shower and toilet connection of the past?

Question

In the past, if someone was showering and the toilet was flushed. The water in the shower would go suddenly hot or suddenly cold.

Why did that happen? What changed to prevent it?

In: 111

Answer 1

Shared water lines where when the toilet needs more water (to fill its tank up) then less cold water gets sent to the shower.

In the US, it became part of plumbing code to add in values to fix this.
https://www.howtolookatahouse.com/Blog/Entries/2019/12/when-were-shower-control-valves-first-required-by-code-to-be-pressure-balanced-and-temperature-limiting-single-handle.html

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EDIT: As the other user mentioned, you could hook up the hot water line to the toilet which would cause the shower to get cold when flushed. I would assume a washing machine and dishwasher, if also sharing the water line, also could cause this.