Buzz
Few things interrupt a peaceful shower like an unexpected temperature shift. While this can happen when someone uses the sink, runs the dishwasher, or does laundry with hot water, it's most commonly linked to flushing the toilet. Here’s why this occurs—and how to stop it—according to two experienced plumbers.
Why does the shower water temperature suddenly change when someone flushes the toilet?
If you notice a sudden change in the shower’s temperature when someone flushes the toilet or uses water elsewhere in the house, it’s caused by a rapid drop in pressure in the cold water lines, explains Mark Collins, a fifth-generation plumber and CEO of 1-800-Plumber + Air. This causes more hot water to flow through the faucet. The temperature fluctuation typically lasts until the other water source, like the toilet tank refilling, is no longer in use.
The main cause of this issue is that the pipes supplying both the shower and toilet are too narrow to maintain sufficient pressure for both at once, explains Hendrik Vandepoll, a plumber with over 30 years of experience and co-owner of Service Force Plumbing in Rockville, Maryland. Ideally, a larger-diameter pipe (e.g., 3/4") should run directly to the bathroom and then split into two ½" pipes for the shower and toilet. However, in many cases, a smaller pipe serves both, which struggles to provide enough pressure when both fixtures demand cold water simultaneously, says Vandepoll.
Why does this issue occur in some homes but not others?
You might have noticed that not every home experiences a sudden temperature change in the shower when the toilet is flushed. But why does this happen in some homes and not others?
Vandepoll explains that this issue is more prevalent—and the risk of scalding is higher—in older homes for several reasons. First, older toilets used larger tanks, which could draw more cold water away from the shower. Older shower heads also had higher flow rates, which could amplify the imbalance of hot and cold water. Additionally, older pipes often have significant mineral buildup, reducing the effective diameter of the pipes supplying the bathroom.
This problem is largely a thing of the past in newer homes. "The main reason is that many states and local jurisdictions now require shower fixtures to be pressure-balancing or thermostatic to avoid scalding," says Vandepoll. The 1990 Building Officials and Code Administrators International (BOCA) National Plumbing Code was the first to mandate pressure balancing or thermostatic mixing valves in new construction. As a result, Vandepoll rarely hears about the shower-toilet flushing issue anymore in the Washington, DC, metro area where his business operates.
How to prevent your toilet from affecting your shower's temperature
In addition to recent changes in building codes, Collins explains that homes built within the last 20 years typically don't experience this issue because shower faucet designs have evolved. New faucets are now required to be 'pressure balancing,' meaning their internal components automatically adjust to maintain a consistent temperature. However, Collins notes that this feature can reduce water pressure at the faucet.
Even so, installing a pressure-balancing shower faucet is usually the most effective solution to stop your toilet from influencing your shower’s temperature. In fact, Vandepoll mentions that most modern fixtures are pressure balancing, so homeowners often unknowingly resolve the shower/toilet problem when replacing old fixtures.
If you're facing this issue with a faucet that's less than 20 years old, Collins suggests that your pressure-balancing faucet may need a new cartridge. 'The internal cartridge in these faucets is designed for replacement,' he explains. 'If your faucet is older, it could be time for an upgrade.'
