eli5: Why is it so difficult to adjust the temperature of water in the shower?

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Like seriously?

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Your pipes might need some tuning from a plumber. But nowadays you can install digital system that heats your water to a specific temperature.

There’s in input lag for it to change the temp and het to the showerhead.
Most people crank the dial, wait, crank it some more, get scalded, cranking it completely the other direction, freeze solid etc.
Just a little dab and patience..

The pipe connecting the temperature controller to the shower head and water in it.
While you change the temperature controller, you change the water temperature flow passing through it, not the water already passed, namely, in the pipe. Many people don’t realize this and use the pipe eater’s temperature to decide how much to turn the controller, the result is often overshooting the controller.

The margin of tolerance is surprisingly small, and different depending on your preference.

The heated water in my building was down for a week so we needed a temporary electric tank where you manually input the temperature.

I like my showers hot, and so every time I took a shower it HAD TO BE on 38.1°C for me. Any less it felt too cold and any more felt scolding. My girlfriend, likes her showers not as hot. Her ideal setting was 37.5°C, which felt just lukewarm to me.

In many people’s homes, scaling accumulates in the hot water tank more so than the hot water line. That means that it’s easier to get hot water through than cold water, and even though you have high cold flow, you probably have low cold pressure. Thrusting high pressure hot water in prevents cold water from flowing, sometimes altogether. Thus, you end up needing to close the valve enough so there’s also a sufficient pressure drop across the valve so they balance out what’s comfortable for you, except the valve design is more for flow control, not pressure control. E.g. 10% open = 10% flow. 40% open = 40% flow.

Another related question. Why are the showers always controlled differently everywhere you move to?

This is a problem the producers of enriched uranium had to solve. Keeping the temperature constant in a varying flow of liquid. The answer came from the dairy industry, Taylor industries and their method for pasteurizing milk in a continuous flow method.

P and ID control was born! (PID) It was top secret in WW2, but now is the basis for nearly all control schemes.

Because for most of us we either have the vague hot and cold scale or the dual valves for hot and cold.

The first one is not only small making micro adjustments hard but has no readings to give actual °C levels.

The second one is dumb and makes you play alchemy with the two taps to find your ideal water.

The main reason I could see this being so unreliable is that unless it’s super new water heaters can’t do precision as well as we want our showers to work at.