Gas and Oil Home Heating Furnaces - old gas furnance won't stay lit

i'm new to this thread and have a problem with a split system central air and heat, heat is natural gas, it has a pilot light and thermo couple. it is an old system, from looking at it, it do not have a circuit board in it like some i've seen. as far as control it has a 24 volt transformer, limit control, a 24v/110v relay and three sensors, one that is in the furnance that has a ceramic base with two wires, one outside of the furnace next to the firsrt sensor,that reminds me of them old metal transistors in appearence, and the third is right at the beginning of the exhaust and it looks like the second sensor. it do not have one of them little exhaust fans that i have seen in soime units.

heres the problem, when i like the pilot, which lites fine, and rise the thermalstat until the unit engages , the furnace flames up for a few minutes but as soon as it time, and sometimes before, the blower kicks in, the furnace will shut down. and if the furnace shuts down before the blower kicks in, the blower still will kick in and run til i lower the thermalstat and it cycles off. i have replace the thermal couple, no change. have not change any sensors, but i did bypass first sensor that is normally close, with no change, and withe that sensor bypass, i check it for continuity during the process and it stayed closed.

this is a friend of mines unit and he said the problem started with having to relite the pilot lite about once a week, then progress to every three days, and then to this.

what do u think guys?

heres the problem, when i like the pilot, which lites fine, ...

...this is a friend of mines unit and he said the problem started with having to relite the pilot lite about once a week, then progress to every three days, and then to this.


There is a contradiction in these 2 paragraphs. We have to get past the pilot thing. Does or does not the pilot stay lit?

If the pilot stays lit, and the flame goes 3 minutes without blower coming on, and then the blower comes on, it could be something as simple as the fan switch is out of calibration, and is only coming on at a temperature when the furnace is at it's high limt temperature. A new thermodisc type? fan switch may solve this. The blower should be coming on sooner than 3 minutes, even if the house is only 50 inside when you first fire it up. (I kinow this for a fact as MY house is 48-50 when I get home and does not take that long.)

Tell us what temp you have in the house and actually time this sequence to let us know exactly how many minutes or minutes and seconds it is until the blower does come on.

From your description it does not sound like your unit is capable of running the blower all the time, otherwise I would suggest you do that in the meantime, just to get heat.

And the mystery of the blower coming on AFTER the flame has gone out is really nothing mysterious. You have built up lots of heat in the heat exchanger, and even though the fire went out, the heat is still in there, so the blower comes on. Like I said, it is like to it's at the limit point. Either that, it could also be that your limit switch is weak and is coming on prematurely, also.

Old mobile home furnaces I have worked on would operate on thermodiscs and I was buying new ones to solve problems like yours. In fact I remember the now defunct store I was always buying these parts from some lady who ran the store, specializing in trailer parts, and bought many of these switches from her.

sorry about the contradition, the pilot light does work fine i think, as long as i dont turn on the heat by rising the thermostat the pilot lite wont go out, but when i turn on the heat, the furnace will come on and run for a while and when or before the blower comes on the furnace will go out including the pilot lite. i am probably exaggarating about the three minutes but i will time it when i get back to the house.
the temperture here varies from 30 to 70 degrees here on a bi-daily basis, and it was 70 degrees yesterday when i was at the unit. question, is the limit control capable of cutting off all gas including the pilot lite gas?

It sounds like an old standing pilot unit with thermocouple and I do not believe this should be going out.

Really pay atention to all of this when you have the chance to observe exactly what is going on. See if the pilot goes out first while the burner is going and if that is what is causing the furnace to shut down. Or, if the burner shuts down and then the pilot is take out with it.

Observe if the pilot flame is sufficiently strong to engulf the thermocouple,. The pilot flame may be borderline weak and maybe you either have to repositon the thermocouple some closer to the pilot hood, or bend on both some to get flame into thermocouple better, or if flame is puny you have to remove the whole pilot assembly and clean the tiny orifice hole at the burner end of the pilot assembly.

It sounds like an old standing pilot unit with thermocouple and I do not believe this should be going out.



yes it is an old unit with the pilot and thermocouple, and i have replace the thermocouple at the beginning, i am almost positive that when the furnace goes out that the pilot and the flame goes out at the same time as if something is shuting it down but i will go back and comfirm as soon as my friend comes in from work this evening. i know that the flame is strong enough and that it is making good contact with the new thermocouple. if i dont turn on the frunance by rising the thermostat the pilot lite will not go out.

Make sure there also is no delayed ignition causing a concussion, that knocks out the pilot flame.

Or any visual observance that just because the burner flame comes on, that suddenly the pilot flame draws away from the thermocouple.

And the part that you say that sometimes the flames go out before the blower motor even comes on seems to rule out any heat exchanger issue, for now. If you said that the flames go out only when the blower comes on, then I'd be suspicious of that.