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Artful Artisan 12-25-2007 09:52 PM

Furnace roars and booms
18 year old Luxaire 120K BTU propane fired downdraft furnace has begun making strange sounds. About 5% of time, furnace lights inside of, instead of on top of burner tubes, making roaring sound until turned off. Other times burner occasionally makes a single loud boom sound (even when burning normally on top of burner tubes) when turning off (even startles the dog!). Two techs visited: one replaced the gas controller -- no change. Second tech checked output pressure from controller and found it normal. He turned the gas valve partially closed, however, and incidence of burning inside burner tubes decreased but did not go away entirely. He thought the burner tubes may be cracked. Don't know what to do other than replace the furnace. Is this situation potentially dangerous? Hesitate to leave the house now with the furnace set to go on.

HvacWiz 12-25-2007 10:06 PM

Did they check for a possible cracked heat exchanger?

Artful Artisan 12-25-2007 10:32 PM

No, it was not mentioned. Could this account for the symptoms?

HvacWiz 12-25-2007 10:44 PM

While not certain, i would suspect that is somthing that needs looked into. A co reading should be done on the furnace while it is running to check if you are at a safe level or not...good luck

sgthvac 12-25-2007 11:06 PM

Yes, a cracked heat exchanger could cause delayed ignition, but dirty burners and low gas pressure could also cause the same problems. Are the burners clean and what did the tech's say the LP pressure was incoming and outlet side? You can take an air compressor and blow out the burners. Incoming pressure should be at least 11" wc and normally 9-10" wc on the outlet side.

USP45 12-26-2007 07:45 AM

I am not fammiliar with a gas furnace, but I am quite experienced as a repairman with oil fired forced hot air units. With the oil type furnace if the pump is sending fuel into the combustion chamber (Heat exchanger) before the ignighter lites it then we get too much vapor in the system and when the ignighter finally does light it, then you get a boom noise. i would look into possible late ignition problem.

Artful Artisan 12-26-2007 10:04 AM

Thanks for your suggestions. I don't believe a CO check has been run. But the burners were removed and blown clean. I do not remember the pressure numbers, but the results were nominal (tech from local propane company). Low input pressure seems unlikely since another exact same model furnace sits beside this one, works from the same gas supply and operates normally. System has fulltime pilot and appears properly positioned. At this point, given the age of the furnace, maybe I should replace the whole rig. Hate to wake-up with a CO headache (or not wake up!). Thanks again for your comments.

bigMikeB 12-26-2007 11:30 AM

it sounds more like there is an obstruction in the heat exchanger tubes, so there is no draft at the burners. I am not thaty familiar with that unit but there maybe plates mounted in the draft box end of the tubes to slow draft that are clogged with soot from poor combustion, it needs to be taken care of before you need a new house.

Artful Artisan 12-26-2007 07:33 PM

Thank you bigMikeB, and everyone else. Will get a tech out here and go into the heat exchanger and draft box for a good look.

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