Why Does the Water Pressure in my Shower Drop When I Flush?

water pressureIf the water pressure of your shower decreases when you flush your toilet, it’s almost always because of one of these three issues:

  • Your shower has a pressure-balancing mixing valve
  • Your pressure reducing valve is on the wrong settings or broken
  • Issues with your bathroom piping


By looking at just how much the water pressure in your shower goes down, you can pinpoint the exact issue.

A small drop that allows you to still shower…

If this is the issue you’re facing, you most likely have a pressure-balancing mixing valve. Switching to a thermostatic mixing valve will solve the issue. What these valves do is maintain the temperature of your shower by mixing the proper amount of hot and cold water. When you have a pressure-balancing mixing valve, it doesn’t account for temperature changes. Instead, it responds based on the water pressure. So when you flush your toilet, the cold water is being pulled away. The valve then pulls the hot water away to balance it. The result? A brief moment where the pressure of your shower is low. 

Switching to a thermostatic mixing valve will solve this issue. These valves can control the temperature of the water separately from the pressure of the water. When you flush your toilet with one of these valves in place, it will sense the change in pressure, but knows that the temperature is unaffected – so it doesn’t change the water pressure.


If your water pressure drastically changes…

If this is the case, there’s an issue with your valve or piping. To figure out which issue it is:

  • Run the shower
  • Run faucets and cold water elsewhere
  • Check the shower pressure while these things are running

If shower pressure decreased…

The water pressure reducing valve is probably broken or incorrectly set.
This valve is on the outside of the home where water enters your house.

You can check the PSI setting on the valve and compare it to the meter on your home. If the numbers are far apart, a plumber can properly adjust it or replace it.


But if the water pressure didn’t change…

Your issue most likely has to do with piping. This is not a do it yourself project. You’ll need an experienced plumber for the job. They’ll be able to diagnose the problem and advise you on possible solutions.

Closing Thoughts

When the water pressure in the shower drops when you flush your toilet, it’s usually due to one of three issues. These issues revolve around having the wrong valve, a broken valve, or poor piping.

While pinpointing the issue can be a do it yourself project for the weekend, it’ll take an experienced plumber to carry out the solution. Professionals at Calix Plumbing in Woodbridge, Va, provide plumbing services for major and minor issues. From leaks, clogs, to pressure tanks, our plumbing experts can get the job done right for you to prevent any problems in the future. Call (703) 986-6919 for a free estimate!