Why Your Shower Changes Temperature When the Toilet Flushes (and How to Stop It)

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Few things disrupt a relaxing shower as much as an abrupt temperature change. While this can happen when someone uses a sink, runs the dishwasher, or does a load of laundry using hot water, it's most commonly associated with someone flushing a toilet. Here's why that happens—and how to stop it—according to two longtime plumbers.

Why does the water in a shower suddenly get hot or cold when someone flushes a toilet?

If you experience a sudden change in temperature in the shower when someone flushes a toilet or uses water somewhere else in the house, it is the result of a quick loss of pressure in the cold water lines, says Mark Collins, a fifth-generation plumber, and the CEO of 1-800-Plumber + Air. When this happens, it allows for more hot water to come out of the faucet. Typically, the temperature change lasts until the other source—including water refilling a toilet tank—is no longer in use.

The key reason for this problem is that the pipes feeding the shower and toilet are too small in diameter to provide enough pressure for both at the same time, says Hendrik Vandepoll, a plumber with more than 30 years of experience, and the co-owner of Service Force Plumbing in Rockville, Maryland. Ideally, a larger diameter pipe (e.g. 3/4") should run all the way to the bathroom, and then be split into two ½" pipes to supply the shower and toilet. "Very often though, a smaller-diameter pipe feeds both, and can't keep up when the cold water supply is suddenly asked to feed both fixtures," Vandepoll says.

Why does this happen in some homes, but not others?

You may have noticed that not all homes have showers that abruptly change temperature when a toilet flushes. But why is that?

According to Vandepoll, this problem is much more common—and the scalding effect far more pronounced—in older homes for a few reasons. First, older toilets used much larger holding tanks, which could increase the amount of cold water pulled away from the shower. Older shower heads had a higher flow rate as well, so the imbalance of hot and cold water could be more extreme, he says. Plus, older pipes may have significant mineral buildup, which could reduce the effective diameter of pipe that feeds the bathroom.

This problem is no longer an issue in most newer homes. "The main reason is that many states and local jurisdictions now require that shower fixtures be pressure-balancing or thermostatic to prevent scalding," Vandepoll says. The 1990 Building Officials and Code Administrators International (BOCA) National Plumbing Code was the first model plumbing code to require pressure balancing or thermostatic mixing valves on new builds. Because of this, Vandepoll says that he rarely hears about the shower/toilet flushing issue anymore in the Washington, DC, metro area where his company is based.

How to stop your toilet from controlling your shower

In addition to updates in building codes, Collins says that this problem isn't typically seen in homes built in the last 20 years, because the design of shower faucets have changed. All new faucets are now required to be “pressure balancing”—meaning their internal parts will self-balance, which allows the temperature to stay the same. The tradeoff, Collins says, is that this also reduces the water pressure coming out of the faucet.

Still, installing a pressure-balancing shower faucet is usually the best way to prevent your toilet from controlling the temperature of your shower. In fact, Vandepoll says that because most modern fixtures are pressure balancing, homeowners often address the shower/toilet problem unknowingly when they replace their older fixtures.

If you are experiencing this problem with a faucet you've had fewer than 20 years, Collins says that it's possible that your pressure-balancing faucet needs a new cartridge. "The internal cartridge on these faucets are designed to be replaced," he says. "If yours is older, it may be time for an upgrade."