Well designed and poorly designed shower control valves

Here is an example of a poorly designed shower control valve.The way that one can recognize a poorly designed shower control valve is that there is only one knob or lever.  The problem with such a shower control valve is that its designer tries to do too much with that one control (the single knob or lever).  The single control is somehow supposed to determine:

• whether or not water is coming out of the shower nozzle, and how much water is coming out, and
• what temperature the water is.

A first evil in this design is that any hard-earned information about what control position yields a good temperature gets actively discarded every time the user shuts off the water.  The next time the shower is used, the user is forced to start all over again with the experiment of trying to figure out what control position might yield a good temperature.

Yes I suppose if there is a repeat user of a shower, for example in a home, the user can attempt to memorize the control position that yields an acceptable temperature.  “When the lever points to just below about three o’clock then it is about right,” that kind of thing.  But this is no good in a hotel where there is no accumulated experience with the particular valve.  And if the control is a knob instead of a lever, it may be difficult to judge visually the angular position of the knob with accuracy from one use of the shower to the next. 

A second evil in this design is that the rotating control cannot both (a) adjust the temperature and (b) adjust the amount of flow.  The usual design is that the first few degrees of rotation turn the water flow on “all the way” and then the remaining range of rotation leaves the “all the way” flow rate in place and only modifies the temperature.

Some single-control shower valves try to use two mechanical degrees of freedom as the user interface for controlling the two things that need to be controlled.  Pulling out and pushing in controls the on/off and flow rate, while rotation controls the temperature.  This approach does address the second evil, because it does permit the user to determine the water temperature and also permits the user to select less-than-full water flow.   But this approach does not fully address the first evil, because usually the act of fiddling with the knob to push it all the way in to turn off the water will inadvertently cause some less-than-fully-controlled rotation one way or another, and the “memory” of an angular position yielding a desired temperature will have been lost.

A related bad design is a two-valve approach where a first rotary valve feeds hot water to the shower nozzle and a second rotary valve feeds cold water to the shower nozzle.  You can usually recognize such an approach because the two valves are in a horizontal line, one to the left and one to the right.  Yes, the user can eventually arrive at any desired greater or lesser flow rate, and can arrive at any desired temperature, by a suitable manipulation of the two valves.  But at the end of the shower, when the two valves get closed, once again any hard-earned information about how the desired temperature was accomplished is actively discarded.  I have to assume that this two-valve approach is simply an accident of history, tied to the idea of how to control a shower if the only kinds of valves that you had available at the time were individual valves that each had one pipe in and one pipe out and were inexpensive.  (The only nod to user customization in those early days was to make it that one valve rotated “on to the left” and the other knob rotated “on to the right”.)

There is, of course, a correct way to design a shower control valve.  I am baffled as to why every hotel does not install this kind of control valve.  The starting point for the design of such a shower control valve is to admit openly that there are two distinct user interface goals.  A first goal is to permit the user to determine whether or not water is flowing, and how much.  A second goal is to permit the user to pick the temperature of the water.  The solution, apparently obvious only in hindsight, is to allocate a separate control to each of these goals.  An example of a correct design is shown at right, where the top control determines whether the shower is on or off, and determines the flow rate.  (It also determines, by rotating to the left or right, whether the water comes out of a shower head or comes out of a second place such as a tub spout or a hand-held spray.)  With the control shown at right, the bottom control determines the temperature.

For a repeat user, one nice thing about this approach is that the bottom knob can be left untouched from one shower to the next and the temperature will be roughly the same from one shower to the next.

I invite the reader to consider that a moment or two of thought, on a one-time basis, when selecting a shower control valve in the first place, can have a nice payoff for hundreds or thousands of pleasant shower experiences thereafter. 

The alert reader will find that some shower control valve systems that answer to this general description (having a separate temperature control) are gobsmackingly expensive.  Some of them are mechanically very complex and some actually require not only connections to pipes but also a connection to electrical power. 

But with a bit of clicking around on the Internet, one can find pure mechanical shower control valves that work very well, that have a separate temperature control, that are reasonably priced, and that are not much more mechanically complex than their simple and poorly designed single-control counterparts.