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Why does my shower get hot when the toilet flushes?

Posted On: November 23, 2016

Imagine yourself enjoying a shower when suddenly you get a blast of scalding water. Someone has flushed and you get that interruption no one asked for.

Why does this happen? This is a question we get asked a lot and we’d like to help you understand why and what can be done.

For so many homes, the plumbing is on a trunk and branch system. That means the home has a larger trunk line that delivers water and feeds off to several different individual fixtures, including your toilet. When something demands water from the trunk, water pressure gets a little lower for the whole house. Since toilets generally take cold water, that leaves your shower mixing more hot water and leaves you with an unwanted blast of heat.

Here are a few potential fixes for this problem.

Increase the Water Supply

This means changes to your plumbing. You could get a larger branch for your bathroom, which will supply more water to it. You could also increase the size of the trunk line which will increase supply for the whole house. That means getting inside the walls of your house, so unless you have some existing problems or you’re doing a major remodel, this might not be the easiest solution.

Distributing Water Evenly

This involves changing your entire plumbing system. Instead of a trunk and branch, you would have a manifold with home runs. That’s a system that has dedicated lines for each fixture that run to a manifold, which then balances water for distributing. Again, unless you’re doing a major renovation, this is a lot of trouble.

Reducing the Flow of Toilet Water

This one is a simple fix. You turn the supply line to the toilet down just a little. That will lower the demand for cold water and make things a little better in the shower. The drawback to that is that the toilet will take a little longer to fill up when flushed and you don’t completely solve the issue.

Mixing Valves

This is a valve on the shower that helps regulate how water mixes when there is a change to water pressure. This thermostatic mixing valve reduces the amount of hot water when there is a change in cold water pressure. That way, you don’t notice a change when the toilet gets flushed.


Whatever solution you choose, we at Genz-Ryan can help get it set up for you. Imagine showering in comfort without the problem of a flushing toilet. If you have had it with that blast of hot water in the shower, set up an appointment and we’ll help solve this annoying issue.

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