Does Flushing the Toilet Make the Shower Cold?

Yes! Surprisingly, flushing the toilet can cause a running shower to pour cold (or sometimes hot) water.

Does your running shower suddenly give a burst of cold water whenever the toilet is flushed? Perhaps, you’ve made it a rule in your home that nobody should flush the toilet when someone is in the shower. But do you know why this is happening?

flushing toilet make shower cold

The only reason why you may get a blast of cold water from your shower whenever someone flushes the toilet is your plumbing configuration.

Here’s how your plumbing setup does that:

Your Plumbing Setup

If you experience changes in your shower’s water temperature whenever someone flushes the toilet, you have an old form of plumbing setup.

This old plumbing setup is referred to as a “trunk and branch” (T&B) plumbing configuration. T&B plumbing configuration has some flaws in regulating water pressure in the shower properly.

With the T&B plumbing formation, both the shower and toilet systems use the same water channel.

Using a system has a way of depriving the other system of adequate water supply, with the toilet being the most favored system.

When you flush the toilet, it triggers the fill valve to refill the toilet tank. The two systems now demand water supply. The toilet tank, which is more favored for water intake due to its position, diverts water from the shower to fill itself up.

This subsequently deprives the bathroom water heater of an adequate supply of water. Meanwhile, to give you your desired warm water, the shower valve mixes hot water from the heater with cold water. But now, there is an imbalance with a shortage of hot water from the heater.

As a result of this imbalance, the shower may give out a burst of somewhat cold water instead of the expected warm water.

This experience is sudden and the water from the shower warms up gradually afterward. You may experience a similar effect if the washing machine or dishwasher is turned on while you’re using the shower.

The washing machine or dishwasher uses warm water. When you turn on the washing machine or dishwasher while the shower is running, it’ll siphon away warm water from the heater. This will cause the shower to pour out cold water.

Sometimes, it may be a burst of hot water from the shower. When the toilet tank is refilling, cold water will be siphoned away from the shower valve. This can make the shower give out much hotter water.

If your home has the T&B plumbing configuration, this sudden fluctuation in water temperature can sometimes be annoying.

Luckily, there are ways to handle this issue-

Don’t Flush While Using the Shower

The easiest and cheapest way to handle this issue is to ensure that nobody flushes the toilet while you’re using the shower.

This rule is easy to obey if your toilet is located in the bathroom.

However, if the toilet and bathroom are located separately, following the rule may not be easy, especially in a large household. In such a case, the next solution should work.

Reduce Your Toilet Tank’s Refill Rate

Most modern refill valves are designed with a high refill rate to refill faster. Fortunately, you can adjust the valve to refill slowly. Though this will cause the tank to take a longer time to refill, the refill effect on the shower will be less noticeable.

Although some people may suggest lowering the water level in the tank as a solution, we don’t recommend this.

Lowering the water level may affect the toilet’s flush rate or capacity. More so, installing low-flow toilets is capital intensive and may be beyond your budget.

Install a Thermostatic Mixing Valve

The last solution to handle this issue is installing a thermostatic mixing valve. The function of the valve is to monitor the pressure of the cold and hot water entering the shower.

Once the pressure of cold water drops due to the refill effect, the valve drops the pressure of the hot water. This action keeps the water from the shower at a constant temperature.

It’s important to note that a thermostatic mixing valve only works well if the cold water pressure is strong and doesn’t fluctuate.

If your home has weak water pressure, this solution may not work well for you. Also, installing a thermostatic mixing valve may be expensive and require the help of a professional.


The sudden change in water temperature while using the shower can be annoying. However, there are ways to handle this issue, as seen above.

The next time you experience a fluctuation in water temperature while showering, try out one of the solutions.

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