I have a Lennox Conservator lll G16 furnace that is over 15 yrs old. The burner lights and I can hear the exhaust fan but the blower does not come on. After about 45 sec. the burner goes out and the exhaust stops. I assume this is a safety feature because the blower didn't work. I had the exhaust fan replaced 2 yrs ago though I don't remember exactly what the part was called. The repairman said it was the fan that blew the gases out the chimney. Any ideas... I'm getting cold.