Why Does Flushing the Toilet Make the Water in My Shower Cold?

Amy Pawlukiewicz
Written by Amy Pawlukiewicz
Updated January 11, 2022
A modern bathroom with tiles on the floor
Photo: Jacek Kadaj / Moment / Getty Images

Highlights

  • The main cause of cold shower water when the toilet flushes is your plumbing setup.

  • When the toilet tank needs to fill, it diverts water from your shower.

  • There are simple fixes that cost nothing.

  • Replacing a valve to solve the problem costs $100 and up.

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There’s an unspoken rule in every household (or maybe it’s spoken)—you don’t flush the toilet when someone is in the shower. Otherwise, you douse them with freezing water and then, possibly even scalding hot water. But what makes that happen? Here’s why flushing the toilet alters the shower water temperature and how you can fix it.

Your Plumbing Setup

If your family is experiencing water temperature changes in the shower when someone flushes the toilet, you likely have what’s called a trunk and branch (T&B) plumbing setup. Essentially, your water flows from one side of your house to another via one large pipe. Smaller pipes branch off from the large pipe and go to your shower, toilets, sinks, and washing machine.

When the plumbing is set up in a trunk and branch formation, sometimes there is only one pipe that goes to the bathroom to supply water to the sink, toilet, and tub or shower. So when one of those fixtures uses water, the other fixtures lose some of their flow.

The Flushing Problem

A finger pushing a flush toilet button
Photo: photographee.eu / Adobe Stock

With the trunk and branch, if you’re in the shower and someone flushes the toilet, the toilet needs to divert water from the shower to fill it back up. Usually when this happens, you’ll experience a burst of cold water as the water diverts, followed by the water in the shower warming up—sometimes too much—as the toilet steals the cold water to fill the tank.

Other Appliances and Fixtures

In homes with a trunk and branch plumbing system, you may experience drops in water pressure or temperature changes when family members use other items in your home like the sink, washer, or dishwasher. Fortunately, there are some solutions available to help solve this issue.

Solutions

While there are some benefits of cold showers, not many people want to find themselves taking one by surprise. Here are some ways you can prevent temperature fluctuations in the shower from water being diverted to other fixtures.

Don’t Flush

The simplest solution to the shower water problem is to make sure no one in your household flushes the toilet while someone else is in the shower. This costs nothing and eliminates the problem, at least temporarily.

Lower the Toilet Tank Water

Since water is diverted from the shower to the toilet to fill the tank after the toilet is flushed, another easy solution is to reduce the amount of water the toilet keeps in the tank. If you put a jug of water, brick, or other object that displaces water into the tank, it will reduce the amount of water your toilet needs. However, less water in the tank means less water to push waste out of your home, which can cause other plumbing problems.

Slow It Down

The rate at which your toilet refills plays a role in the water pressure and temperature change in the shower, so another easy fix is to slow down the water pressure to the toilet. You can do this by turning the valve under the toilet to the halfway position, allowing the toilet to fill more slowly. You may also hear noises like your shower whistling as a result.

Install a Thermostatic Mixing Valve

Some plumbing systems have what’s called a pressure-balancing valve, which balances the pressure between the hot and cold water so when the cold water pressure reduces, the hot water follows suit. Installing a thermostatic mixing valve for your shower will allow you to control water temperature and pressure separately in the shower. This is a job for a local plumbing professional and the cost starts at around $100.

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