burner relights during cycle

backprop

Good day to all.

I have a Comfortmaker gas furnace, ca 1985. It is inspected with the A/C annually, and gets good marks. It's in a small house, and the entire gas bill (heat+hot water+dryer) rarely rises above $120/mo. in winter.

It has a White-Rodgers cycle pilot system with a 36C84 valve and 3098-522 mercury flame sensor.

Occasionally, during a cycle, the main valve will close and the burner will shut down. The igniter immediately goes into action until the pilot and burner simultaneously relight. The furnace then runs normally.

Here is a YouTube video of it occurring. This was the first and only time it happened during that cycle. As you may hear, the blower was already on. Once the burner relit, it completed normally. Sometimes this won't happen at all (even for weeks at a time) and other times it might relight several times during a single cycle; you can hear the valve opening and closing from the living area.

As a side note, when the thermostat calls for heat, the pilot always ignites. There's never an issue of only the pilot going out.

This thread has been really invaluable in helping me understand how the cycle pilot system works to some extent, but I can't quite make the connection to what's going on with my furnace.

At first I thought it might be the mercury flame sensor. I believe this part is rather unique, as the WR cross-reference manual indicates it is the only one of its type and has no replacement. But then I looked at the gas valve schematic and the above URL, and thought the issue might be the pressure switch.

In short, should I call the gas company to come out and check the pressure? I've had one heating contractor say (by phone) that they can replace the whole valve unit, and another tell me they wouldn't, and I'd need a whole new system given the symptoms. I'd rather keep this one going a few years given the shape it's otherwise in. But what should my next call be? Any other diagnostic steps I can do to decide?

Thanks for your help.

unclejohn

That will be a tuff one to find the issue on since it only happens very intermittently. Could be any safety momentarily opening and closing.

Anthony Mobilio_3

Mercury flame sensor

backprop

Can you help me understand why? I understand how the flame sensor can cause the valve to shut off. But how does it then immediately turn the main valve back on without calling for the pilot again first? It seems counterintuitive that a "weak" switch could shut off AND back on "by itself" as shown.

Not doubting you at all, just want to put the sequence together in my head.

Ironman

I concur that if it only happens momentarily it would be tough to diagnose with certainty.
You could try checking you flue to make sure there are no restrictions. Also, does the inducer seem to be up to normal speed?
It could also very well be the mecury switch or a faulty limit switch. A COMPETENT technician on site with the proper instruments would have a better shot at finding it.
In all honesty, the furnace is 10 years past its normal life expectancy and I would seriously doubt the integrity of the heat exchanger. You would be prudent not to invest much $$ in that furnace and look towards replacing it with a more efficient one. You'd never expect a vehicle to last with as much run time on it as that furnace has.

Anthony Mobilio_3

The response time to the temperature of the mercury flame sensor is to quick, it is cooling down to switch the sensor back to pilot, also make sure the pilot is clean and the flame is in good contact the sensor

backprop

Ironman, there is no inducer on the unit. I will check my maintenance records to make sure the flue has been checked but I believe it has been during its maintenance.

I don't plan on sinking a lot of money into the furnace for sure, but all the components including the heat exchanger have regularly been inspected, and if a relatively simple part is required, I may be inclined to fix it.

Anthony, to part of your recommendation: the flame is on good contact with the sensor. In the video, if I look closely, I can see it glowing red.

The limit switch was replaced in 2012. Not that or couldn't be bad, but it is newer.

ratio

Check the pressure switch, keeping in mind that it's possibly working correctly. From what I've seen here, the only conditions that would allow the main valve and pilot valves to open simultaneously are (pressure switch opening and then closing without sufficient time for the flame sensor to cool off), or the same situation with failing anti-short-cycle electronics as well.

Please give more thought to replacing her. She's lived a full life, you've been in the black for the last 15 years. That Cycle Pilot is scary indeed. I wouldn't want it in my house, not after watching it open the main valve just in case we're ready for gas. There's an opportunity to dump a lot of gas into the room before we know that the flames aren't flaming.

JUGHNE

I danced with one of these that was 30 years old. Intended to change it over to electric spark, major money for the HW kit.

The new pilot would not place properly and caused delayed ignition. So the furnace was changed to a standing pilot with T-couple.
It was an old furnace in great shape, elderly lady did not want to go with a change out. Been working good for 2 seasons.

T-couple have quite a few seconds to shutting down gas valve. Seems like a long time when your used to the quickness of todays flame sensors. This was the standard for many years.
But you have natural gas with a good draft on the flue pipe...yes?
That draft would pull out any unburned raw gas that went into the heat exchanger if there was a flameout.

This could be an economical option for you if you plan on a change out soon. You will probably need the furnace much longer than my customer will.

backprop

Thanks everyone. The contractor's coming out this week. We'll give her a proper burial if needed.

unclejohn

First off are you a rugger? Second do you have a wiring diagram.

backprop

I'm not sure what a rugger is, so that's undetermined

I do have a wiring diagram - in fact the original schematics that came with the furnace, all tucked away in a little manila envelope nearby.

Ironman

If it has no inducer, then it doesn't have a flue pressure switch.
It may have a gas pressure switch.

backprop

Yes, thank you. That's what I was referring to in the OP - the gas pressure switch.

unclejohn

A rugger is one who plays rugby. In rugby there are backs and props, and hookers. I was a hooker. Insert joke here.......
So looking at the drawing that ratio has the igniter is constantly powered and only stops because the flame grounds it out. So when the pressure switch in the valve opens because it's bad the sparking starts immediately and since the pilot is still in the hot position the main valve opens and wah- la the burners and pilot lite togeather which is a good thing. You need a new valve and a new furnace as soon as you can swing it.

backprop

new valve AND new furnace!?!? I'm going to pick one or the other ...based upon the contractor's recommendation tomorrow. Thanks for all the help.

backprop

And no, I don't play rugby. I actually had no idea of the nomenclature - quite a coincidence. backprop is an artificial intelligence term - a method for training neural networks. Though I claim to be neither artificial nor particularly intelligent.