Hot Water Fluctuations In The Shower by Pete Busch If you have an older home you may have noticed that when your taking a shower the water temperature will fluctuate, sometimes a lot. When a toilet is flushed the pressure is reduced on the cold water supply pipes. This will reduce the amount of cold water going to your shower resulting in very hot water spraying all over you. The same holds true if someone turns on the hot water somewhere else in the home. Your shower.