# ThermoCoupler and Thermopile



## Fire Bug (Dec 12, 2008)

Hi Everyone,
 Can someone tell me the difference between the Thermocouple and the Thermopile and what their functions are exactly.
 I know the Thermocoupler produces mili volts from the heat that is put on it from the pilot light and this is sent to the gas valve to prevent a explosion if the pilot light is not lit and the gas valve is opened, but I am not sure what the Thermopile does exactly.

Thanks,
John


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## DAKSY (Dec 12, 2008)

Hey, John...

 Can someone tell me the difference between the Thermocouple and the Thermopile and what their functions are exactly.

They are both considered miliivolt generators. The Thermocouple has two small pieces of dissimilar metals inside the outer shell. When these metals are put under heat, they expand at different rates & essentially "rub" against each other generating a small (millivolt) amount of DC electricity. This millivoltage is in the range of about 25-30 (roughly) mv...
This voltage will hold open two magnets in the gas valve, which allows the gas to flow to the pilot hood...
A thermopile can be thought of as a "pile" of thermocouples...maybe 25 - 30 pairs of dissimilar metals which act the same way, but generate more mvs...
Normal range is from 325 - 600 mv. This voltage allows for the larger magnets in the burner side of the gas valve to stay open & keep the gas flowing to the burner..  

 I know the Thermocoupler produces mili volts from the heat that is put on it from the pilot light and this is sent to the gas valve to prevent a explosion if the pilot light is not lit and the gas valve is opened, but I am not sure what the Thermopile does exactly.

Not exactly...if either one of the systems fail, or get agitated enough to move the pilot flame away from the T-C or T-P, that side of the system will have the magnets close & stop the gas flow...
This is why they're considered "Safety Valves..."
This generally happens w/i 30 seconds...Turn off the pilot in one of your gas appliances & count to 30...I guarantee you'll hear a "click" as the magnets close...

HTH


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## Fire Bug (Dec 12, 2008)

Thanks Dasky,
 This is pretty cool stuff!!
 Let me see id I got it staight, the thermocouple controls the magnets in the gas valve that allow the gas to flow into the pilot light and the thermopile controls the magnets in the gas valve that controls the gas flow to the burner.
 If one fails, will they both fail? For example, if the thermocouple fails while the burner is lit, will both the burner shut down and the pilot light shut down?
 I am sorry if I seemed to be confused by this, but I think that these items seem to be the most complicated devices of these gas stoves, inserts, and fireplaces.

Thanks again,
John


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## DAKSY (Dec 12, 2008)

If the T-Couple fails, it'll take out the pilot & as a result, the T-Pile will lose mv & shut the burner down.
If the T-Pile fails, the burner will generally stay lit. They are independent systems - to a point..


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## PaulRicklefs (Dec 13, 2008)

Fire Bug said:
			
		

> Thanks Dasky,
> This is pretty cool stuff!!
> Let me see id I got it staight, the thermocouple controls the magnets in the gas valve that allow the gas to flow into the pilot light and the thermopile controls the magnets in the gas valve that controls the gas flow to the burner.
> If one fails, will they both fail? For example, if the thermocouple fails while the burner is lit, will both the burner shut down and the pilot light shut down?
> ...



Thermopiles generate power to be switched through a mv thermostat, a toggle switch or shorted at the gas valve to supply power to the main burner circuit on the gas valve.

Thermocouples generate power to be used to hold open the pilot circuit which then lets gas flow to the burner circuit valve.

They are two different systems that do two different things sort of.  If a thermopile fails and there is a thermostat connected (or the TH is jumped on the valve) to the system, the pilot light will stay lit but the main burner won't light because there is no power to signal the burner circuit in the gas valve to open.  So the thermopile is used to run the main burner circuit only.

If the thermocoupler fails, the pilot light will go out and will also not let gas to the burner circuit.


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## Fire Bug (Dec 13, 2008)

PaulRicklefs said:
			
		

> Fire Bug said:
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 I think I might have it staight now. 
 If the thermopile fails and the heating unit is connected via thermostat on a remote for wall mounted, the power will not reach the big magnets in the gas valve and the magnets will not open or stay open to allow gas to suppply the burner of the unit.
 On the other side of the coin, if the thermocoupler fails, no power will reach the small magnets in the gas valve and the valve will not open or stay open causing no gas supply to the pilot and also no gas supply to the  burner circuit.(thermocoupler controls two circuit, thermopile controls two).
 Thanks for the very detailed explanaton and the time you put into it.

John


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## PaulRicklefs (Dec 13, 2008)

Fire Bug said:
			
		

> PaulRicklefs said:
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