Pilot Lit, Gas Valve Closes When Simulating Thermostat


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Old 11-03-15, 07:55 PM
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Pilot Lit, Gas Valve Closes When Simulating Thermostat

Hello all,

We just bought our first house and it is equipped with in-floor gas furnaces. The pilot light stays lit so it would seem the thermocouple is fine. The thermopile was replaced at the same time that the previous owner had a new gas valve installed (in the last year or two). The thermostat wasn't working so I tried simulating the thermostat by bridging the TH terminals on the gas valve. The furnace fired up just fine...once. I gathered up my tools to crawl out form under the house and tested it again so I wouldn't have to crawl back if it wasn't working. The thermocouple clicked and the pilot went out. I relit the pilot and tried bridging the terminals again several times. Click, dead each time. I tested the voltage generated by the thermopile and it is putting out voltage within the range it should be. I even tried using a 1.5V battery to kick the gas valve on. Click, dead.

Any thoughts on what might be the issue? I'd rather not start changing parts out willy-nilly and my google searches aren't leading to anything enlightening. Any input is much appreciated.

To the best of my knowledge I have a Robertshaw 700 Series Gas Heating Control Millivolt Control Valve.

Thanks!
 
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Old 11-03-15, 08:10 PM
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I tested the voltage generated by the thermopile and it is putting out voltage within the range it should be.
What range would that be?
 
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Old 11-03-15, 08:12 PM
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Take the 2 leads from the thermopile/aka pilot generator off the valve and connect them with alligator clips to your meter. Do not hold them in your hands or juice will flow thru you and give wrong reading. Should have at least 500 mv for the system to work. A healthy generator will produce over 600 and up to 750 mv.
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Old 11-03-15, 08:17 PM
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To check Resistance of the gas valve, connect one wire to the valve as shown.
1. Resistance between the THTP & TH terminals must be 11.5 ohm 0.2

2. Resistance between THTP & TP must be 10.0 ohms 0.2

See pdf here


http://www.keatingofchicago.com/medi...llvoltGasV.pdf
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Old 11-03-15, 08:47 PM
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Thermopile was putting out ~650 mV. The valve's manual says it should work with >350 mV. I did not check resistance because no typical values were given. I'll try that next, thanks for the input.
 
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Old 11-04-15, 03:22 PM
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The 650 MV is a good voltage output.

The gas valve commonly has three terminals --- TH, TH/TP and TP.


Measure the DC millivolts between the TH and TP terminals when the thermostat is calling for heat.

That is the voltage actually being applied to the gas valve to open the gas valve main burner gas control.


If it's 100 MV or more, the gas valve should operate OK. If it doesn't you probably need a new gas valve.


If my memory is good, you can measure the millivolts between the TH and TP terminals which will give you the total voltage drop through the thermostat and thermostat wires. If that voltage loss is high, look for defects in the wiring or thermostat which may be causing the problem.

In particular, post the make and model number of the thermostat here. It's quite common for the wrong thermostat to be installed which will cause big millivolt drops to the gas valve.

It should be a millivolt thermostat, not a thermostat designed to be used with 24 VAV equipment.
 
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Old 11-04-15, 09:22 PM
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So I crawled back under the house to do some of the electrical testing you all suggested and the thermopile didn't check out this time. I pulled it out and did a bench test on the gas stove and it got up to just over 300mV. Looks like I need to replace it.

When we were doing the inspections, I noticed they changed out the thermostat since we had last been through the house; turns out it was a 24V system. I double checked the new programmable thermostat I bought and it works for millivolt systems.

Thanks for your help!
 
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Old 11-04-15, 09:28 PM
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Basically any thermostat that uses output relays and runs on batteries can be used on a millivolt system.
 
 

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