The goal of this ible is to show you a free way to minimize the impact that flushing the toilet has on your shower water pressure and the temperature swing that comes with that.
There are more expensive options that involve replubming, but this is a simple and free lifehack anyone can do.
Step 1: Material
What you need is... nothing! That's some huge savings when you compare to the pro solutions like a balancing valve at each shower.
Step 2: Close the Valve
All you need to do is close the intake valve to your toilet, and re-open it a bit. When flushing, the flush pressure comes from the top reservoir; the hose refilling it that actually burns you doesn't actually need to go fast; unless you need some speed flush-to-flush action. Have the top of the reservoir open to see how quickly it refills, and put it as slow as you feel comfortable with.
The difference is pretty astounding; instead of a nearly 7c / 13f swing, it went down to about a 2c/4f difference.
Step 3: Test 1: Regular Situation
In my first test, with no fix, the water goes from 33c/91f to 40c/104f.
Step 4: Test 2: Aggressively Fixed
In the second, with the extra slow refill (like 20 minutes to refill the tank), the temperature changes by about 1c/2f, going from 33c/91f t0 34c/93f.
Step 5: Test 3: Goldielocks Solution
In the third test, the base temp goes from 34c/93f to 36c/96-97f; so a change for about 2c/4f for a reasonably quick filling toilet. It will get noticeably warmer, but nothing dangerous or painful.