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We have a very old boiler system (HydroTherm R150) in our 1800’s home. Last year it worked without issue, this year we’re not able to get it started.

The pilot light was shut off, I opened the valve for it and was able to light and adjust it without issue. However, the only way I’ve found to get the burners lit is to bypass the thermocouple mechanism. On the component in the second photo, if I put a jumper wire on the two terminals then the gas valve clicks and the burners light.

HydroTherm R150 boiler system

Thermocouple switch/mechanism

Is it safe to assume that the thermocouple has gone out and is preventing power from reaching the water tank thermostat?

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  • Sounds like maybe there is a safety feature in play. Commented Nov 7, 2021 at 2:31

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Shorting it as a test as you did is a good way to verify the circuit. Do NOT leave the bypass (short) in place, things could go very bad in a very short time. I would start by replacing the thermocouple unit, that should solve your problem. From your picture it is on its last legs if it is working. I would consider that a safety device, it is there to prove the pilot is lit before allowing the main gas to flow. I have not seen any gas boilers or furnaces that do not incorporate this safety feature in some way. Some do it with IR, some sensors detect the ioznation of the gas flame, and many others. They all want to be sure if the gas is flowing it is burning.

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That device that you jumped out and the burner fired is a "Baso" switch. Try this; remove the thermocouple from the Baso switch and wipe off the end terminal on a rough surface to clean it, the end of the thermocouple that screws into the Baso switch. (it usually takes a 3/8 open end wrench). Re-install the thermocouple. Make sure that the pilot flame envelopes the top 1/3 of it and that the pilot flame is blue. Push in the reset "red" button to see if the burners fire. If not, first, buy a thermocouple since it is the cheapest and then a Baso switch.

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