I'm not sure if this question is for Physics SE, but I'd go ahead and ask.

I was wondering how the heated showers work (the ones with the hot and cold knobs). I have always been curious and this may clear my head on the questions such as :

  1. Why is the temperature so volatile? (A slight turn to Hot makes it REALLY hot, and the inverse as well)
  2. How is the temperature regulated?

closed as off-topic by ACuriousMind, Brandon Enright, John Rennie, David Z Jul 24 '14 at 10:15



  • $\begingroup$ I do not get which kind of device you mean. With just two valves, or with temperature regulation too? Are you asking why that regulation works pretty slow until the temparature get's constant? (Could be that the question is better suited for diy.stackexchange.com) $\endgroup$ – Volker Siegel Jul 24 '14 at 2:39
  • $\begingroup$ I think there is an interaction between the faucet regulator and the water heater, thinking of a gas heater - that may make the question somewhat interesting and on-topic. But you write "with hot and cold knobs" - the variant I'm thinking of is different, it has a temparature knob, and a water flow rate knob, which are independant (or at least pretend to be). $\endgroup$ – Volker Siegel Jul 24 '14 at 2:47
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    $\begingroup$ The temperature is not so "volatile", as you describe it, in the showers I've used in my life. $\endgroup$ – NeutronStar Jul 24 '14 at 2:47
  • $\begingroup$ This is a question about plumbing, not about physics. $\endgroup$ – David Richerby Jul 24 '14 at 8:14

The showers I have used that have two knobs have something akin to a globe valve for each line enter image description here As you unscrew the handle, there is more opening between the plug and the body, letting more water through. The resistance of valves like this is quite variable when the plug is near the body. Once the plug is withdrawn a certain amount the resistance is small. If you were happy with the temperature that came from maximum flow through each of the hot and cold lines, it would be almost constant. Unfortunately, you probably want a temperature that uses all of one pipe you can get and just a little of the other. For me, I open the hot all the way and use the amount of cold to get the proper temperature. Changes in the geometry of the cold valve can change the flow significantly, leading to a change in temperature. In my shower, the water takes a noticeable step down in temperature after a certain time of use. The two valves are close together and connected by copper pipe. I take the change to be caused by the body of the cold valve heating due to conduction from the hot side. That increases the opening and necessitates an adjustment to the valve.

  • $\begingroup$ I see. So there is a repository of pre-heated water, and the heated water would flow if the "Hot" knob was turned? $\endgroup$ – Mark Gabriel Jul 24 '14 at 2:53
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    $\begingroup$ Yes, there is a water heater with a tank that keeps the water heated. Typical US home ones are 30-50 gallons (120-200 litres). Typical temperatures are 120-140F (49-60C). When you turn on the hot water, it takes a while to get hot because the water stored in the pipe between the heater and the shower has cooled. $\endgroup$ – Ross Millikan Jul 24 '14 at 3:30
