My condo association uses propane gas (buried in the ground in a tank). I have an Ultra II 97 furnace which is about 12 years old. It heats beautifully. If the furnace hasn't fired up in a few hours and it's cold out, there is major combustion and a big boom as the pilot lights which typically either blows off the furnace front door or at least blows it out. It's only when the pilot lights. I've had numerous furnace repair people here who have cleaned and checked everything, taken the furnace apart, replaced various parts but still haven't stopped the problem. The problem began last fall when our condo association had a new gas company take over providing our gas service. The furnace repair people said my only recourse if they can't resolve the problem is to have a new furnace installed. I hate to go to that expense as it runs beautifully otherwise and heats very well. I'm only afraid of the noise if the flame that shoots out upon lighting should catch something on fire. Can anybody think of another reason for this loud combustion upon lighting the pilot??