Why does my shower water pressure drop when the toilet flushes?

Water Pressure Flushing Problems

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

  1. You have a pressure-balancing mixing shower valve that needs to be replaced with a thermostatic mixing valve.
  2. Your pressure reducing valve is set too low or is not functioning properly.
  3. Your bathroom's piping is constrained.
  4. Your bathroom piping is too small.

If you notice a noticeable drop in water pressure, you most likely have a pressure-balancing shower mixing valve and should replace it with a thermostatic mixing valve.

Shower mixing valves are in charge of regulating the temperature of the water in your shower. They function by "mixing" hot and cold water automatically.

Pressure-balancing valves typically have only one handle and respond to changes in water pressure. It is the simpler of the two valves because it cannot detect temperature changes and only controls the water temperature indirectly.

They work by adjusting the ratio of hot to cold water pressure. As a result, when you flush your toilet, cold water is drawn from your shower. The pressure-balancing valve detects a drop in cold water pressure and responds by restricting hot water pressure.

As a result, you will be blasted with hot water for a short period of time, followed by a noticeable drop in water pressure until the toilet bowl fills back up (about 30 seconds).

What is the solution? Use a thermostatic mixing valve instead.

Thermostatic mixing valves typically have two handles: one for controlling water pressure and the other for controlling water temperature.

They function similarly to a pressure-balancing valve in that they blend hot and cold water, but they control the water temperature directly without affecting the water pressure.

If your water pressure slows to a trickle or dramatically decrease, you probably have one of these problems:

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

If your shower pressure decreased, you most likely have a problem with your water pressure reducing valve.

These valves are located directly where the water enters your home and look like this:

The factory setting for most valves is 50 psi. Check the meter attached to your pressure reducing valve to determine your home's psi settings.

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 call plumber near you inspect your pipes to diagnose the problem and provide professional advice on the best solution for you.

Related reading:How to fix water pressure in home

Does Water Pressure Effect Toilet Flush?

A typical residential toilet has a tank that holds 2 to 4 gallons of water. When you push the tank's lever, this water is what causes the commode to flush; water pressure has nothing to do with the unit's ability to flush.

Why Is My Water Pressure Low After Flushing Toilet?

The classic example is when a toilet flush causes both a change in water temperature and a drop in pressure. This type of valve combines hot and cold water at a specific pressure level. When you flush the toilet, cold water is diverted to refill the tank, causing a pressure drop in the cold water line.

What Causes Problems With Water Pressure?

The waste pipe, siphon jet, or rim jets are usually partially clogged, or the water level in the tank or bowl is too low, resulting in a lack of flushing power. Clear the obstruction and adjust the system to correct the water levels in those cases.