Why Does My Shower Change Temperature When Someone Flushes a Toilet?

hand flushing toilet handle

It has happened to all of us who’ve lived in an older home. You’re in the middle of a nice, relaxing shower when someone flushes a toilet, and suddenly your hot shower feels like molten lava. Though you might just take this plumbing issue as a fact of existence, there’s actually a way to solve it--and no, it doesn’t involve not flushing.

The solution requires a little something called a thermostatic mixing valve. Below, we’ll explain what it is, how it works, and how it can help solve your shower woes. But first...

Why Does My Shower Temperature Change When Someone Flushes?

To understand why a flushing toilet can affect water temperature in a shower, you’ll first need to visualize how your bathroom’s plumbing is set up. In many bathrooms, there is one cold water supply line and one hot water supply line leading up to all of the plumbing fixtures. That means that the shower, sink, and toilet all share one hot water line and one cold water line.

If you’re like most people, when you shower, you use a mixture of both hot and cold water to achieve the perfect temperature (because hot water alone would be scalding). This balance between hot and cold can remain undisturbed--as long as nobody uses a lot of water all at once from a water line you are using for your shower.

So what happens when someone flushes a toilet? To flush, a toilet requires a lot of cold water all at once, especially if it’s an older, less efficient model. Unfortunately, that toilet shares a cold water line with your shower. When it flushes, it takes cold water pressure away from your shower. Consequently, you have much more hot water flowing to your shower now than cold, which makes the water that comes out extremely hot.

What Is a Thermostatic Mixing Valve and How Does it Work?

Instead of placing a ban on toilets anytime someone showers, you can update each of your home’s showers with a thermostatic mixing valve. These valves can detect a drop in water pressure (like the drop in cold water pressure when someone flushes), and they react by reducing the hot water flow to match the cold water flow. This keeps the temperature of the water that comes out consistent and reduces your risk of getting scalded.

Need help updating the plumbing in your home? At Albright's Mechanical Services, we offer a broad range of plumbing repair, replacement, and installation services. Contact us today online or by phone to learn more about how we can help you: (410) 834-0148!

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