So I go to troubleshoot a mammoth unit that has been rigged by a coworker 3 months ago, and now had no heat. unit is going to get replaced by a seasons 4 unit, next week. Found Honeywell R4795 burner control bad. Picked up and replaced. Stood 10 ft back and a 12 ft flame (8ft height) backfired and caught me. Evidantly, found that during rig job, a jumper was installed to main gas valve from burner blower motor, so during purge cycle for 10 sec 1.5" gas line was wide open dumping gas... Boom!