Over a decade ago the plumbing code began requiring that all shower valves be “anti-scald” to prevent people from being burned by the sudden burst of hot water that can result from a flushing toilet or faucet diverting cold water. Since then, most shower valves are single handle and use either “pressure balanced” or “thermostatic” technology to be anti-scald.
A pressure balanced valve works by balancing the water pressure coming in on both the hot and cold lines. Once you turn it on and set it at the desired temperature the valve will operate to maintain the same water flow though each inlet. If it senses a change in pressure of water in the cold line it will adjust to decrease the flow though the hot line to prevent scalding. It is reacting to the amount of water coming in, not the temperature.
A thermostatic valve does respond to hot and cold. The cartridge is designed with elements that react to changes in the temperature and will increase or decrease the flow of hot or cold water to maintain the desired setting. Even as your hot water supply decreases the valve will continue to work to maintain the selected temperature.
In general, pressure balance valves have a lower “gallons per minute” (gpm) flow than thermostatic valves which is important to understand if you are installing a multiple outlet shower. Most pressure balance valves cannot support more than two shower heads at once and sometimes not more than one. In addition, the majority of pressure balance valves are what are called “cycling” valves in which you turn on a single handle that starts at cold and gets warmer as you “cycle” to hot. There is no volume control, it is on or off. This isn’t usually a problem in a standard shower but may be less desirable for a bathtub. There are some manufacturers (Rohl, American Standard and Moen for example) who do offer pressure balance valves that include volume as well as temperature control.
Thermostatic valves offer higher flow and more precise temperature control. Thermostatic valves can be purchased with or without an integral volume control, in other words some thermostatic valves adjust only temperature and have no on/off function. While that may sound odd it makes a lot of sense if one is designing a custom shower with multiple heads and body sprays. Instead of having just one main on/off valve there is a central thermostatic valve combined with outlet specific on/off controls to allow the user choice of which and how many sprays to use. In other word each shower head, hand shower or set of body sprays has it’s own on/off control. These sorts of thermostatic valves have very high flow rates (16-20 gpm) and so can power multiple outlets effectively. Grohe, Hansgrohe and Rohl are some of the companies offering thermostatic shower components.
The above is a general description of pressure balance and thermostatic valves. Deciding which is right for you project can be done by determining what kind of performance you are looking for. For most simple showers cycling pressure balance is probably fine. If you have a tub or want to be able to adjust the shower flow (perhaps when bathing children) then you may want a pressure balance valve with a volume adjustment. Thermostatic is obviously the way to go if you want multiple outlets but many people choose thermostatic for simpler showers because they want the increased performance (better flow and temperature control) that a good thermostatic valve offers.