Gas pilot out and thermopile reads 600+mV, is it worth trying a new one?

Question

Gas hot water furnace. Pilot won't stay lit, goes out as soon as you release the button even after 60+ seconds.

The valve is a Robershaw 7000M8ERHC, with a Robertshaw 1951 thermopile. The thermopile document says it should produce 250 to 750mV. Under heat I measured up to 600mV and still rising slowly when I stopped measuring.

It's not a standard 25mV thermocouple that I can buy off the shelf. I tried that anyway, it didn't work, it didn't even fit.

Tomorrow when the local plumbing supply store is open I may try to buy a proper replacement, but the thermopile seems to be working so I'll probably need a plumber.

Further info

Further notes

• There is no corrosion or buildup. The end of the thermopile is slightly blackened in color but with no visible buildup.
• Ruskes asked (and deleted) about a transformer. There is no transformer in the thermopile circuit, but his question led me to notice that there is a shunt switch that was hidden inside the valve's thermopile socket, that is connected to the boiler's high temperature relay. When that relay shuts off the boiler, a second circuit in the relay also bypasses the thermopile so the pilot switches off. For a moment I thought @Ruskes had led me to the problem. But no .... that switch is working fine. The circuit is closed, I measure millivoltage on the switched side of it.
• I think I'll go with Fresh Codemonger's advice, and just blindly replace this. I will not DIY a valve replacement!

Answer 1

Thermocouple is cheap, buy one put it in if it doesn't fix the problem you've got a spare for next time.

Getting someone out to swap the gas valve is going to be a few hundred at least.

Answer 2

For what it's worth I'll add my own answer.

The other comments and answers were helpful in getting me here.

1. If you know what model gas valve, and if possible what model thermopile or thermocouple, you have, you can look up the specs and determine what voltage(s) should be present under different conditions.
2. If you take measurements and find the thermocouple is performing as it should, and you also find it free of physical defects and you see the pilot flame is good: then you should not expect that blindly replacing the thermocouple will help anything.
3. The one "universal" thermocouple at the hardware store is not universal at all. Electrically there are different voltages and behaviors. The "universal" ones produce about 25mV when hot, but others produce up to 750mV. Physically there are different connector types at the valve end. There are two-wire (spade) ones, coaxial ones, and within coaxial there are different sizes.

Sourcing a thermopile that is not "universal" may be difficult, and if you can tell through measurements that your old one is working it may be a better approach to proceed as if the valve is bad.

Answer 3

It has been a while but that sounds a little on the low side. Check to be sure the thermocouple is actually in the flame and the flame is high enough and blue. If it is not adjust it so it is. If that does not resolve it is probably the valve or thermocouple that has failed. First thing and the most cost effective is to try another thermocouple and be sure it is in the flame. At this point you need to decide to get professional help to replace the valve or heater. I would not recommend doing this unless you have had experience doing this as a mistake could end up as a big bang.