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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!
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  • lets talk about the model and the schematic
    – Traveler
    Oct 27, 2022 at 23:45
  • Is the pilot flame blue or yellow (blue is normal)
    – Traveler
    Oct 27, 2022 at 23:55
  • Did you clean the thermostat from build up (soot)
    – Traveler
    Oct 27, 2022 at 23:56
  • 1
    Without knowing your model, I read that the thermocouple has to have 750 milli volt
    – Traveler
    Oct 28, 2022 at 0:28
  • 1
    go and change the thermocouple on a blind hunch
    – Traveler
    Oct 28, 2022 at 1:18

3 Answers 3

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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.

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  • Upvoted because I accepted this advice and unfortunately it didn't fix the furnace but it led me to a more conclusive place .... calling a plumber tomorrow.
    – jay613
    Oct 28, 2022 at 2:46
  • But I don't have a spare ... the new one doesn't fit and isn't suitable.
    – jay613
    Oct 28, 2022 at 2:47
  • thermocouples last up to ten years. is this a furnace or a hot water tank? if it is a furnace you'll need a new one, if it is a hot water tank you might not. I'd order one if it is a furnace. Oct 28, 2022 at 23:37
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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.

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  • If it's producing 25mV in a hot flame, then it's a thermocouple, not a thermopile Oct 29, 2022 at 2:11
  • The existing (old) one is producing 600 or more mV (still rising slowly when I stopped measuring). I bought a generic replacement because they had one at the store ... that produces 25mV and doesn't physically fit. Waste of $12 but that's fine. If I order a correct replacement thermopile it'll take several days minimum, and I don't think it will help. It's time to call a pro who will just get the job done quickly. It's cold.
    – jay613
    Oct 29, 2022 at 15:12
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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.

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  • Measured properly now, under proper pilot flame (instead of a lighter) and it's 600mV or higher. I think that's in range. I do not intend to diagnose or replace the valve itself as DIY ... I'll be calling a gas installer tomorrow. This will be $$$$.
    – jay613
    Oct 28, 2022 at 2:48

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