Jules the Dolphin

Hi All, Recently purchased a 1986 Dolphin with 38k. Most everything works but the furnace is not emitting heat. Propane is hooked up and works to light the hot water heater and stove and the furnace comes on but only blows cold air. I thought maybe it just needed to be left on for a while to cycle through the air and allow the propane to flow but its been going for about 30minutes and still only has cold air coming out. Any advice for trouble shooting this? Thanks!