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Old 12-16-2013, 06:27 PM   #1
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Thermostat on Room-sized Gas Heater


I have a 15-year old White-Rodgers 2-wire external thermostat (millivolt, mercury filled ampoule) that can operate a Vermont Castings gas heater in the dining room. But I don't think it's working because when I turn up the thermostat for more heat I don't hear a click and the heat (the flames) does not come on. Does the thermostat need to be replaced? (The pilot light is always on.)

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Old 12-16-2013, 06:35 PM   #2
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Thermostat on Room-sized Gas Heater


could be a dozen or more reasons. weak powerpile generator that produces the millivolts or poor wiring connections etc etc. is the mercury in the bulb gone dark black? it should be silver and move quickly. if black then replace it and a basic Honeywell programmable tstat from HDepot or CTire or Rona should do the trick. mercury ones are obsolete and the spring type replacements won't work with your system very well. otherwise call a Pro from a fireplace specialty store.

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Old 12-16-2013, 06:42 PM   #3
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Thermostat on Room-sized Gas Heater


The mercury is still silver and moves quickly. I measured only 600 mV between the two thermostat terminals, the manual says it's a 750 mV thermostat - could that be a problem?
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Old 12-16-2013, 06:46 PM   #4
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Thermostat on Room-sized Gas Heater


600 is good. down to 500 still works. remove the wires after labelling and taking a picture of them on the gas valve after turning the pilot out. then scrape the terminals on the valve with a sharp screwdriver or knife and same with the terminals on the powerpile and the thermostat wires until they show bare copper and reassemble. powerpile is the thingy in the pilot flame and produces the millivolts. 750 millivolts is .75 volt so we are dealing with a VERY small voltage and poor connections are the major problem with millivolt systems. tighten the connections very tight and snug. powerpile generator
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Last edited by yuri; 12-16-2013 at 06:49 PM.
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Old 12-16-2013, 06:54 PM   #5
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Thermostat on Room-sized Gas Heater


Thanks. This could take a while to try. But if I don't try it I need to wait 2 weeks for a professional appointment (they're that busy!). I have the manual and can remove the cover of the unit to get at the switch wires at least. Thanks again.
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Old 12-16-2013, 06:58 PM   #6
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Thermostat on Room-sized Gas Heater


Make sure you take some pics. If you mess up the wires then it will be unsafe and won't work and doubt anyone can help after that. Well built stove. They are not that hard to take apart, but take pics so it is easier to re-assemble.
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Old 12-16-2013, 07:00 PM   #7
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Thermostat on Room-sized Gas Heater


Good. OK, I have a cell phone that can do that. Plus, I'll draw a diagram and label it thoroughly. Thanks for your advice!
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Old 12-16-2013, 07:07 PM   #8
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Thermostat on Room-sized Gas Heater


You would be amazed at how many people we get here who don't turn the power off when changing a thermostat or label wires and then want our help.
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Old 12-31-2013, 02:42 PM   #9
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Thermostat on Room-sized Gas Heater


The gas heater was fixed yesterday. The contacts in the switch on the unit (therm, off, on) needed to be 'cleaned' because of the oxidation on them. I didn't need a new switch & wiring or anything! Thanks Yuri for weighing in on this (from a very cold Winnipeg these days).

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