Why Does Flushing the Toilet Affect the Shower?

If you have ever been in the unfortunate situation of being in the shower, then someone flushes the toilet, then you know how disruptive it can be. The water pressure changes and the water temperature are altered abruptly. But why does this happen?

Flushing a toilet affects the shower because the usual cold water flow in the plumbing system is altered. The toilet and the shower use the same cold water line for most plumbing systems, so the action of one fitting affects the other. The problem can be prevalent when your building has old plumbing with malfunctioning shower valves. 

Everyone would like to enjoy a long, hot shower after a long day. And definitely, no one wants the experience to be ruined by a flushing toilet.

Keep reading to learn what exactly happens when the toilet flushes when you’re in the shower and how you can avoid this uncomfortable experience.

What Happens When You Flush a Toilet While Showering?

When you flush the toilet when showering, two things happen: One, the flow of cold water in the shower and the toilet change. The other is that the water temperature from the shower also changes in response to the varying water pressure. 

During the actual flushing process, the water from the shower is siphoned into the toilet to meet the high water demand. This reduces the cold water flow into the shower making the little water coming from the shower hotter. 

The hot water blast effect is uncomfortable, not to mention that it can lead to burns.

shower water becomes hotter when you flush the toilet

What Exactly Happens That Shower Water Becomes Hotter When You Flush a Toilet?

Usually, when you use the shower, you set the temperature knob in the middle between the hot water and cold water lines. This ensures that the shower delivers roughly equal amounts of hot and cold water.

When you flush the toilet, cold water from the shower’s cold water line is directed to the cold water line of the toilet. This causes the temperature to adjust in favor of the available hot water. As a result, little or no cold water comes from the shower, and only uncomfortably hot water is released.

The degree to which the water gets hotter depends on the hot-to-cold water ratio delivered to your shower by the cold water lines. 

My Shower Gets Hot When My Toilet Flushes. How Can I Fix This?

Avoid Flushing while showering

The easiest solution to this problem would be to just avoid flushing the toilet while the shower is in use. However, this may not work for everyone. If you still want to flush the toilet while the shower is in use, there are some adjustments you can do to help eliminate the flow rate and temperature effects.

new shower valve being installed in the bathroom

Install new shower valves

The first adjustment you need to do is to upgrade your old shower valves into new ones. New shower valves that are pressure-balanced sense when the pressure in the water pipes changes and adjust the water pressure in the shower head to maintain a consistent flow.

This means that you can now flush the toilet without having to worry about the water pressure and temperature changing.

Reduce the refill rate of the toilet

The flow rate of your toilet may be the reason you’re getting hot water when you flush. And the principle behind this is simple– if the toilet demands a lot of water from the cold line at a fast rate, the rate at which the water coming from your shower also increases. 

You can reduce the refill rate of your toilet to eliminate the hot water problem. You just need to adjust the shutoff valve below your toilet to its halfway position. 


Does using any other water fixture in the house affect the shower?

If the pressure balancing element of your shower is malfunctioning, the shower’s water pressure and temperature will always be altered whenever someone else in the house uses a water fixture. 

washing hands in the sink

You need to ensure that your shower’s pressure balance valve works as required. A properly functioning shower water valve senses the difference in water pressure between hot and cold water lines and adjusts the water flow accordingly. 

With a pressure valve, you don’t have to worry that the shower water flow rate and temperature will change when someone uses a water fixture such as a dishwasher in the house. 

Can flushing the toilet make the shower water cold?

Yes, with plumbing systems where cold water is directly siphoned from the trunk pipe or storage tank. 

Flushing the toilet creates a high demand of cold water to push everything in the toilet out as usual, but a blast of water is released both to the toilet and into the shower. The result is an increased cold water flow in the two cold water lines. When the ratio of cold water to hot water that gets to the shower is high, the water becomes colder.  

A similar scenario may occur when you use a hot water fixture such as a dishwasher when taking a shower. Hot water may be diverted from the shower leaving it with freezing cold water.

Can the cold water lines of the shower and the toilet be installed independently?

The installation of the cold water lines for the shower and toilet can be done independently, but it’s not always recommended. This is because this type of installation magnifies problems such as leaks and corrosion. 

But, considering that a shared cold water shower leads to the hot water problem when you flush the toilet, these disadvantages could be overlooked.

Wrapping up

Flushing a toilet affects the shower as the water is diverted from the shower cold water line into the toilet cold water line. 

Many people still have the old-fashioned plumbing system that doesn’t properly regulate the water pressure. This can cause the water to come out too slowly when you flush your toilet while showering. So, if you’re one of these people, upgrade your plumbing system to avoid the shower hot water problem when you flush your toilet.

That said, the simple solution to this problem remains to avoid flushing the toilet when showering!

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