Gas water heaters have a thermostat inside the water of the tank, which is then connected to the front control at the bottom of the water heater. On the front control there is a dial with three settings: ON, OFF and PILOT. Above the dial is a knob with a recessed screw which controls the gas flame. Below the bottom of the dial are three tubes. The largest tube, always in the middle, is the natural gas supply tube. The medium-sized tube, usually the one on the right, feeds natural gas to the pilot light switch. The smallest tube, usually the one on the left, is called the thermocouple. The thermocouple allows you to control the amount of gas fed to the pilot after it has been lit, therefore controlling the size of the flame. Lastly, the burner assembly is attached to the tubes and control; it can be removed for cleaning, but do not take apart the control itself.
If your pilot light will ignite but not remain lit, then the thermocouple needs to be replaced. If the thermostat inside the heater ever reaches 190° F (210° F on many older heaters), the front control will shut off and the flame will die. If this ever happens, you will need to install a new control.
To make your water hotter on a gas water heater, turn the dial to the right. Be sure to do this in very small increments at first to avoid scalding. Perhaps only 1/4 of an inch movement at a time would be best. Cooler temperatures can be achieved by turning the dial to the left. If you or your family ever go on vacation, its a good idea to turn the gas water heater's dial all the way to the left. This will place the gas water heater in vacation mode, using substantially less natural gas.
Unlike gas water heaters which use natural gas and fire to heat the water in the tank, electric water heaters use two horizontal elements, similar to the metal grill pieces found on top on older household stoves. One element is in the upper half of the water heater and one is in the lower half. There are two kinds of elements: high watt and low watt. Low watt elements are better because they produce less sediment, but they are more expensive than high watt. In electric water heaters, the element in lower half of the tank will do most of the heating, while the upper element usually operates only when a large amount of fresh cold water is introduced into the tank. Only one element will be working at any given time. There is also an adjustable thermostat attached to the lower element found on the outside of the tank. Thermostats are usually preset at 120° F.
To adjust the heat on an electric water heater, find the lower electric element's door. Its usually oval in shape and simply needs to be unscrewed with a household screwdriver. Once the door is removed, its simply a matter of adjusting the thermostat to a higher or lower temperature. It should be noted that the upper electric element has a thermostat as well, but sometimes its not adjustable. Its just as well since the lower electric element does the bulk of the heating work in an electric water heater. Adjusting the lower thermostat has far more influence on the overall heat of the water.
Like gas water heaters, electric water heaters also have a high-temperature shut off switch. This is attached to the upper element and will turn the element off if the temperature reaches 190° F. Some tanks have only one element. If this is the case it will always be at the bottom of the tank. In single element tanks, the shut off switch is attached to the bottom element. If the tank ever shuts off, there is a red reset button similar to a GFI on household electrical outlets.