Heil gas furnace comes on but I only get cool air, furnace is lit and I can see a flame but no heat is coming through the ducts. The ducts in the basment is not warm. The exhaust ducts is hot, what might the problem be?

Disconnected supply line, bad zone valve, if so equipped, very dirty filter, blocked cold air return. Can you hear the blower start up?

I checked the settlement trap, no build up. I replaced the air filter. I cleaned the pilot with a wire bush. I can hear and see the blower start up and the fan is turning. The pilot will click and light and stay on for about 30 seconds then shut off for 2 to 3 minutes and try to light again ( some times it will sometimes it won't, when it dose come back on it only stay lit for about 10 to 15 seconds.