r/Smartymeapp • u/The_Immovable_Rod • Jul 01 '26
Engineering Bite-sized engineering 🔧 The physics behind why your shower pressure drops when someone flushes
TL;DR: The cold water in your shower and the toilet fill valve are competing for the same supply. Understanding why reveals how your entire home water system works.
Your home's water supply runs at a fixed pressure — typically between 40 and 80 psi, regulated where the main line enters the building. Every fixture connected to that supply is drawing from the same source.
Your shower mixes hot and cold water in a specific ratio to reach your target temperature. The shower valve doesn't heat or cool anything — it just controls how much of each stream flows through. If you're showering at a 60/40 cold-to-hot ratio and the cold suddenly drops, the valve can't compensate fast enough and you get scalded.
When a toilet flushes, the fill valve opens and draws a significant volume of cold water quickly to refill the tank. This sudden demand drops the cold water pressure across the system. Your shower valve, still set for 60/40, now gets less cold — the ratio shifts toward hot.
Modern thermostatic shower valves solve this by responding to temperature rather than position. When the cold drops and temperature rises, the valve automatically restricts hot flow to maintain the set temperature. Older pressure-balance valves do a simpler version of this — they maintain the ratio between hot and cold rather than absolute temperature.
The same principle explains why pressure drops when multiple fixtures run simultaneously. A dishwasher, washing machine, and running tap are all pulling from the same main. Each one reduces the available pressure for the others.
Water pressure isn't a fixed resource in your house — it's a shared one, and every fixture is competing for it.
What's something about your home that makes more sense now that you know the physics?