boiler control

Old 08-13-02, 08:13 PM
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boiler control

I hope you guys can help. I have a 60's vintage Thatcher boiler/tankless water heater. Model # GO 33, Series 33,with a Economite Model # E20 , Seriel # 66596 controlled by a basic Honeywell merc. thermostat. The heat came on and wouldn't shut off with the thermostat all the way down. I shut off the elec. pwr. and pulled the 'stat' from the backing plate and rechecked it and the motor fired and kept going even after the normal water heating cycle. There were no shorts or chaffes in the wires at the 'stat'. Any help would be greatly appreciated. Thanks. woeisme
Old 08-18-02, 10:20 AM
lynn comstock
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The heater fires all of the time. The circuit is completed through the thermostat. Turn off the power and disconnect one wire at the stat. Turn on the power and the fire should be off. The stat has failed in the ON position. This is potentially capable of creating a steam-powered explosion. Replace the stat.
Old 08-18-02, 06:38 PM
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Lynn, If you read my question closer you would note the boiler still fired with the T-stat disconnected which opens that circuit. The aquastat will also close the circuit when needed or if it is shorted closed. I replaced the relay and the system works fine now. The stat didn't need replacing. Thanks anyway.
Old 08-19-02, 09:50 AM
lynn comstock
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I didn't get the picture that you tested the boiler (not the thermostat).

I guess that I thought the "thermostat" that you were speaking of was the aquastat (water temperature thermostat). The function of the thermostat is unclear to me from here. I would doubt that there are 2 separate thermostat pathways to turn on the heater.

Also I don't Know how the relay fits into the wiring. My guess is that the thermostat controls the burner indirectly (through the relay). Was there a temperature limiting thermostat working to keep the heater from runaway firing?

I would like to understand better so that I could do better next time. Thanks for the feedback.

Last edited by lynn comstock; 08-19-02 at 10:09 AM.
Old 08-19-02, 11:04 AM
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sounds like

the valve itself may be stuck in the open position. if no voltage is present at the valve, and the unit is firing, bad valve. is this appliance natural gas? i have seen this before on both residential gas furnaces and rooftop units. after 30 years, thart valve has opened and closed an unthinkable number of times. with a problem such as this, i would call a pro, he may find signs of problems yet to come.

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