that it's winter, this is happening. Also, without the filter, it smells like dust coming through the vent.

I have an American Standard gas, in the attic, and the AC is store outside.

Thanks in advance for any input.

Low airflow will cause a furnace to overheat and go out on limit. You really should have a filter or the cooling coil and blower wheel will get plugged with dirt. Check everything for obstructions, check the blower compartment to be sure the insulation didn't get into the blower, open all registers and try again. Sometimes limit switch's get weak and need replaced. Find out what the temperature rise is, heat duct temp' minus cold air return temp' = rise.

What should the temperature rise be? We didn't leave the filter out, just had to replace it with the cheap one which I don't think catches as much dirt.

What furnace do you have? Generally 35-70 degrees for a modern furnace.