Old Bryant Frunace crapping out

[Old Furnace]

I have an older (40 year old) bryant furnace that is not lighting. The pilot light remains lit and the blower is going but no hot air. I can see that the burners are not igniting. Any *

[Freon]

You have a standing pilot light. Have someone call for heat at the thermostat while you are at the furnace observing. You should hear the gas valve open... a distinctive metal sound. Then you should hear the flow of gas to the main burners which should light. Tell us what does and what does not happen.

[Old Furnace]

Ok I gave the thermostat trick a try and of course the damm thing worked perfectly. Throught the day today when mt wife was home she said it just blew cold air for a long period before it would finially kick in and heat up. I have a feeling that at one point it will just not kick on at all. What more can I *

[Old Furnace]

The unit is a Bryant series SE-G Upflow - 125-394U

[how]

Intermitant problem with call for heat and no main gas.
It could be a faulty thermostat not calling for heat...Jumper W & R at the thermostat base to bypass.
Problem thermostat circuit....Jumper the thermostat W & R connections in the furnace to bypass.
Faulty transformer/gas valve or limit safety circuit....Need meter to measure 24 V.

[Old Furnace]

Thanks for the responce. I have tested the thermostat, and it works fine. The unit when working well responds well to any change I make to the thermostat. I just cannot pinpoint why the unit on occation will run and run but not fire up the burners. Tonight it worked perfect but I'm worried it just will not last.

[how]

I think I mis understood your furnace complaint.
Are you saying that sometimes when there has been a call for heat, the burners are not firing but the fan is running on.
Do you have your thermostat and thermostat wiring set up for remote blower * Probably not.

When your wife says the furnace is blowing cold air, how close is the room temp in the thermostat room to the temp that the thermostat is calling * (dif between the room temp and what the thermostat is asking *) Are there times when you are trying to get more heat and you can'*

Failing fan/limit controls are the usual cause of a run on blower on a 40yr old furnace. The limit side of those same controls can also prevent the main gas from coming on.

[Old Furnace]

To ansewr the first question, Yes, when the temp drop below the thermostat setting the unit tries to kick on as normal. The fan kick on and in some cases the unit tries to fire with a loud boom noise but doesen't always fire up the burners. It seems after several attempts the unit will fire up.

To answer the second question, I havent really momitored this closely but i have noticed a two degree difference between the setting and room temp before kicking on. This seems to be working as normal with the responce for the call for heat.

In addition i have noticed some ash around and by the floor of the unit. The problem is this is sporadic and we still have heat but the unit is injured and limping to the finish line.

[how]

You may also have other furnace problems but the most common cause of a "boom" sound and the discovery of ash around the furnace on a 40 year old furnace is probably one of two causes
..A dirty crossover between the burners. When the burners are burning you should see a ribbon of flame conecting them all together.
Any dirt/rust/debris in this crossover will cause one or more burners to not ignite immediately with the others. In a call for heat when enough un ignited gas builds up in the burner chamber it finally ignites in a concussion that makes the boom you hear and dislodges ash from inside the heat exchanger or the furnace vent pipe.
...The other possibility is that your gas valve doesn't shut off completely every time. It allows a small amount of gas to fill the heat exchanger so that when there is a call for heat both the burner gas and the excess accumulated gas ignite at the same time.

[MTI STUDENT]

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