Water tank heater won't shut off i have the thermostat turned all the way down but it still keeps boiling the water out of control

over heating possible bad gas control valve , relief valve bad or water pressure too high or overheating will cause it to leak also lack of thermal expansion tank

Thermostat is likley problem , or limit switch control they regulate temp.

1) For gas: Turn off gas to heater:
http://waterheatertimer.org/images/Gas-water-heater-89-valve-2.jpg

2) For electric: Turn off circuit breaker.
http://waterheatertimer.org/How-to-troubleshoot-electric-problems-with-water-heater.html#veryhot

3) For gas: Copy following link to identify gas control valve thermostat, and troubleshoot by type of gas control.
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html#intellivent

4) For gas: Gas control valve thermostat has probably gone bad.

5) Add comment and give clear description of water heater.

6) Call plumber.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Thermal fuse is a heat-sensitive safety feature that shuts down system when overheating is detected.
In electrical wiring, the thermal fuse reacts to high temperature caused by too many amps on the electric circuit. When temperature exceeds the fuse amp rating, then the fuse trips and cuts power to electric circuit.
Example of thermal fuse is a household circuit breaker.
Gas water heaters have a different type of thermal fuse located in the combustion chamber or on the combustion chamber door, and also inside a copper probe inside water tank. Water heater thermal fuses will trip when water temperature exceeds fuse rating, or when heat inside the combustion chamber exceeds temperature rating of fuse. This action will cut off gas supply to water heater.
So a thermal fuse is a safety feature.

Thermostat is not a safety feature.
Thermostat is a measuring tool.
Thermostats measure temperature.
Example of thermostat is outdoor thermometer, or cooking thermometer, or Heat-AC control located on wall inside house.
Thermostats can give a simple temperature reading, or they can cause a separate control to turn on-off depending on application.
For example thermostat on Heat-AC will read room temperature, and when temperature exceeds setting, this triggers a switch that turns Heat-AC on-off.

Many appliances have both thermal safety fuse and thermostat.
Take for example a gas water heater. A copper tube extends into the water tank. Inside the copper tube are 2 things: A thermostat probe to measure water temperature, and a thermal fuse called an ECO for energy cut off. The ECO is a thermal fuse, and is a safety feature.
http://waterheatertimer.org/images/Gas-valve-ECO2-358.jpg
The thermostat reads temperature of water, and turns water heater on-off to keep water hot. A person can adjust water temperature by adjusting the thermostat dial on front of water heater.
However if the thermostat fails, or some other problem occurs, and water inside the tank gets hotter than 180 degrees, then the ECO or thermal fuse will trip. When the thermal fuse trips, this turns off gas to the water heater so water cannot get any hotter. The safety device prevents water heater explosion. The water heater must be repaired since this particular thermal fuse cannot be re-used afterwards, and must be replaced.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

Hello Mary
Thermal fuse is a heat-sensitive safety feature that shuts down system when overheating is detected.
In electrical wiring, the thermal fuse reacts to high temperature caused by too many amps on the electric circuit. When temperature exceeds the fuse amp rating, then the fuse trips and cuts power to electric circuit.
Example of thermal fuse is a household circuit breaker.
Gas water heaters have a different type of thermal fuse located in the combustion chamber or on the combustion chamber door, and also inside a copper probe inside water tank. Water heater thermal fuses will trip when water temperature exceeds fuse rating, or when heat inside the combustion chamber exceeds temperature rating of fuse. This action will cut off gas supply to water heater.
So a thermal fuse is a safety feature.

Thermostat is not a safety feature.
Thermostat is a measuring tool.
Thermostats measure temperature.
Example of thermostat is outdoor thermometer, or cooking thermometer, or Heat-AC control located on wall inside house.
Thermostats can give a simple temperature reading, or they can cause a separate control to turn on-off depending on application.
For example thermostat on Heat-AC will read room temperature, and when temperature exceeds setting, this triggers a switch that turns Heat-AC on-off.

Many appliances have both thermal safety fuse and thermostat.
Take for example a gas water heater. A copper tube extends into the water tank. Inside the copper tube are 2 things: A thermostat probe to measure water temperature, and a thermal fuse called an ECO for energy cut off. The ECO is a thermal fuse, and is a safety feature.
http://waterheatertimer.org/images/Gas-valve-ECO2-358.jpg
The thermostat reads temperature of water, and turns water heater on-off to keep water hot. A person can adjust water temperature by adjusting the thermostat dial on front of water heater.
However if the thermostat fails, or some other problem occurs, and water inside the tank gets hotter than 180 degrees, then the ECO or thermal fuse will trip. When the thermal fuse trips, this turns off gas to the water heater so water cannot get any hotter. The safety device prevents water heater explosion. The water heater must be repaired since this particular thermal fuse cannot be re-used afterwards, and must be replaced.

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

I believe 640 is typical tank-type atmospheric-vented gas water heater.
Gas water heater problems can get complicated. New heater sells for $350 at local box store.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

If thermostat is not controlling temperature accurately, then (1) gas control needs replacement (2) water heater has sediment build-up (3) combustion and burner parts need cleaning. (4) All three things above, plus additional causes such as bad venting or not enough combustion air.

Usually recommended that plumber replace gas control valve thermostat:
See typical tank-type thermostats:
http://waterheatertimer.org/images/Typical-gas-valve-500.jpg
http://waterheatertimer.org/How-to-adjust-temperature-on-water-heater.html

Clean sediment out of tank:
http://waterheatertimer.org/Clean-sediment-out-of-water-heater.html

Clean combustion chamber and burner parts and troublehoot venting and air supply:
http://waterheatertimer.org/How-to-install-gas-water-heater.html#troubleshoot
http://waterheatertimer.org/images/Clean-burner-300.jpg
http://waterheatertimer.org/pdf/Bosch_Ventilating_tight_houses.pdf

If you'r really lucky you may find a leaking fitting on top of the water heater thats draining down inside the shell and out the bottom, otherwise its time to replace a rusted out water heater.
Sorry

The thermostat controlling the tank heater is defective or wired incorrectly.

Not sure but check the inlet water at the heater, the filter may be clog up a little that restrict the flow. Did u use the heater at all during the winter? If not, the heat exchanger may be clogged with built up in there. U need to descale it. Good luck. Chinhcu