I have a (fairly standard) natural gas water heater that is only hooked up to a water supply line and gas - no electric. I'm wondering how the thermostat is able to open/close the gas valve without electric?

Based on some Googling, I've found that the thermostat itself is a thermistor, which provides variable resistance based on temperature. That implies some electric current coming from somewhere. But where? And what provides the actual energy to open/close the gas valve?

Haven't been able to find this anywhere...

2 Answers 2


There are three things with "thermo" that you need to be aware of. Thermistor, Thermocouple, and Thermopile.

You know what a thermistor is. A Thermocouple generates voltage when its heated, and the voltage varies with the heat. A Thermopile is a collection of thermocouples (in a single package) that generates more voltage.

So, the voltage to open the valves comes from thermocouples and the heat comes from a pilot light.

You know how when you light a water heater you have to hold down that button for a long time to get it to stay on? Well the button manually opens the pilot light valve and you have to hold it on long enough for the thermopile to heat up and take over the job. Once it's up to heat, it keeps the pilot light valve open and provides voltage for the other controls. Also, if the pilot light goes out, voltage drops, and all the valves remain safely closed until it is re-lit.

  • So, my water heater has a thermocouple, which other sources suggests creates ~75 millivolts of electricity, which is tiny. How are those 75 millivolts enough to keep a valve open? Not saying you're wrong, just interested. Commented Nov 29, 2017 at 16:19
  • I'm having a hard time finding a reliable source to tell you how thats possible. If you look at this picture, the only two things in the thermostat portion are an ECO (emergency cut off) and a temp probe. The thermostat only has an electrical connection to the valve, so theres no wax based thermostat control like in a car's cooling system. I think the key is in opening a very tiny valve with the voltage and allowing that flow of gas to open a larger valve, but I can't find anything to back that up.
    – JPhi1618
    Commented Nov 29, 2017 at 16:41
  • Apparently, the solenoid valves only require a couple mV to operate... youtube.com/watch?v=DLxhKrSEDwE Commented Nov 29, 2017 at 16:50
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    @EdBeal - Well, that's a little unnecessary. First, I actually did Google further which is how I found that YouTube video. Second, I accepted the answer and stated I was asking a follow-up out of curiosity, not because I didn't trust the answer. And third, it's not obvious to somebody who's not an electrical engineer that a couple mV is capable of opening and closing a mechanical valve. Commented Nov 29, 2017 at 21:09
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    One thing missing here. The main gas valve is operated hydraulics as a result of the expansion of the gas in the temperature probe that is immersed in the tanks water.
    – Paul Logan
    Commented Nov 29, 2017 at 21:44

If I could try to explain part of the magic; a long time ago people started to realize the when two dissimilar metals come into contact, the chemistry's of the two metals would react with one another. This reaction we call electrolysis. This reaction frees up some loose electrons. When you put this reaction under heat the free electrons are multiplied. The engineers went to work to refine and enhance this process. They developed what we call the modern day thermocouple. The thermocouple is simply two different metal coupled together. When we place this thermocouple in the pilot flame it will provide for us about 75mV. The makers of these gas valves then designed and built an incredibly sensitive plunger that is lifted by a very small yet detectable electromagnetic force developed by a coil of extremely fine wire, this wire is truly like human hair, powered by this 75mV. This plunger allow gas to pass for the pilot light. Once the pilot light is shown to be burning, this process allows the hydraulics of the compressed gas in the temperature probe to open the main gas valve. The wonders of modern engineering.

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