A shower control valve manages the flow and temperature of the water in your shower. There are two basic types of shower control valves. One valve has a single flow and temperature control handle, the other has two or more handles to control temperature and flow.
Single-control Handle Valves
A shower control valve with the single control has either a knob or lever that is turned, flipped, or pushed to initiate the flow of water to the shower head.
Most valves are marked with symbols for hot and cold water, allowing you to easily adjust the temperature of the water. Moving the handle further towards the hot water symbol increases the water temperature. To decrease the temperature, you move the handle towards the cold water symbol.
To facilitate the hot/cold blending of the water, there are two water lines connected to the valve. Inside the valve itself, there is a sleeve with an opening. When the valve is closed, the opening is against the body of the valve. As the handle is turned towards the hot side, the cold water begins to run into the opening in the sleeve. Further turning the knob aligns the hole over the hot water intake, while simultaneously leaving the cold water fully exposed to the opening in the sleeve. Turning the knob further to increase the flow of the hot water begins to close the opening over the cold, until you reach the hot water stop, which results in only the hot water feed being exposed.
This type of shower control valve makes it difficult to control the flow of water to the shower head, resulting in a constant pressure.
Multiple-control Handle Valves
A shower control valve with multiple handles works in a fashion similar to the single valve control, except you have discrete controls for hot and cold water. Most multiple handle shower valves consist of two handles, one hot and one cold. In some showers there may be an additional handle that controls the water pressure to the shower head or allows the shower to be shut off while the hot and cold water are engaged, but these are less common than the two-handle control valves.
The valve in this design has two lines, one hot and one cold, going to individual sides of the valve. Corresponding to each side are handles that control the flow of water entering the shower head. Inside the valve, there are sleeves with openings that are exposed to the incoming water as the handles are turned. This type of valve gives you greater control over the water temperature since each side is controlled individually. The water entering the shower head comes from the hot and cold sides and merges in a single feed to the shower head. This generates the pressurized stream that comes from the shower head.
A multiple-handle shower control valve also allows you to control the water pressure that flows to the shower head. This provides you with greater variability in the force of the shower, while allowing you to maintain a constant temperature.