Need more information generic answer to start read instructions ---if a single lever can adjust the cold and hot by adjusting the tabs on the mixing valve ---
Https://lifehacker.com/why-the-shower-gets-hot-or-cold-when-you-flush-and-how-508252782 It could be caused by an over-enthusiastic thermostatic mixing valve, which reduced the hot water flow too much. Using up all the water in the hot water tank would also lead to a cold shower.
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Need more information generic answer to start read instructions ---if a single lever can adjust the cold and hot by adjusting the tabs on the mixing valve ---
Https://lifehacker.com/why-the-shower-gets-hot-or-cold-when-you-flush-and-how-508252782
It could be caused by an over-enthusiastic thermostatic mixing valve, which reduced the hot water flow too much. Using up all the water in the hot water tank would also lead to a cold shower.