Gas Furnace Flame Jets Start, then Die Out


I'm trying to get an idea of what my problem might be. The thermostat triggers the furnace, one by one the gas flame jets start going, then they cut out, then one by one they try to start up again, then they all die out except for an orange glow, then the blower comes on and shoots out the cold air. I turned off power to my furnace for half an hour, and when i turned it back on, and it worked as expected. Anybody have any thoughts to the problem?
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On Feb 26, 11:20 am, snipped-for-privacy@yahoo.com wrote:

well it could be a number of things. If you are very low on gas (on LP gas) or your pressure regulator (at the supply head) is frozen it can easily cause this problem and is most likely. You could also have a bad thermal cutoff switch telling your furnace's gas valve to cut off.
best bet is the first option. It really sounds like you're low on gas pressure. If you find that your gas supply is just fine and there are no problems there- your best bet is to call a licensed professional to have them service your furnace.
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On 26 Feb 2007 08:20:18 -0800, snipped-for-privacy@yahoo.com wrote:

Take a look at your fan belt and see if it actually drives the fan when the motor comes on. When the burners fire up and and fan does not run, the heat exchanger gets over heated fast. The temperature overheat sensor then cuts off the burners. But the part where you said that by switching off the furnace and then turning it on restored normal function will need a recurrence of the problem to nail down the problem.
Watch for this. The delay in the fan starting up may be due to the fan belt being old and glazed, due to incorrect tension or even worn bearings. When the opportunity arises you might as well undo the fan belt and take out the squirrel cage assembly to do preventive maintenance. Use the opportunity to vacuum and wipe clean the furnace compartment and to clean the squirrel cage. Note the direction of the vanes and reinstall the same way. Take out the bearings and check against the shaft for wear. The bearings' holes may be ovalled.
Should you need to replace the shaft or the bearings do not use any grease or lube oil. They are dry bronze bearings and any grease or lube will be cooked dry by the heat and cause problems. Replace with a new belt anyway as the cost is negilble. Tension the belt so that the weight of the motor can just rotate the squirrel cage without slipping.
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hmmm, fan belt, I'm not sure his unit is that old. I haven't seen a fan belt on a residential furnace ever.
I agree with the low pressure of the gas, the regulator maybe.
Another thing that was eluded to is this, the thing that starts your fan in heat mode is a temperature sensor in the furnace. When the gas first ignites, it heats up the heat exchanger and a temp sensor senses that the furnace has warmed up and then turns on the fan. Also, when the gas shuts off, the fan continues to run until the temperature sensor in the furnace gets down to the shutoff point.
How long do your burners shoot out flames before they shut off? If it's more then about 15 seconds, then the furnace temp sensor that tells the fan to come on may have not been working and the burners stayed on until the high limit temp sensor cut them out, the furnace cooled some and the burners came back on but then the high limit cut them out again. I'm guessing maybe the temp sensor that turns the fan on was flakey or if your unit is new enough, the electronic board malfunctioned and the reset cleared it.
The high limit temp sensor is a safety and keep the burners from running too long without the fan blowing, ie, the furnace would get so hot it would burn up the house.
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What is the furnace brand? Take the door off and see if there is a LED on the board that flashes while it goes through its sequence of operations, if so find a trouble shooting guide and follow the instructions. Have you had anybody work on it in the past? If so, did they change out the control board or fan motor? Some furnaces when you change the intergrated control board need a separate wire that comes in another kit, if you change the board in A/C you wont notice until Heat that you did it wrong. It could also be caused by someone changing the fan motor and not reconnecting the ground wire properly. Could be the negative pressure switch getting weak and not keeping contact. If you have L/P make sure it was converted, if not converted, the manifold may have gotten clogged with ash and the over temperature switch may be tripped. If you have natural gas don't worry about any frezzing valves or whatever that guy was talking about. My guess would be the negative pressure switch, they are easy to check if you have the right tool.
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Do you have a condensate pump (for humidifier runoff) connected to it that might be clogged?
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