Why Does My Shower Water Pressure Drop When The Toilet Flushes?



Hand under shower head running water

If your shower water pressure drops when the toilet flushes, you most likely have one of these 4 problems:

  • You have a pressure-balancing mixing shower valve and need to switch to a thermostatic mixing valve
  • Your pressure reducing valve is turned too low or isn’t working properly
  • Your bathroom piping is restricted
  • Your bathroom piping is undersized

Wondering which particular issue is haunting your shower experience? Start by looking at how dramatically your water pressure drops.

Thermostatic mixing valve vs. Pressure balanced mixing valve

Photo source: Signature Hardware

If your water pressure drop is noticeable but there’s still enough water to continue showering…

You probably have a pressure-balancing shower mixing valve and need to switch to a thermostatic mixing valve.

Shower mixing valves are responsible for controlling the water temperature of your shower. They work by automatically “mixing” hot and cold water.

Pressure-balancing valves usually have just one handle and respond to changes in the pressure of your water. It’s the more basic of the two valves because it can’t sense temperature changes and only indirectly controls the water temperature.

Basically, they work by adjusting the ratio of hot to cold water pressure.

So, when your toilet flushes it pulls cold water from your shower. The pressure-balancing valve senses the drop-in cold water pressure and responds by restricting the hot water pressure.

The result is a short period where you’re blasted with hot water followed by a noticeable drop in water pressure until the toilet bowl fills back up (about 30 seconds).

The solution? Switch to a thermostatic mixing valve.

Thermostatic mixing valves typically have two handles: one controls the water pressure, and one controls the water temperature.

They work similarly to a pressure-balancing valve in that it blends hot and cold water, but they directly control the water temperature without any changes in water pressure.

Want to learn more about the difference between these 2 valves? Check out our article, Why Is It So Difficult to Get My Shower Temperature Just Right?

Want to make the switch to a thermostatic mixing valve? Contact an ATL plumber to help you install your new valve.

If your water pressure slows to a trickle or dramatically decreases…

You probably have one of these issues:

  • Your pressure reducing valve is turned too low or isn’t working properly
  • Your bathroom piping is restricted or undersized

To test which is your particular issue, follow these steps:

  1. Turn on your shower.
  2. Turn on faucets and other cold water appliances elsewhere in the house (don’t flush toilet or use appliances in the bathroom)
  3. While those appliances are running, check the shower pressure. Depending on whether the pressure decreased or not, follow the appropriate instructions below…
Various water pressure valves

Photo source: Youtube.com

Pressure valve set to 50 psi

Photo source: Nextag.com

If your shower pressure decreased…

You probably have an issue with your water pressure reducing valve.

These valves are located right where the water enters your house and looks something like these:

Most valves have a factory setting of 50 psi. You can find out your home’s psi settings by checking the meter attached to your pressure reducing valve.

If yours is much lower than this, have a plumber take a look. They will either adjust the psi to a stronger setting or replace the valve if it’s not working properly.

If your shower water pressure didn’t change…

You most likely have an issue with restricted pipes or undersized piping in the bathroom.

In either case, you’ll need to have a licensed plumber look at your pipes to diagnose the issue and give you professional advice on the best solution for you.

Related Reading