won't light

[brad]

Have a old BRYANT gas furnance model 125389TB serial no 10L14855 that has been a good furnance,but now the burners wonit come on.I have the pilot lit,but burners won't come on when you move the thermostat to heat.I need some advice and troubleshooting help,please.

[JWWebster]

It could be the gas valve solenoid is open. Remove one of the thermostat wires from the gas valve (power is off) and check for continuity between the 2 terminals. If you have an open circuit (no continuity) then the solenoid is bad.If you can tap on the valve and make it come on the the gas operator innards are bad. Gas valve replacement is imminent. If that is good I would look for a resetable clixon nearby and check it for continuity.  If it turned out to be that I would push the button and reset the safety, brush off the burners, vacuum out the heater and check the smoke stack for an obstruction.

[brad]

Thank you for the reply and very good info.This is a very old heater,about 40 to 45 years old.I can not find a resetable clixon or any kind of safety switch on this thing.The gas comes into the unit from the gas line thru a shut off valve on to the burner valve thru anouther oil filled type of valve.The pilot is straight off main gas inlet thru a small shut off to the pilot.No buttons or anthing to push,just off or on.What do you think...time for a new furnance?

[JWWebster]

That might be a millivolt system. I would replace the thermophile if that is the case. It cost way more for a thermopile but that thing carrys enough current to operate the heater>

Millivolt operation

[brad]

Thanks for the info and help.I'll see about changing out that thing.

[brad]

Let me make sure that I understand....the thermopile is what controls the gas valve,right or wrong?The pilot will light and stay lit even if the thermopile is bad unlike a thermocoupler.Is this right?

[JWWebster]

The thermopile puts out enough energy to operate the gas valve. All that is needed is a cheapasz 2 wire wall thermostat to shut the heater on and off at the desired temp. If that thermophile gets weak over time then it won't operate the gas valve but it might hold a pilot safety magnet, so it folls a lot of folk. This heater is worth repair even if you had to get a new gas valve because of the simplicity of it.

[JWWebster]

Take some pictures and post em