What Really Happens When You Flush A Toilet While Someone's Showering

One long-standing family prank that has graced the scenes of our favorite TV shows from time immemorial is flushing the toilet while someone's in the shower. The implied consequence of this is that the shower would get super hot and screams would ensue, as well as a well-deserved smack on the head. Unless you're a physics major, understanding the mechanism behind this prank may be difficult. Sometimes, it even seems like a fable people make up just to make others laugh. However, this is a very real thing. When you're showering with even-tempered or warmer water and someone flushes, the cold water diverts to the toilet, leaving you with searing hot water. The simple solution is to stop anyone from flushing while you're bathing.

This phenomenon occurs when your toilet, bath, and shower are all connected by one central pipe that's responsible for carrying cold water called the trunk and branch plumbing system. There are central pipes for both cold and hot water. These main lines then branch out at a point to every water fixture in the house. This is bad news for the innocent soul in the shower because the cold water that was diluting the boiling hot water would instantly disappear. While there are steps you can take to stop this from happening, your first call should be to weigh the pros and cons of your current plumbing situation to know if it works in your best interest.

Oftentimes, your local plumbing ordinances will dictate how your pipes should be installed. Having a trunk and branch system of plumbing is not the end of the world. It's more traditional than other newer forms of plumbing but there are reasons why your locality may have instituted codes ensuring that everyone installs their pipes this way. On the upside, this plumbing system uses cheaper installation methods and materials, which is good for the homeowner. It is also conventional, which means that the chances of hiring a plumber who doesn't know how to fix it when there's an issue is low. 

On the flip side, a trunk and branch system not only causes this "flush-and-get-burned" disaster but also leads to low water pressure in the whole house. With all the cold water in the house coming from just one source, the water pressure tends to be low when someone is running it in another room. It's not effective. The water wastage associated with the trunk and branch system is another one of its disadvantages. When you turn the hot tap on, you have to go through several seconds of cold water before the hot water shows up. Thankfully, you can do something more tangible to curb the flushing problem and the other downsides of just locking the bathroom door when you shower.

The cheap solution to not getting blasted with hot water during a shower is just making sure no one is flushing the toilet at the same time. However, this may not be sustainable for people with larger families and limited bathrooms. Not every plumbing system is set up in a modern way, so your hands may be tied to an extent. You can still take measures to prevent this from happening so you don't get burned by hot water. The second most affordable way for you to stop this from happening is by marginally shutting the toilet fixture so that the amount of water rushing into it to fill up the tank would be reduced. This way, if someone does happen to flush while you're showering or bathing, the cold water taken from your bath wouldn't make it unbearably hot.

If you have a little bit more money to blow, you can opt for new pressure valves. The movement of water through your house's pipes is regulated by pressure valves. These valves are activated when you open or close the shower taps. Installing modernized pressure valves that can balance the pressure regardless of who is using what tap or flushing which toilet would alleviate the issue entirely. Make sure to talk to a plumber and check with your local plumbing laws for guidance and permission to make any upgrades.