Why Your Shower Changes Temperature When It Flushes the Toilet (and How to Stop It)
Few things ruin a relaxing shower more than a sudden change in temperature. While this can happen when someone uses the sink, runs the dishwasher, or washes clothes in hot water, it is most often associated with someone flushing the toilet. Here’s why it happens and how to stop it, according to two veteran plumbers.
Why does the shower water suddenly become hot or cold when someone flushes the toilet?
If you experience a sudden change in shower temperature when someone flushes the toilet or uses water elsewhere in the house, it’s the result of a rapid loss of pressure in the cold water lines, says Mark Collins, a fifth-generation plumber. and CEO of 1-800-Plumber + Air . When this happens, more hot water comes out of the tap. Typically, the temperature change lasts until another source, including the water that fills the toilet tank, is no longer used.
The main reason for this problem is that the pipes supplying 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 co-owner. from Service Force Plumbing in Rockville, Maryland. Ideally, the larger diameter pipe (such as 3/4 inch) should go all the way to the bathroom and then split into two ½ inch pipes to supply water to the shower and toilet. “However, many times the smaller diameter pipe feeds both fixtures and fails when suddenly there is a need to supply cold water to both fixtures,” says Vandepoll.
Why does this happen in some houses and not in others?
You may have noticed that not all homes have showers that change temperature dramatically when the toilet is flushed. But why?
According to Vandepoll, this problem is much more common—and the burn effect is much more pronounced—in older homes for several reasons. First, older toilets used much larger holding tanks, which could increase the amount of cold water drawn from the shower. Older shower heads also had higher flow rates, so the hot and cold water imbalance could be more severe, he said. Additionally, older pipes may have significant mineral deposits that can reduce the effective diameter of the pipe supplying the bathroom.
This issue is no longer an issue in most new homes. “The main reason is that many states and local jurisdictions now require shower units to be pressure-balanced or thermostatic to prevent scalding,” says Vandepoll. The 1990 National Plumbing Code , adopted by the International Committee of Building Officials and Code Administrators (BOCA), was the first model plumbing code that required the use of pressure balancing valves or thermostatic mixing valves in new buildings. Because of this, Vandepoll says he rarely hears about shower and toilet flushing issues in the Washington, D.C. metro area, where his company is based.
How to prevent the toilet from controlling the shower
In addition to building code updates, Collins says this problem is not typically found in homes built in the last 20 years because shower faucet designs have changed. All new faucets must now be ‘pressure balanced’ – meaning that their internal parts will self-balance, allowing the temperature to remain constant. The trade-off, Collins says, is that it also reduces the pressure of the water coming out of the faucet.
However, installing a pressure-balancing shower faucet is usually the best way to keep the toilet from controlling the shower temperature. In fact, Vandepoll says that because most modern fixtures are pressure balancing, homeowners often unknowingly solve a shower and toilet problem when they replace their old fixtures.
If you’re experiencing this problem with a faucet you’ve had for less than 20 years, Collins says your pressure-balanced faucet may need a new cartridge. “The internal cartridge of these faucets is designed to be replaced,” he says. “If yours is old, it might be time to upgrade.”