Old gas furnace
Indestructible, so replacement is unlikely
It needs a thermocouple once in a while. Pilot refused to stay lit when I tried to fire it up 2-3 days ago, so got a new TC today. No Joy.

So, not much else to fix, the **** is very hard to turn so I bought this new valve, identical to the current unit:

http://www.ebay.com/itm/201172363727...%3AMEBIDX%3AIT

Should be a simple matter of some pipe dope on the threads and ck for leaks after install, right?

Any other tips? Water heater and drier still work so I am thinking it's not a gas supply problem.

 

If pilot did lite was it lazy and yellow, pilot orfice needs cleaning, orfice is at pilot where tubing connects to pilot. Thermocouple needs hot blue flame to prove pilot.

 

You could double check the new TC with a voltmeter, leave the TC unscrewed from the vas valve, clip one lead on the 'button' other clamped on the tc copper body, should be around 30mV open circuit. if its getting good flame wrap around it your good there. make sure the center contact at the bottom of the 'well' the TC screws into hasn't gotten pushed down from overtightening thru several part changes over the years. pull it up gently with an o-ring pick if it is, Ohm out the holding coil of the valve on theses same terminals where TC screws in....cannot recall that value off top oh my head. There's a start Chris..
Brett

 

The one that I saw had an electric eye on it if it did not see a flame it shut off.The eye would get sueit on it and need cleaned.
railguy

 

Meh
New valve in, seemed OK at first
Same story now
Pilot is bigger [like almost 2" across] and blue and comes out both sides of the shield, but, after 5m or so, the big flame just goes out, and there is no pilot either. I don't have a yellow tip on the pilot flame like this guy:

'http://www.youtube.com/watch?v=UNd2TAddg78"]Pilot Light Won't Stay Lit - How to Replace a Broken Thermo Couple on Furnace - YouTube

My house is Nat. Gas, from the city, not LPG.

Relights easily, repeat.

Oh, by the way this is OLDSCHOOL
there is no electric or ohms on the thermocouple- it's a Bourdon? tube pressurized copper line to a fitting on the main valve. I have a new one in there, and two previous units on hand. Maybe the sensor tip is too high up in the flame? Lacks instruction on where to place the tip of the thermocouple.

Well, I could be wrong. This guy shows a thermocouple putting out 20mV with only heat at the tube and a voltmeter at the valve end. Interesting.

'http://www.youtube.com/watch?v=suen7QlJl4o

New valve:
http://www.ebay.com/itm/201172363727...%3AMEBIDX%3AIT

Thermocouple type:
http://www.ebay.com/itm/New-Universa...item3f3c4121b8

So, TC is new... main valve is new.
AFAIK, the transformer and thermostat only serve to call on the big flow, so even if that failed, the pilot should stay on. It would just be like the house 'stat saying "time to shut off now".

I thought maybe the blower or the switch that tells the blower to come on when heat builds up was bad, but operating the blower on Manual-ON mode has no effect on the flame or lack thereof.

Time to call the furnace guy?

 

Try this first.

http://www.fixya.com/myhome.aspx

I still say that there is a sensor that senses a flame if there's no flame or the sensor is bad, it turns the gas/fuel off.I believe the problem is somewhere in that area.
railguy

 

Chris Post me a pic of the gas valve w/ thermocouple installed please. The TC is a voltage generator, by virtue of several junction points within the 'tip' when immersed in a flame the dissimilar metals experience an exchange of electrons causing a potential difference, or voltage to act upon the pilot valve solenoid coil within the gas valve, so that when you push down on the 'Pilot' **** the TC voltage then 'holds' the coil...and the valve pilot circuit open. Allowing the valve to unlatch the safety and when the main valve is called for to produce heat it can open. There are a couple assemblies that can cause the problem...its easier if you provide a picture of the install to tell you what to do next. If you try what I suggested before (post#3) you will see the flame wrap necessary to produce voltage, about 1/2" wrap of flame around the tip end will do nicely...let me know sir.
Brett

 

I don't know much about this but I did sleep in a Holiday Inn once. When I was a kid I helped a plumber and once a guy had the same problem. It was the gas line clogged with dirt. There was a nipple where the pipe Teed off from the supply to the furnace that hung down about 5 inches. It was full of dirt. A good clean out and all was well.

 

If you have a REALLY old school furnace their is a combo control Fan control hi limit..
and if the burner is coming on for 5 minutes...does the blower ever start by itself? if not..and you have one of these controls, then the problem has nothing to do with the gas valve or TC, its because the blower isnt coming on at the right time, try leaving the fan in 'on' position (and is the manual switch you turned on...on a control that looks like this?) so blower is running, and restart the pilot etc...post outcome.

 

I have never cleaned that pipe out in the 25 yrs or so here.

The good news- got it going.

All this time I never knew that the thermocouple was THAT kind, that generates a voltage. So, I measured one of the old thermocouples while heating it with a profane [sic] torch, Ayep, 30 mV hot, about 0 mV at room temp. Huh.

Dismantled, found new TC not really all the way up in its holder. Pushed it in all the way. Not better. Removed the NEW TC and put in the one that was verified 30 mV at hot...

ALL BETTER.

Another reminder, like working on old cars sometimes:
new != perfect
[new is not equal to perfect]
sometimes a proven old part is better. For the main gas valve, though, not so much.

I dismantled the old main valve out of curiosity, found nothing really wrong, but many interesting components and fine engineering and machine work.

 

Glad to hear all is well now.

 

Also with a furnace that is that old, it's a good idea to check the heat exchanger for cracks. I've seen them "suck" the pilot out due to a cracked exchanger. If it has cracked, then it is emitting carbon monoxide into your house. Another good idea is to have a CO detector if you don't already have one.

Glad to hear you got it going