Importance of the Valve
When taking a shower, the shower valve is the plumbing component that lets you turn the water on and off while being able to adjust water temperature to meet your needs. Depending on the way that your shower is set up, you may have a single valve which regulates both the water pressure and the temperature, or you may have more than one valve to allow you a bit more control over the temperature and pressure. If your shower has a single handle for turning the water on and off, as well as controlling the temperature then you are operating with a single valve. If you have two or more handles that allow you to adjust hot and cold water independently, you most likely have at least two shower valves.
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If your shower has a single handle then you also have a single valve that is controlled by that handle. The valve will have two water lines coming into it, with one being filled with hot water and the other with cold. Turning the handle will adjust the valve, letting water in from one or both of the water lines. Most single-handle showers allow the cold water access when the valve is first opened, with hot water being added to the cold as the valve is opened further. Continuing to turn the handle will open the valve even more, allowing you to add more hot water to the mix and make the shower water warmer. When the valve is opened as far as it will go then the cold water line will be blocked almost completely so that only hot water is coming through the line.
Two or More Handle Valves
If your shower has two or more handles then you almost certainly have at least two valves. Your shower will have separate handles for hot and cold water, with each handle controlling a valve that has a single water line attached to it. You can control the temperature of the shower water by adjusting the amount of hot and cold water that you allow to come through the different lines, turning the handles of each until you reach the water temperature that you desire. Though not common, you may have a third handle which controls the water pressure and can turn the shower off completely; if you do, then it has a valve behind it that is fed by the water lines controlled by the other two handles.