I’m sure it’s happened to y’all at one time or another: you finish your morning run or workout, and you’re enjoying a perfect shower. Just as you’re ready to rinse the conditioner from your hair, someone in the house (kids, spouse) decides that’s the perfect moment to flush a toilet. Your delightfully warm shower instantly turns into a scalding torture chamber. When the screaming stops (both the pain screaming and the “who did that?” screaming, because everyone in the house knows that the shower turns into a hot geyser when a toilet flushes) you might find yourself wondering, ”why does the shower get hot when you flush the toilet?’
Well, as a diversion while recovering from the burns, I did a little checking. Turns out the answer is in the way the plumbing is set up in your house. Most homes have a “trunk and branch” system. One big pipe runs through the building, and smaller pipes “branch” off it to supply fixtures like sinks and toilets, or appliances like dishwashers. When one cold-water using fixture suddenly needs more cold water, as when a toilet flushes and refills, it reduces the supply to other “branch” pipes, including the pipe that supplies cold water to the shower. That perfect mix you landed on when you adjusted your shower is temporarily interrupted.
The simplest way to avoid this is to announce, in a loud and authoritative voice, “I’m going to take a shower. Don’t flush a toilet until I’m out!” Unfortunately, this doesn’t often work when others in your home (read teenagers, spouses) have their faces in their screens and respond with meaningless grunts indicating they heard your voice say something, but it couldn’t possibly have been as important as the game they’re playing or the text they’re typing. Sigh.
To avoid this, you can try slowing down the rate the toilet tank fills by slightly closing the supply valve to the tank. It’ll take longer to fill, and it might make noise, but it should reduce the sudden shift from “ahhhh” to “OWWW!” in the shower. Another trick is to put a brick in the toilet tank so it will use less water. The downside is that it may take two flushes to clear the bowl.
If these tricks don’t solve the scalding shower when the toilet flushes problem, it’s plumber time. A professional plumber can install something called a “thermostatic mixing valve,” that keeps the right balance of pressure between the hot and cold supply to the shower even when the toilet is drawing on the cold water supply. Now, if the overall water pressure fluctuates significantly along with the temperature, you might have something else going on causing low water pressure in your pipes. You know I’m all for do-it-yourself projects to save money, but don’t mess around with the pipes – leave that to a pro. You’ll thank yourself the next time you’re enjoying a perfectly balanced shower.