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?
Best Answer
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.