Why Does Flushing the Toilet Affect the Shower?

Have you experienced taking a refreshing cold bath when suddenly, the shower water gets hot because somebody flushed the toilet? You know it isn’t a coincidence, but why does flushing the toilet affect the shower?

This incident happens when you have an older plumbing system, particularly a trunk-and-branch configuration. Flushing the toilet siphons the cold water away from your shower faucet, causing the hot water in the plumbing to come in and replace the lost fluid.

But don’t fret; this article has the explanation and solution to this problem.

Why Flushing the Toilet Changes Shower Water Temperature

If the water becomes too hot in the shower when the toilet is flushed, it’s because the cold water is moving to the toilet tank for a refill. As a result, the water temperature of the other fixture in use—the shower—momentarily changes.

The traditional trunk-and-branch design is the common culprit. This plumbing configuration consists of a large pipe connected to the main supply line, commonly ¾ inch in diameter.

This large pipe is the trunk that runs from one end of the house to another, with smaller pipes typically of ½ or ⅜ inch in diameter branching out towards fixtures to deliver water.

The pipes for different fixtures are interconnected. Therefore, when one unit requires cold water, like when the toilet tank is refilled after flushing, the cold water in the plumbing system will temporarily decrease in amount and pressure, affecting the shower’s water temperature.

With this system at home, the toilet and the shower won’t “steal” cold water from each other, since the pipes are not interconnected.

If you want to transform your trunk-and-branch into a home-run manifold, we recommend discussing it with a professional plumber, since it’s a lot of work (and money, too!). But if you seek simple troubleshooting, continue reading below for some tips.

How Else Can You Solve This?

As we mentioned, the trunk-and-branch plumbing system is traditional. If your house is old, you might have this configuration. Upgrading your plumbing can be rigorous and expensive, so plan and budget effectively with the plumber’s help.

In the meantime, we listed manageable solutions that may work for your shower to prevent scalding from a sudden flow of hot water.

1. Don’t flush the toilet while showering. The easiest way to keep water in the shower from turning hot is to not use or flush the toilet while you take a bath.
2. Lessen toilet tank water flow. While this will make the refill time slower than usual, closing the toilet’s shut-off valve a bit may help prevent shower water temperature fluctuations. However, this should only be a temporary solution, as the lack of tank water may make flushing inefficient.
3. Upgrade to an efficient toilet. A more effective way of lessening the toilet tank’s required water is to upgrade to an efficient fixture that requires less water to flush (such as a 1.28-gallon unit).
4. Install a thermostatic mixing valve (TMV). If you never want to make your shower water hotter even when flushing the toilet, invest in a thermostatic mixing valve. It regulates the water temperature by mixing cold and hot liquids to ensure comfort and safety.

Conclusion

Why does flushing the toilet affect the shower? Flushing makes the shower water hot because in older plumbing like the trunk-and-branch configuration, the pipes that supply water to the fixtures are integrated.

Interconnected pipes cause cold water to be siphoned when you flush the toilet, temporarily taking the cold water from the system. In return, the shower is left with hot water, and you have to wait for a while for the preferred temperature to come back.