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I have an old (late-50s era) forced-air furnace system with a standing pilot. Over the past few years it's started randomly shutting off (an audible click occurs as though the thermocouple has cooled and tripped it). I've replaced the thermocouple and even the entire pilot/thermocouple assembly and had an HVAC-employed neighbor verify my work.

It doesn't coincide with windy days and wasn't a problem 4-5 years ago, so it's unlikely to be some kind of backdraft issue with the vent.

I'm now suspecting the (ancient) control system's transformer is on its last legs. My reasoning for this is because the Nest thermostat (1st generation) now and then reports it's lost power then randomly comes back on. This doesn't seem to coincide with the pilot shutoff and is much rarer, but I'm thinking because the Nest has got a very small power draw and I'm assuming holding open the safety valve is by far the larger draw, that's usually the first to trip.

Is this a reasonable theory? Should I look into replacing the control system?

[Removed alternate limit switch suspicions and pic.]

Joshua Nozzi
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    Frankly, you should look at replacing the 70 year old furnace. Yes, it's pricey, but the amount of savings from improved efficiency will pay for the replacement cost reasonably quickly. Of course, I'm sure you know this and just aren't willing to bit the bullet _quite_ yet... – FreeMan Nov 06 '20 at 14:46
  • @FreeMan Thanks, yes, it's not something we can do quite yet but I agree completely. For now I'm trying to resolve re-lighting the pilot several times a day. I have an update coming, though... – Joshua Nozzi Nov 06 '20 at 15:21

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Usually the pilot light is normally only controlled by the thermocouple/ sensor on systems of this age probably only a millivolt controlled gas valve.

What I have seen quite often is the burners get dirty some rust on the cast iron burner holes and even spider webs on them can stop the flow then a bit of gas is not burning properly and when the call for heat is gone the main valve shuts off (not the pilot) and then there is a puff of gas that flashes and that pressure blows out the pilot on shutdown.

I usually pull the burner elements and clean the tops with a wire brush , then clean each opening with a wire just small enough to go through (a small drill bit also works if there is a lot of rust. Don’t cut any more metal away.

After completing each element I reinstall them light the pilot if it went out and adjust the flame. Each burner usually has a opening that is adjustable on the back I take them all the way to blue and work to a small orange tip, then I start over, sometimes the adjustments on the first ones will be off. If they look good I will cycle the furnace a few times to see if it is going to blow itself out again.

There can be other things but with what you explain I would go there (clean) first.

As far as the age I have worked on “ancient furnaces” some solid cast heat heat exchangers (from the 30’s) that will probably last another 50 years one had all the replaced motors listed on the side and the 1 gas valve that was replaced because they wanted to add a thermocouple safety.

If the heat exchanger is in good shape you loose a little efficiency but a higher efficiency furnace will never pay for itself compared to running some of the old beasts that I have found that will probably continue with a new motor every every decade or two, check out the burners most come out in sections when cool lift pull away from the manifold (sometimes if you hit a stop rotate~90 while pulling to the back, once they clear the manifold pull it them out to be cleaned and reinstall in reverse order.

Ed Beal
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  • Thank you, Ed. This was a very informative and detailed answer. The burner in this furnace is a single horn-shaped thing that bends 90° upward. There is a ... diffuser? ... (a metal plate on a rod) that is inserted into the top to spread the flame evenly. This is very hard to photograph, but I can say with and without the diffuser, the flames are heathy. The main burner aperture is just a single large nozzle aimed upward at the diffuser. Could that still lead to the unburned gas blowout scenario? – Joshua Nozzi Nov 07 '20 at 13:02
  • Let me ask, though, do the dripping wire terminals not concern you at all? LOL, I feel like this is _a_ problem even if it’s not _the_ problem I’m looking for. – Joshua Nozzi Nov 07 '20 at 13:39
  • The dripping terminals could be several things in my experience it is oil or an oily non conductive substance after looking more closely at the red wire there is some insulation damage that may be the root cause. I haven’t had many issues with open burners like that they usually haven’t lasted as long as the cast ones. – Ed Beal Nov 07 '20 at 15:16
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I reconsidered FreeMan's advice and just had the first of a parade of HVAC pros out to quote me for a full system replacement. I told him about the problems I described in my OP and he had a look. It seems he agrees with my initial assessment: the transformer in the control system is marginal and voltage drops are causing the gas valve to freak out (my words).

The power leads from the transformer to the valve are in good working order and he gave me my second professional opinion that I'd installed the replacement thermocouple / pilot assembly correctly. He said any power loss or power dip from aging systems and/or a failing transformer, combined would cause the gas valve to shut down everything, including the pilot. I learned my furnace used to be oil but was converted to gas decades ago (so the gas valve is certainly newer than I thought).

Based on the first quote, it's likely I'll just go ahead and replace the whole system, so we'll probably never know exactly what the cause was. I'm done playing around in my furnace.

Thanks all for your advice.

Joshua Nozzi
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