I do not understand the advantage of the popular "hot" and "cold" controls in showers, as opposed to "temperature" and "quantity" controls.
Is it simply because the latter option is harder to implement? Why isn't it more common? It would make much more sense for the user to be able to control the temperature and the quantity as discrete items rather than how it is now.
As with many other devices (eg the QWERTY keyboard) the hot/cold tap persists not because it's the most usable design, but because of:
That said, there are temperature/pressure controls available today, and they are AWESOME to use. You can do a search for thermostatic shower faucets (or mixers, or valves) to find out more.
Pressure-temperature faucets are used with varying frequency around the world (see comments below), and I've seen them in Japan and various countries in Europe and Asia.
These faucets are particularly amazing for filling baths consistently because water has a pretty high specific heat capacity so getting the temperature wrong by just a few degrees while filling a bath means spending quite a of time waiting to get the right temperature or adjusting the water mix.
Here are two common designs....
Here is a popular format with end-mounted temperature and flow knobs common in Japan and elsewhere:
These faucets often have a lock, label and/or a click affordance around 38°C to provide additional ergonomics around safe water temperatures.
For more background on how these work, this brochure from Delta Faucets is quite informative.
I have not seen these in Europe (that's not to say they aren't available or used), but have seen them in many places in Asia. Here is a typical format with temperature and pressure knobs:
These systems use varying combinations of electric/mechanical/electromechanical innards to provide separate pressure and temperature controls from a single cold-water input.
There are other pressure-temperature formats available including some crazy high end designs like this one with 4+ shower heads, steam option, custom lighting, ambient music, remote control, storable user presets, and more:
I suppose it's mostly a question of how much money you want to invest into your fittings. In most cases you'll have one pipe for hot and one for cold water. The knobs then just open and close those pipes – I can hardly think of any easier / cheaper solution. However there are actually different solutions that do exactly what you describe:
Visiting Canada in the late 80ies I was confronted with shower controls for 'temperature' and 'pressure'. I don't know if that was a tailored system, but it was confusing anyways. One difficulty was also, that depending on the pressure, the temperature would slightly change – so you always had to adjust both knobs.
At least in Germany you can buy 'mixing batteries' for about twice the price regular fittings would cost. Moving the hand gear up and down controls the pressure – moving it right-left controls the temperature. Leaving the handle in the centered position you'll always have a comfortable 'warm' mix.
From a UX point of view I would recommend the latter – it also minimizes the problem of getting dirty hands clean while not getting clean fittings dirty.
ps: for everybody who "have never seen a shower that has separate hot/cold knobs like you describe" I just have to add this photo of a 'regular' (cheap) shower fitting in Germany/Berlin with two basic knobs hot/cold:
I can honestly say I have never seen a shower that has separate hot/cold knobs like you describe, and I've lived everywhere up and down both the east and west US coast.
Every shower I've ever seen has two concentric wheels. The inner one controls temperature while the outer one controls pressure.
Here's what they look like (although the labels around the outside seem all wrong... unless they mean to say the shower is only comfortable when off...):
Hot and cold knobs work great, and everyone intuitively understands that turning the knob makes more water come out, ie more water pressure. The practical reason is it gives maximum control with the fewest parts. Also, the range for usable water pressure is not very large, and dedicating an entire knob to it seems to be a waste. Your solution would require more complex combination hot/cold valves for the temperature control, yet offers no practical benefits.
From a UI standpoint, what happens in your system when I turn the temperature knob, but water strength is at zero? Nothing comes out? That is confusing to someone who knows turning either knob make water come out. I fail to see how that is better, when the original system gives feedback regardless of which control is used.
As far as controlling the temperature and pressure separately, that's exactly what hot/cold knobs already do. Too hot, add a bit of cold. Not enough pressure, turn up both knobs the same amount.
That sell shower controls similar to your description and I wouldn't call them better. I have them at my house, and I cannot stand them and wish I had hot/cold valves.
Separate Hot/Cold valves is a simple analog solution that just 'works'.
edit I may have oversimplified some aspects, but the basic thing it boils down to is COST. The fancy temperature controlled shower head show above is much costlier than simple hot/cold valves. I think some folks get spoiled designing UIs where fancy controls are 'free'. We are dealing with actual, physical items that break here, not UI controls. Home builders look for the most cost effective solution that works and is easily repaired. That is traditional hot/cold valves
They don't any more. They used to because it is the most mechanically simple implementation of temperature control. However, this is only historical.
None of the other answers mention the legalities of this. The International Building Code (what most US local codes are modeled on these days) mandates that all new shower controls must be temperature regulating design.
International Residential Code Chapter 27, P2708.3 Shower control valves:
Individual shower and tub/shower combination valves shall be equipped with control valves of the pressure-balance, thermostatic-mixing or combination pressure-balance/thermostatic-mixing valve types with a high limit stop in accordance with ASSE 1016 or ASME A112.18.1/CSA B125.1. The high limit stop shall be set to limit the water temperature to not greater than 120°F (49°C). In-line thermostatic valves shall not be used for compliance with this section.
I found it very amusing reading all these answers which I assume were written by people in the US.
I live in the UK and visit France and Spain very regularly. I believe a resident of any of these countries would be appalled to find a hot/cold shower in a hotel room in Europe - I have not seen such a thing for many years.
All the talk about the the difficulty in manufacturing temperature/flow showers should bear in mind that there may well be 1 billion of them in Europe, and I don't think we can generalise that Europeans are generally wealthier than Americans.
To put in context, I recently had a new shower installed and the shower valve is a ceramic disc, thermostatically controlled mixer with 4 independent flow outlets. The valve cost around £600.link. This very much the "Rolls Royce" of shower mixers, but http://www.screwfix.com/p/swirl-vino-thermostatic-mixer-shower-flexible-exposed-chrome-effect/39565 this is a much more basic model for £45 with all the external kit included. I've seen the valve alone for £40 before. Many people are now installing the next evolution where the temperature and flow are controlled digitally and are adjusted using buttons. Each family member can program their own preference so a single button press will set up the shower to their requirements. I know someone who has had one of these for about ten years.
The point being that these are standard kit in Europe (and Japan by the sound of it) so I think it's safer to ask the question "Why do Americans persist in using hot/cold showers?"
Some systems in South America (called "calefón" in my country) require certain level of pressure to get the hot water running (for example, you can turn the knob 30º without the "calefon" getting activated in order change the water temperature), so in that case it's a device limitation.
In Australia, most showers have separate hot and cold knobs, and the more modern remainder are 'mixing batteries' like what tillinberlin describes.
There are some big UX problems with hot/cold taps:
When turning on the shower it takes time to adjust the taps to get the right temperature and pressure.
People must be taught to turn on the cold tap first and then the hot tap, to avoid scalding.
Sometime, rarely, in houses with low pressure the hot must be turned on first to get any hot water. Maybe this is because the pressure to hot water system drops too much if the cold is on first.
Usually the hot tap is on the left and the cold tap is on the right, but sometimes this is reversed! This often leads to the hot being accidentally turned on first, scalding the user.
There are some advantages to hot/cold taps though:
If you want a cold shower you only have to turn one knob. :P
The mechanism is simple with a very long life.
There is no little lag when changing the temperature.
To be honest, I never knew temperature/pressure knobs existed. Will be getting one for the new bathroom. TYSE!
While most countries have the mechanical knob as mentioned in all the other comments, Japan in fact, does have the option to control the water temperature and quantity.
You can even set an alarm that rings when your water has reached the desired temperature.
As for quantity and relatively pressure control, the toilets give you the option to select between three settings; low medium high.
This question assumes that “temperature” and “quantity” knobs are easier to use.
And yes, those types of controls are easier to use for me at least, but for my elderly mother, those types of controls have never been easy to use.
My mother could never tell which knob was for pressure and which knob was for the temperature. Part of the problem I think is that these types of controls are not all uniformly identical. I suppose that the shower fixture that cost $350+ would probably work for everyone, but the problem is that not everyone is willing to spend that much money on a shower fixture.
So when confronted with that choice at the store when you're already over-budget on your bathroom renovation, it's sometimes just easier to go for the cheapest $69 model (with only a 2.8 stars rating and mostly bad customer reviews) just to get the job done.
Why? Because of all sorts of reasons outside of UX.
But I'd ask the reverse...why would you have both a temperature and quantity option? When showering, there usually isn't a gradient of amount. It's either on or off. No real need for a spectrum there.
As for temp, temp is created by adjusting the individual amount of cold and hot. In otherwords, the temperature is the 'quantity' variable. There is no temperature control. One merely is controlling the quantity.
This is a necessity throughout history as plumbing has mostly always been analog.
Now in the age of digital, your suggestion may make a whole lot more sense. But it simply didn't make sense with an analog valve system.
In Britain, at least, there are architectural/plumbing issues in the past that we had to deal with. This guy explains it better than I could: https://youtu.be/HfHgUu_8KgA
It is funny that there are so many answers to this question without getting one that has to do with the actual function of a shower valve.
Let's go over the 3 examples that we have as examples already:
1.
This shower system has electronic mechanisms to calibrate the temperature and flow. The pros include easy user interface, looks cool (maybe because its different), and allows user to turn a little knob.
The cons are many though. First this must be calibrated correctly. After calibration the systems hot/cold water flow is changed it will become uncalibrated. Also what if you favorite temperature is between two of the settings? Now you are either a little too hot or a little too cold. And then the most obvious thing is cost/repairs. The low end ones I have seen are $500 plus compared to $50 for a more traditional setup.
2.
I kept the image even though a don't agree with it. Most modern versions of this have a decent size comfortable zone and this is adjustable. Note that in the application the water will divert through one valve like, the same as example 1. The water will be mixed together before being shot out. The pros include familiarity, ease of use, and the ability to tune your temperature tighter.
The biggest con with these is that most do not come with any sort of flow rate control. It is either on or off. The anti-scald mechanism is another feature that these have and not sure if it is a pro or a con - to some it will be one and to others the opposite. I for one like to be able to set the temperature as high as I want - especially useful when cleaning. The last con also has to do with the anti-scald mechanism. When the flow rate is decreased (think toilet flushing or someone doing the laundry) the valve will automatically process slower to reduce chance for temp changes. Which means you get a limp shower for a few minutes.
3. Good old knobs. The key to the knobs is that they do not (normally) have a shared mixing valve. Each operate independently of one another. The pros include they are cheap, they are intuitively easy to use (other than some people have hot on the left), only with knobs can you get the EXACT TEMPERATURE AND FLOW, and they are easy/cheap to install.
There are many cons on the knob side either. Some may say how they look, but I have seen some cool knobs. Another would be that you have to turn two separate things. If you have children the fact that there is no anti-scald mechanism is a con. Children can turn knobs and burn themselves easier. However the fatal flaw of the knobs is that you don't know what you get until you get it. You turn the hot and cold and just guess that the temperature might be right. You feel the water and adjust until it is right and continue to do so every shower there after.
Which is the best? Each of these choices offers different levels of work, different levels on install money, and different levels of controlability by the end user. Like any UX question, design should not trump functionality.
