Need help with Duo-Therm furnace

[austinado16]

I'm installing an older Duo-Therm furnace and was wondering if anyone here had experience with them.  It's all hooked up and I can light the pilot, but after holding down the gas knob long enough, when released, the pilot goes out.  

I've checked for millivolt output from the thermocouple and it puts out 10mv which is correct.

So what does the thermocouple energize inside the gas valve?  And could that be something that's either stuck inside the gas valve, or is there something on the gas valve end of the thermocouple that isn't functioning?

[tknick]

Sounds like the thermocouple is either bad, or not aligned properly.  I'm going with alignment b/c you said the output voltage is correct.  Sometimes they can work themselves out of alignment because of the vibrations during travel and therefore not heat up enough to keep the gas flowing.  Take a look at it and see if the tip is in the center of pilot when lit.  If not, bend it back into the flame.  Hopefully that'll solve the problem.

The symptoms you describe sound like classic thermo-couple problem for any gas appliance.  It's working when you hold the pilot knob down because you are effectively bypassing the thermocouple.  The assumption being you do not need a safety shut off because the operator is standing her pushing the knob down.  If the flame went out, you would obviously turn the gas off.  The thermocouple is there so when no one is around if flame goes out it gets turned off automatically and no big BOOM.

travis

[wavery]

I get this problem a LOT in our apt building. All of our apts have gas fireplaces and no one uses them. The thermocouple gets all carboned up and refuse to work. I used to replace them but a few years ago, a Gas Company repairman showed me a little trick. He scraped the the bulb with a a piece of emery cloth and it worked. I do it all the time now and it always works.

[austinado16]

Thanks for the tips guys.  Unfortunately, the sensing end of the thermocouple is right in the flame.  The 10mv is also a good indication of that.  I held the pilot on today until the sensing tip was glowing red and still the pilot went out when I released the knob.

There's some sort of magnet valve in there right?  When you're holding the button down, you're depressing that valve and then when the thermocouple makes it's 10mv, it becomes capable of holding the magnetic valve open.  That's the clank you hear when you shut the gas off, the thermocouple cools, voltage drops off and then the thermocouple can no longer hold the magnetic valve open.

Sound about right?

So I guess my question is; is the valve part of the gas valve assembly, or is it something on the gas valve end of the thermocouple?  (I haven't replaced a thermocouple before so I don't know what they look like on the gas valve end.....or what the gas valve looks like inside there.)  Guess the next step is tear into it and see what's going on.

[AustinBoston]

So I guess my question is; is the valve part of the gas valve assembly, or is it something on the gas valve end of the thermocouple?

It's the part that the thermocouple wires/tube attach to.

Austin

[wavery]

The "clank" that you here is the gas valve solenoid. It is an electromagnet . When the the thermocouple is heated, it will send a signal to the solenoid when the pilot button is released. It should get 12v (minimum). If it's getting less than 12v, you might want to check for a bad ground (if your battery is over 12.2v).

[austinado16]

Haven't pulled my thermocouple yet, but looked at the ones for sale on ebay.  There isn't anything at the end that goes into the gas vavle on them.  Mine actaully has 2 wires that come out of it's gas valve end and they go up too a temp sensing component which is an overheat safety.  That component is good (checking with a ohm meter) and I am getting 10mv in the wires, just like the owner's manual says.

So what does the gas valve end of the thermocouple do inside the gas valve.  Is it electronically holding something open for the pilot's gas supply.....and at the same time sending an "Okay" signal to the gas valve's main electromagnetic goodie?

[austinado16]

Progress!

Removed the pilot solenoid assembly from the main gas valve assembly, and took the thermocouple out of the pilot solenoid.  Now I understand how they work:  Thermocouple creates millivolts (10mv in my case) and the end of the thermocouple that's screwed into the pilot solenoid transfers that voltage to the magnetic windings of the pilot solenoid (or safety solenoid in the case of an LP fridge). The windings are then "charged up" and create the magnetic effect that is strong enough to hold the valve open.

When I pulled mine apart, I found moisture corrosion on end of the thermocouple where it screws into the pilot solenoid.  In my case, I've got a second temp sensing safety, where 2 wires engage the thermocouple end in a seperate little plastic connector.  They make sort of an electrical "U", through a heat sensitive coupling.  My contacts were all corroded, and since it's millivolts, there was no way for the current to get through the corrosion.

I cleaned the corrosion and tested the operation of the pilot solenoid right there in my hand by heating the thermocouple probe with a BBQ lighter while depressing the plunger.  Sure enough, after about 10secs there was enough voltage to power up the magnetic windings and hold the plunger down.  

So.....I should have a working furnace now.  Time to put it all back together and "fire" it up.

[austinado16]

Success!  Man, does it crank out the heat!  I like how the blower fan runs for quite while after the thermostat kicks off.  Neat little unit, especially considering it was built in 1980.

Geez....with a working fridge, furnace and a new-to-me carryout stove, I think it's time for a little excursion!