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I am trying to diagnose a problem with the water temperature in a large building. Both the shower and tub have the same problem so it is not a valve issue. The shower has a completely different valve system than the tub but the problem is the same for both so it is not valve.

There are two problems. One is that it is difficult to set a proper middle temperature. It is either scalding hot or freezing cold. Also, the water seems to "remember" the temperature. So for example, if I turn it to hot and it gets scalding hot then I try to add cold water, even if I turn the hot water off and have nothing but cold, it is still too hot, and vice versa. If I have cold, then turn on hot fully, it stays cold, even though I had hot on previously so there should already be hot water in the lines; it seems impossible. It is almost like turning on one temperature is blocking the other one somehow.

The second problem is that the temperature changes relatively rapidly. So, even if I get the temperature in the tiny sweet spot where it is correct, within a minute or two it will start drifting cold. This is happening so fast that it is not a tank depletion issue. I think the building might have insta-hots. Are these problems typical of insta hot systems?

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  • I wouldn't expect an insta-hot to be an issue unless you are suggesting that is what is being used for the bath too. If so, I would expect it to be an issue. While you state it is not a valve issue, it still could be. For one, what is the temp setting for the hot water? The hotter the setting, the more difficult it will be to mix, not to mention too hot of water can damage a plastic valve component. Also, if it is insta-hot, they are not generally meant for bath, so the main q here, is the unit properly sized for the use? Going to need more information on what the hot supply system is
    – noybman
    Commented Oct 14, 2017 at 14:24
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    If the water circulating in the building supply lines is very hot (say 150 F to 160 F or above) there may be a thermostatic mixing valve in the wall behind the shower/tub that is designed to limit the temperature going into the hot supply of the shower valve to something like 120 F. If the circulating hot water is too hot, then this thermostatic valve could be unable to control or be damaged. How hot is the hot water from the kitchen faucet? Does it have the same control problem as the tub/shower?
    – Jim Stewart
    Commented Oct 14, 2017 at 17:34

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I figured out the problem: insta-hots, or what are called "tankless" hot water systems, have a pressure drop across the heat exchanger. If water is fed to both the cold and hot line at the same pressure, then there will be a pressure differential between hot and cold, the cold being higher. If both supplies are going to a common valve, then the cold will back into the hot supply. This can cause weird variations in shower temperature. For example, if a shower is started and the temperature is set, it will creep colder and colder because over time, more cold water creeps into the hot water line.

To solve the problem, it is essential that the pressure of the hot water coming out of the tankless system matches the pressure of the cold water system.

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  • Are you sure that you aren't just getting more cold water because it is being fed at higher pressure? There is little back pressure from the shower head, so the water sources will just mix coming out. When the shower valve is shut off, the hot and cold should each be blocked; the cold shouldn't be feeding into the water heater output. If the shower has a "suspend" button that temporarily stops the output, cold still shouldn't backflow into the heater because that is being fed by the same pressure.
    – fixer1234
    Commented Oct 29, 2017 at 1:36

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