Why Does Flushing the Toilet Change the Shower Temperature?

Sharing a bathroom is a common consequence of modern living. The more people in a household, the better the chances you’ll be in need of relief when someone is showering. In addition to being slightly awkward, it can also be painful for the person in the stall. Flushing a toilet can send scalding hot water over the hapless occupant. But how and why does that happen?

Elizabeth Yuko of Lifehacker recently offered a succinct explanation. When a toilet fed off the same water supply line as a shower is flushed, cold water pressure drops while the toilet tank is being refilled. It’s not an influx of hot water that sizzles the shower occupant, but a rapid loss of cold water that can no longer dilute what’s coming from the hot supply line. The result: lots of yelping and jumping around.

So why can’t the shower handle it? The fault usually resides in the bathroom’s supply lines. In older homes, there’s often one narrow line feeding both fixtures. When the toilet is emptied and requires refilling, the line can’t maintain enough pressure to fill the tank and keep the shower water at a comfortable temperature.

Toilets aren’t the only culprit: Any appliance that uses an influx of cold water can also turn a shower sizzling, including washing machines, dishwashers, and sprinklers.

The problem is less pronounced in newer homes and apartments, as modern building codes often mandate fixtures that can maintain thermostatic pressure by themselves. Some newer shower valves may also do pressure balancing on their own. These newer valves can sense when cold water pressure drops and will then throttle hot water so the temperature remains consistent. The drawback here is that the water pressure will drop temporarily.

If you're unable or unwilling to upgrade your shower valve, you can try tightening the water supply valve running to your toilet, which will slow the amount of water replenishing the tank. Obviously, this means the tank will take longer to fill, but at least no one will need skin grafts.

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