The subject of shower valves has been somewhat simplified in recent years, due to code regulations that require protection against sudden temperature fluctuations. This has eliminated much of the plain single-lever, two-handle valve business that once dominated this field. (That is not to say that we no longer see single-lever and two-handle shower valves - but those that are sold today are usually equipped with this additional protective capability.) Why the need for such protection? Just recall the last time you took a shower without it, and someone flushed a toilet nearby. There are two basic technologies used in maintaining consistent temperature output - thermostatic and pressure balance. As a basic distinction between the two, here's what you need to understand:
1. Thermostatic valves provide this protection by sensing and responding to variations in actual output temperature - whatever the cause.
2. Pressure balance valves do their job by sensing and responding to fluctuations in the relative input pressures of the hot and cold supplies. Unlike thermostatic valves, they do not compensate for any factor other than this.
Let's take a closer look inside each type.
ThermostaticTypically, there is a stem that controls two ports at the same time (hot and cold supplies). On this stem there is a thermostatic control device that moves the valving discs in a relatively independent fashion once the temperature selection has been set. This thermostatic element causes the two discs to move as a unit in the direction required for compensation. An example: Let's say that when our setting is established at 100°F, the two discs are positioned about the same distance from their respective seats. Suddenly, our output temperature rises, so the disc assembly responds by moving in the direction of the hot seat in order to further throttle that inlet, at the same time, further opening the cold inlet. This movement brings our output temperature back to the desired setting. If we later experience a drop in our output temperature, the reverse will take place (the disc assembly will move in the other direction). The two common technologies used for the thermostatic control element are a “wax capsule” type or bi-metal. Both translate variations in water temperature into mechanical movement that is used to position the control discs. One last point in regard to thermostatic valves: The control handle does not usually provide the functions of shut-off or volume control. In other words, the turning of the handle will select temperature, but will not provide those other requirements. Products that do offer the other functions do so by means of a secondary control (a separate valve - sometimes contained in the same overall housing, sometimes installed separately).
Pressure BalanceWhere thermostatic valves respond indirectly to relative pressure levels in the hot and cold water supplies, pressure balance types do so directly. This valving module is located upstream (ahead of) the basic mixing valve in the assembly. The typical pressure balance valve, then, has two distinct valving modules: pressure balancing and mixing. While pressure balance technologies vary - they all perform the same basic function. When a fluctuation occurs in the relative hot and cold input pressures, the mechanism responds by throttling down the higher and further opening the lower. The two most common design approaches to pressure balance valving are the “piston” or “shuttle” type, and the diaphragm type. The action of a pressure balancing valve seems to be a difficult concept to grasp for many people, but let's see if we can clear this up a bit. One key to understanding the action in such a valve is to visualize the mechanism cut in half - that is - to see the hot side separated from the cold side at first. If it were possible to do this, you would see that the natural tendency for the valving on either side would be to turn itself off as a result of the water pressure applied to the mechanism. This is true of every pressure balancing valve made. The thing that keeps the valving for each supply from shutting itself off is the valving on the other side, pushing from the other direction. Thus, with inlet pressure about equal on the hot and cold sides, you have a standoff and the mechanism stays put.
Let's look at a specific example now to see how this happens. In the piston type of pressure balancing valve, the cold supply water comes through an inlet port into the control area. Pressure from this cold supply is transmitted through small passage in the piston and into the cavity at the end. This pressure in the end cavity wants to push the piston away from the end, and this would result in closing off the inlet port. What keeps this from happening? The exact same thing going on at the other end of the piston - pressure from the hot supply is pushing the opposite direction. So the piston stays where it is, partially throttling both inlets to produce an equal output. But now we have a drop in cold supply pressure. Now, the “cold end” of the piston, which had been preventing the hot end from turning itself off, is no longer able to hold off this force entirely, so the piston moves somewhat toward the cold end. This results in a further throttling of the hot inlet, and a further opening of the cold. The piston will now stay in this position until normal pressure is restored to the cold supply, at which time it will move back to its original position of equilibrium. With diaphragm designs, the same basic thing happens, except that incoming pressure is applied to a resilient membrane rather than a piston. A valving poppet is connected to each side of the diaphragm, which moves in and out in relation to an inlet seat. Here again, each side of the valve wants to turn itself off, but is prevented from doing so by the other side.
Unlike thermostatic valves, pressure balance types do combine the on-off function with mixing into a common control. Some also include volume control, as well, either controlled by the same handle (single lever) or by means of a separate valve and control handle.