By consolidating the balance of hot and cold water into a central control, single-handled showers provide an easier way to achieve the desired temperature. While these fixtures are available in an endless array of styles and finishes, each single-handled shower uses one of two underlying mechanisms: the pull-out valve and the turn-around valve.
The Pull-Out Valve
In this faucet, the handle is attached by a thin metal rod to a plastic tube. Toward the far edge of the tube, two narrow rectangular holes are cut opposite each other into the plastic. This plastic tube fits snugly inside a rubber-lined pipe, which also has two long, narrow openings that are cut at the center. Each of these rubber-lined pipe openings lead directly to the cold water and hot water lines, respectively.
When the handle is pushed in, the tube's holes don't line up with the pipe's hole, sealing off all the water. However, by pulling the handle out, the two pairs of holes partially overlap, allowing water to rush into the tube (which leads to the shower spout above).
By turning the handle to the left or right, you increase the hole overlap for one side while decreasing the overlap for the other. This increases the amount of one water temperature flowing into the tube, which pushes the average temperature of the "mixed" water either warmer or cooler.
The Turn-Around Valve
The turn-around valve also uses the "tube-within-a-rubber-lined-pipe" system, except that the inner tube features a single, long rectangular opening and the hot/cold openings in the pipe are on the same side (spaced about 1 mm apart).
When the faucet is off, the tube's opening is completely sealed off from either the hot or cold water inlets. By turning the handle, you first align the tube opening solely with the cold water inlet, causing chilly water to flow from the shower. As you continue to turn the handle, the tube opening starts to overlap with the hot water inlet as well as the cold. The water mixes in the tube for a tepid shower. Further turning will have the tube overlapping solely with the hot water inlet, putting you at the mercy of the water heater's thermostat.