Have A Heil Furnace Model # NULK105AH02 : Serial number: H51118647, 105,000BTU
Been having problems getting the burners to stay lit. We've cleaned it completely, put a new electronic switch on it. Still not working properly. It goes off propane, instead of natural gas. We are getting black soot coming through the heating vents when it does run for the short period. It starts fine and warm air comes through but generally after about 10 minutes or less, the burners go out and only cold, very cold air blows out.
Have had 3 furnace professionals out here and they can't figure it out either. No answers so far and it's very cold out, our outside temp. is about -22 below regular air temp. and with windshield really darn cold. So any help would GREATLY be appreciated.

You didn`t mention if the filter was clean,, or if the flame senser had been cleaned,, I`m sure if the pro`s were there they should have checked these 2 items,, but they both are very important

Filter is clean. Flame señor cleaned also,
Furnace has been cleaned inside out.
Still no heat. Thought we had it running, warmed up for about 15minutes than blew cold air again only and temperature started dropping again.
Thanks for the info though

Propane + sooting through vents = serious problem! It's very likely that you have a failed heat exchanger or some other leak in the combustion section of the unit. If this is the case, your health and safety may be in jeopardy.

Do the furnace blow back if heat exchanger is cracked