Hi,
I am new to this forum, and do not much about plumbing. What could be the cause that every time someone flushes the toilet the water going to the shower turns hotter? What can it be done to resolve this issue?
Thanks,
C

No good solution because it requires a pressure balanced shower valve to be installed.

or the one that is installed is pressure balanced but is not functioning properly

If the system is a well system, a bad pressure tank can do the same thing. What is happening is that the water system inside your house has a finite volume of water that can flow through your pipes at a given pressure. If a fixture that is closer to the source of the water is turned on, the water will attempt to exit there first, reducing the pressure in that line and thus the driving force of that water. It is like being the last person in line at a buffet; there might not be anything left when it comes to you. Now in your case, the hot water is still flowing at the same speed, and the cold is dropping off, leaving you with almost all or all hot water at the shower.
There are a few ways that the problem could be solved, and some are better than others.(situation dependent)

1. Repipe the house. When the volume of water in the pipes is greater, it is less prone to pressure drops when a fixture is turned on. It is this reason that most older houses with galvanized piping get repiped; it isn't usually due to leaking, it is the narrowing of the pipe walls that cause problems.
2. Install a pressure balanced shower valve. In the event that the pressure from one supply drops, the pressure from the other side of the valve is dropped to match. Your shower water may come to a dribble, but nobody will get burned.
3. Install a thermostatic shower valve. Very similar to option 2, but will maintain a temperature, and is not pressure dependent. In most cases it is a better option than option 2, but slightly more expensive generally.
4. Repipe and install a new shower valve.

"What could be the cause that every time someone flushes the toilet the water going to the shower turns hotter?"

One of the common causes is that the piping isn't large enough.

What happens is that there is only so much volume that can come through the piping and so when one flushes the toilet the pressure/volume drop on the cold side since the cold side instead of only having to supply the shower now also at the same time has to supply the toilet.

Hope that makes sense?

Before we can jump in with a tangible resolve... we need more info/facts. Do you have a picture of your shower valve and can you tell us how old it is? The facts we have... they less we can speculate... and be wrong.

Best Wishes

You should get a plumber over to scope it out. The first thing I would look at is the water piping - the material, the age and condition. If it's galvanized pipe installed when the house was built in 1923 then you're probably looking at a re-pipe.

If you have a pressure reducing valve, that could be failing. It could aslo be something as simple as a main valve that's not full open. Your interests would be best served by getting someone on-site that's familiar with plumbing.