Water heater gas valve
In this pic. you can see the limit and temperature probe.
This gas valve has the pilot button integrated into the control knob.
Some valves have separate knobs for the pilot.
The pilot button on the valve is separate from the control knob.
The control knob must be turned to the pilot position before the pilot button can be pressed.
This control valve is on a newer style water heater (has a sealed burner compartment).
Notice its the same type valve as on the older style water heater in the picture above.
This is a newer style water heater with an unusual control valve.
Notice the two white wires coming out of the bottom of the valve.
The wires are connected to a roll-out switch(thermal fuse). If there is a flash fire in the burner area this switch will shut the burner and pilot light off.
To replace this valve you would need an exact replacement in order to wire up the roll-out switch.
Gas Control Valve Troubleshooting
A water heater gas valve controls the pilot light, water temperature and it has a safety limit to shut everything down.
To check the pilot control, follow the lighting instruction on the water heater. Do not try to light the pilot if you smell gas.
Press the pilot button down and light the pilot.
If the pilot lit but will not stay lit when the pilot button is released the could be the problem.
If the thermocouple has been replaced and it has been properly installed (tip of the probe in pilot flame, thermocouple screwed in tight at the control valve, thermocouple line not kinked) and still the pilot will not stay lit when you release the pilot button then the gas control valve is at fault.
If the pilot will not light at all with the pilot button down check the gas supply.
You have gas and the pilot still will not light, the pilot line or orifice may be clogged.
Water Heater Temperature Control
Water temperatures in a water heater can vary greatly without ever moving the temperature control.
In a gas water heater where the thermostat is located in the bottom of the tank, the water at the top of the tank (where water is drawn off when a faucet is turned on)can be 20 to 30 degrees hotter than water in the bottom.
This is caused by short cycling. What this means is that just enough hot water has been used to cause the thermostat to come on. Cold water has been dumped in the bottom of the tank near the thermostat. So, while this water is heating, the temperature in the top of the tank is rising.
If you find your water much hotter at times this is probably the reason and not the water heater gas valve.
Ok, with that being said, water heater thermostats can malfunction. If your water is too hot and you can hear the burner running turn the thermostat all the way down. If the burner stays on, turn the gas supply off and replace the gas valve.
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Should I remove the plastic sleeve on my water heater gas valve thermostat? | Yahoo Answers
No that stays on there and will protect the copper element from corrosion.
Put it in just like it is.