I have an intermittent problem with my central heating system and now that it's cooled off again it's back. I have a Carrier "dual pack" HVAC unit, roof mounted. This is a heat/AC unit in one box up on the roof. When it is functionning properly and I turn up the thermostat, I hear a faint hum overhead, not the regular blower but a quieter one. This is followed in a few seconds or so by a clicking or sparking sound and a "fwoomp" from the gas lighting. The gas runs for a while and then the main blower comes on. Right now, when I turn up the 'stat, I get the low hum and that's all, no click/spark, no "fwoomp", no heat. It's my uninformed theory that the low hum is a blower pulling down the atmospheric pressure in a burner housing or something like that, and that when the pressure drops the spark is an electronic igniting device. So possibly the burner housing (if that's the right word for it) doesn't seal, or the pressure sensor is bad, or the spark igniter is bad. If anybody out there is an HVAC expert I'd appreciate any feedback. I have an appointment next Monday to have it checked out, and just hope it keeps malfunctionning until then so they can diagnose it, but in the meantime I'd like to know a bit more about how these things work and what the most likely problem is, not to mention the cost of repair. I only use 2 rooms in the house and have electric heaters for them so I'm not gonna freeze, but would sure like to get the central system back on line. The unit is about 15 years old, cools fine in the summer, but has had this intermittent heating problem the last 2 winters (this'll be the 3rd) TIA for any help or advice!