Why Does the Shower Heat Up When the Toilet is Flushed?

You know the drill: when someone is in the shower, you don’t flush the toilet … If you do, the poor person who is showering gets a face full of scalding hot water!

It’s unpleasant, painful and probably due to your water pressure in your home. That’s right, water pressure is to blame for your suddenly boiling shower. But how does it work? How does water pressure affect your shower’s temperature?

Usually plumbing is configured in a trunk and branch system. The water runs from one side of the building to the other in a large pipe, with small offshoot pipes that connect to different plumbing fixtures, like your shower, washer, toilet or sink. If you have lower water pressure in your home, when a fixture takes some of that water from the trunk pipe, it means there is less water for the rest of the fixtures. When you flush the toilet, it calls for cold water to fill up the tank. The toilet draws water from the trunk pipe. If you’re showering during this time, the shower will have less cold water to draw from in the trunk pipe, and compensate with more hot water. This heats up your shower!

This doesn’t only happen with your toilet. It could happen if someone turns on the washing machine, the sprinklers or gets a drink from the kitchen sink.

The easiest way to prevent a scalding shower is to limit how fast water fills up the toilet tank. You can do this by closing the supply valve on the wall behind the toilet slightly. This will make the toilet tank fill more slowly and allow for more water pressure to accommodate the shower. Never place a brick or other heavy object in the tank to decrease the amount of water used to fill up the tank. It may cause your toilet to overflow instead of flush, or not get solid waste down the s-bend.

Simply twist the supply valve behind your toilet to change your water pressure.

You can also purchase a pressure-balanced valve that will provide water at a constant temperature in your shower, regardless of if your water pressure changes in the hot or cold water lines. A pressure-balanced valve recognizes the drop in cold water when a toilet is flushed, and makes sure the hot water drops the same amount. The water temperature is the same, but the pressure of the water may be lessened. Jerry's Plumbing, Heating, & Air Conditioning can install this type of valve in your plumbing system.