Why do I have to keeping holding the pilot button in-order to keep the pilot lit?

The gas line coming out of the valve and going to the burners looks pretty twisted up. I'd be fairly uncomfortable using it that way in someone else's house.

Hmmm… I thought Millivolt systems had wires from the Power Pile / Thermopile unit, that looks like a thermocouple system to me. Is (was) the tip of the thermocouple sufficiently in the plot flame ? It may have just needed a thermpcouple. It looks like you have gone way too far too fast.

Wow you took the Gas valve apart, the real question here now is how many folks heads just exploded !!!

Anyway If the gas valve is actually defective the whole gas valve gets replaced, you can't get repair parts for them.

BTW the fire extinguisher IMO is too close to the work area situation. If something bad happened that you actually needed the fire extinguisher and you backed away from the fire by reaction you may not be able to get to the fire extinguisher.

What was going on here? Was something leaking ?

No guarantee the replacement thermocouple was actually good, the quality of replacement parts in this country are at an all time low, its a race to the bottom.

I suppose since it looks like you have the pilot valve solenoid removed from the gas valve assembly you could test the solenoid and the thermocouple as an assembly. With the thermocouple attached to the solenoid assembly, heat the thermocouple tip up with a propane torch or some like flame and see it the solenoid holds the valve in the open position when pushed into the coil body, like pushing the Red button. When letting it cool down after the test don't let the spring launch the parts across the room (never to be seen again) if they are not captive.

Sorry about missing the putty thing I was distracted when I read the original post

the only definitive way to verify thermocouple condition / function is to use a multimeter with mv readout capability i would expect 25-35 mv from a good thermocouple a (thermopile much higher) . just because you screw the t-couple into the magnet and test it by depressing and heating the tip of the thermocouple with a torch is not fully conclusive the magnetic (solenoid) could be bad..

to the guy questing if this is a millivolt setup or not. what voltage does a thermocouple output?

are we going to talk about the soot?

can you show us the old part that you replaced?

some thermopiles have a coaxial connection like a thermocouple instead of the typical 2 spade terminal connection that most thermopiles have. if you replaced a thermopile with a thermocouple it won't produce enough power to open the gas valve.

the gas valve should be replaced now that it has been disassembled

can you show us the label on the gas valve? it looks like it is millivolt but the valve wouldn't get hot as a result of a call for heat unless it is a 24vac system with a transformer somewhere. did it work after the electrician touched it?

are we going to talk about the soot?

Apparently back in the day the Essex TF-5 or TF-555 can be outfitted either with a Thermocouple or Thermo-Pile system.

@109A_5

A thermocouple produces a millivolt DC output. A healthy standing-pilot thermocouple will typically produce somewhere around 25–35 mV open circuit, while a thermopile/ Powerpile produces substantially more as I previously stated —commonly around 500–750 mV. No disagreement there??

i think the safety valve would have a coil for a thermocouple if it is a 24vac main valve and it woul be a coile for millivolt on the safety valve if it has a millivoilt main valve.

So… with the whole electrician, gas valve heating up thing, it is most likely is a Thermocouple / 24 VAC gas valve.

Which means troubleshooting the Thermocouple / Pilot valve solenoid system, two parts, what's wrong ?

@Nola803 do you have a multimeter ?

Let's say it is a Thermopile / Powerpile powered gas valve and the OP screwed in a thermocouple into this assembly pictured below so it won't work. How does the 750 milliVolt power supply get to the Main valve solenoid coil?

With 24 VAC @ 0.3 Amps or 7.2 VA (as an example) I would expect a gas valve to warm up. With a 750 milliVolt gas valve running on a Thermopile / Powerpile I would not expect it to warm up much.

Putting 24 VAC on a 750 milliVolt Main valve solenoid coil may destroy it, and even so would probably not cause the present issue.

I'm just so happy to see the fire extinguisher there.

I have not seen any photo if a 24V. transformer. if there is in fact a 24V. transformer and it was connected to the millivolt thermostat terminals, then that valve is toast. That little solenopid coil that works on millivolts got a real charge out of the redesign. If your electrician did you this favor, thene he owes you a new gas valve. You need a new valve.

This is a chart of different Millivolt Pilot Generators that are available for that valve. Uni-Line Powerpile. The most common one is the two wire connection model but your valve may have needed the coaxial cable design.

Now that you have destroyed the gas valve, you might as well put it back to a millivolt design so it will work during a power failure as it was originally designed.

This is the gas valve you should purchase: Robertshaw 700-501 along with this Pilot Generator that matches the Robertshaw valve. During a power outage the burners will still operate but the fan at the top will not operate. That fan needs power top operate. The Gas valve has multi function acapability. it comes with 1/2" x 3/8" bushings and it can be used as a strait thru or an 90° orientation. Perfect for your application.

Here is a simple wiring diagram to follow when connecting the wires from the thermostat to the gas valve.

We had a similar wall heater where the pilot would not stay lit. It had a thermal switch up near the top that would shut it down if it got too hot. The wiring to that switch turned out to be the problem. When the thermal switch was bypassed at the valve everything worked fine. Soldering the wires onto the crimped connectors and wiggling them around a bit on the switch solved the problem.

The second picture clearly shows a thermocouple, not a thermopile or pilot generator. It was removed from the pilot assembly. I see only two wires going to the gas valve. There's a fan next to Billy Bass so there's 115 vac there. Its a 24 volt gas valve.

For me @OffGridICF , the fact that you disassembled a part that's not meant to be disassembled, tells me that as much as I love the ambition, you're in over your head. You said the unit heater is likely from the 80's. You're in for a new gas valve, minimum. (Natural or LP?) That's at least a couple/three hundred dollars for the valve if it can be found. If you have a licensed contractor do it, you're looking at a lot more $$. Even with the extra $$, its highly advisable because its not just a gas valve replacement. Its that, plus checking static and manifold pressures. Making adjustments if needed. Test the pilot assembly. Checking the burners. Draft. And doing heat exchanger and combustion tests so you know you're safe. With actual proof.

But forget about that because we all know its time to replace it. Nobody WANTS to do it. But logic dictates that an 80's unit heater + costly repair = new unit.

Or you could go ductless. That's what we call a variable.

If you decide to DIY the gas valve, also pick up a low level CO detector. 👍

And if it is LP, not all pipe compound is acceptable.

Just thinking as I'm going. If it is LP, and the new valve needs a conversion kit, that's definitely not DIY. Please. Manometer.

For me @OffGridICF , the fact that you disassembled a part that's not meant to be disassembled, tells me that as much as I love the ambition, you're in over your head. 

@HVACNUT , it wasn't me :-) That was @Nola803, the OP. I've been in over my head plenty of times, but I've never disassembled a gas valve. Now that I've been suspected of it, I guess better get it on my to-do list ;-)