Is my old gas solenoid replaceable on my 1959 Lennox furnace

[brian sansone]

My 60 year old furnace has been faithfully working perfectly....until recently
I replaced the fan last year.
The fan control switch went out.
After kinda familiarizing myself with it, I'm worried about the gas solenoid valve.
From just looking over the system and schematics, it seems possible that the gas valve could fail, in the open position, and there would be no other safety system to cut the gas off. If it was running (which i guess is the only way it could jam open) It would over heat, and not even the limit switch would be able to stop it. It cuts power to everything, but the pilot is just a pilot. It burns 24 / 7.
If for some reason the pilot went out.......It would be an open path from the supply through the gas chain, and into the house. That scares me.
I cant afford to replace the system at the moment.
Should I preemptively replace the solenoid. Are proper replacements available for old furnaces?

 

[AC Power]

I have a fan forced, natural gas, Lennox furnace from 1957. Of course, the motor has been changed, the pillow blocks have been changed, the impeller, high limit and fan switch has been changed, and I replaced the gas valve (as you call a "solenoid", I'm assuming). True, many pieces have been changed, but I don't want a new furnace. Solid state boards and variable speed DC motors and built-in VFD's...no thanks!

Anyways, your pilot flame has a "flame proving device", AKA, a thermocouple. This device is made of a copper tube (wire within) about 18-24" long. One end is positioned into the pilot flame, in the general area of the main burner. When this end is suffocated with the pilot flame, it generates power. This power is delivered (by the wires inside the copper line) to the gas valve. Should this voltage drop to zero, the gas valve closes completely and will not open even if the stat (thermostat) is calling for heat. No gas can flow in this scenario. On the other hand, if the gas valve is receiving millivolts of power from the thermocouple, the gas valve remains in the ON state, that is, for pilot flame gas. When the stat calls for heat, the gas valve (solenoid) open the main line, delivers gas to the burners, they ignite, and you start the heating cycle. All is good.

From what you've told me....your system is ok. Don't fall for this "cracked heat exchanger thing". I have plenty of personal knowledge on combustion should I have to prove it.

 

[brian sansone]

I was asking about the gas valve becuase I couldnt find a replacement. And an HVAC guy basically said "no way, it needs to be up graded" I would be surprised to find that an electro-mechanicle gas valve for a furnace is not available. There must be millions of old working lennox furnaces out there.

About the flame sensor. Mine doesnt work anymore, until it heats up. Then it get going. Ive been starting my furnace by jamming a screw driver between the contacts. One I hold it there for a few minutes, and the sensor sits in the main blaze, it starts working correctly. Last year, after I got it going once, it worked all winter. But I ordered a replacement. ( can you clean a thermocouple?)

The picture in on the site I ordered from was my switch. But the one that showed up has four poles, instead of two, like mine.
I just found the schematics for the new one, and it looks like it should work just fine. Is it a "universal" replacement?

It sound like you know your stuff. Why does my gas valve only have a single wire running to it, from the flame sensor switch?
Is it grounded through the metal gas lines?
Thanks for the info!

 

[AC Power]

RE: Gas Valve availability: Most all gas valves all look the same. For residential applications, you've a 1/2" NPT female IN and same for OUT. Take a picture of your valve, go to a gas fitting supplier (not Home Depot), and they've have one for you.....they are nothing odd. I would suggest you get a gas fitter to change it for you.

RE: Flame Proving Device (AKA thermocouple): Use you replacement thermocouple. Jamming screwdrivers into contacts is not the best idea unless your trained as to what circuit you are bypassing, and usually we only do that temporarily in an effort to test equipment....we always remove jumpers (screwdrivers included) when we leave site. Thermocouples can be cleaned, but it won't remediate a dead thermocouple...it will just make it clean looking!

RE: Picture? I don't see a picture, unless I am missing the location in this website regarding where to "click" to see a posted pic. Anyways, if you've purchased a switch with 4 poles, and your application only requires 2...then just use 2 (of 4) available poles. Make sure your not misunderstanding the pole identification...ensure you are OPENING and CLOSING the same set of contacts for each pole. I suspect some of these replacement switches are just as you say...."Universal".

RE: Gas valve only one wire: It actually has two wires. There is a single insulated conductor inside what your calling "a wire". It appears to you to be one wire, but the bare copper "wire" you see is actually the second conductor used to emit power (millivolts) from the thermocouple to the gas valve. In this case, the base copper "wire" you see is grounded to the furnace/gas valve/tin work. All the metallic items in the furnace, you ductwork, the electrical boxes, metallic gas pipe, copper water pipe.....they are all grounded. Gas pipe is grounded only on the consumer side (your side, that is, in your house). There is a dielectric union where the gas pipe enters the house (if metallic) that maintains cathodic protection from your gas utility.

I hope this helps.

Any more questions...let me know.