Gas furnace will sometimes explode

______________________________________________________________________ Greetings, My gas furnace is of unknown age, and I don’t know the make or model number right now (I am at work.) Anyway, the problem is this: When the thermostat calls for heat, the ignitor starts to spark, and the pilot will light. Sometimes, once the pilot is burning, the main burners come on, but fail to be lit by the pilot. (The following are gusses) The gas from the main burners will build up inside the unit, and eventually light, but the built up gas will explode, and it blows the access panel off the unit. About 3 years ago, I replaced the pilot module due to the wires rusting. The unit would work after replacment, but that is when the problems started. The replacment module visually looked just like the one that went bad. It is installed in the same place, and it does produce a strong spark, and the pilot will light and stay lit. The sparking noise quits once the pilot lights. When it does light, I can hear the main burners flowing gas. Like I said, most of the time, the burners light and all is well, but sometimes not. It almost seems like the pilot might not be making a flame that is high enough in the gas stream to cause the main to light-off. I really don’t think that I installed the new pilot unit wrong, as it looks just like the old one looked. I did notice that the small supply line to the pilot is bent. Not a kink, but the diameter is smaller in one area (about 1.5″ long area) Could this be causing a reduced gas flow, resulting in a shoter pilot flame? I am talking about the pipe running between the main gas valve and the pilot assy. I have removed the main burners and cleaned them with my wire brush, and they are clean, and the openings are clear. Thanks for you help, Replies: acefurnacefixer “Re(1):Gas furnace will sometimes” , posted Mon 19 Dec 16:53 ______________________________________________________________________ We need the nodel number and make so we know what type of burners you have, then we can advise you on what may be going on(obviously you have a seriously delayed ignition). Yes! I CAN fix that! Excuse my spelling……I am paid to fix it not spell it. http://www.white-rodgers.com/common/ptech/thermo/Classic_80_prog_5_2.h tm explodingheater “Re(2):Gas furnace will sometimes” , posted Mon 19 Dec 20:27 ______________________________________________________________________ The unit is a Bryant (per the outside of the access panel) I am not sure I am seeing the correct model number, I can’t see it anywhere stamped on the unit. One the (very damp and hard to read) wiring diagram from the inside of the access panel, if I am reading it correctly, is a Model 585C. There are 5 main burners, about 1.5 feet long each (length is a guess from memory of cleaning them) The pilot assy is screwed near the middle burner, where the burners attach to me main supply pipe (the big pipe from the gas valve) The part I replaced is the piece that connects to the small gas line. The other end of this small gas line connects to the main gas valve. Are ALL the burners identical, or might one have a “cutout” or something to allow the gas a shorter path to the pilot? I sort of remember taking ALL the burners out about 2 years ago, and I might have gotten them back in the wrong place. Anyway, it’s a thought. I also noticed (while looking for the model no.) that the pilot supply tube comes out toward the botton of the pilot assy. The tube has to make a sharp bend to clear the floor of the unit. This bend is where the pipe is narrower. It is not kinked, as the pilot lights easily when heat is called for. Could it be this narrowing of the pilot tube is causing the pilot flame to be too short (due to lack of gas, but enough gas to make the thermocouple say OK), and not make it to the main gas everytime (well, until there is more that enough gas in the enclosure, which makes a loud bang when it lights off) Thanks for the help. I can send/post?? a digital photo if it would help. Archangel “Re(3):Gas furnace will sometimes” , posted Thu 22 Dec 00:12 ______________________________________________________________________ I wouldn’t be concerned at all about the short bend in the pilot tubibg. The pilot assembly itself has a restrictor or small oriface that only allows a small amount of gas through. One thing to check, if you have a gas leak detector, your main gas valve may have a small leak and it’s pooling up in the combustion chamber. Also do what acefurnacefixer said, dirty areas around the sweet spot will cause the gas to go into another direction, and also check the orifices in the manifold and make sure they are clear of any debris. There might be a small buger of old pipe dope hanging in there. Assistant Trades Supervisor HVAC 17 years exp. Licensed aircooled “Re(3):Gas furnace will sometimes” , posted Tue 20 Dec 18:33 ______________________________________________________________________ I would suggest that you have your furnace cleaned by a service company, trying to clean the burner is NOT a DIY project. The amount of air and gas mixture needs to be checked, flue and heat exchanger needs to be cleaned.If you are sensable and are concerned with your family safety call a pro. acefurnacefixer “Re(3):Gas furnace will sometimes” , posted Mon 19 Dec 21:42: ______________________________________________________________________ You have what we call “long” burners…….a minute piece of fuzz or dust in the “scoop” can cause a burner to not light. Remove all the burners……..run very hot soapy water through them……allow them to dry completely (they must be 100% dry). If you look down the inside of the burners you will see the “scoop”, it creates the crossover. If there is dirt at all in any one of the scoops they will not cross over and you will get the delayed ignition (explodstion).once they are clean and DRY putthem back in and see what happens. Yes! I CAN fix that! Excuse my spelling……I am paid to fix it not spell it. http://www.white-rodgers.com/common/ptech/thermo/Classic_80_prog_5_2.h tm [this message was edited by acefurnacefixer on Mon 19 Dec 22:56]

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