If there's a sound you don't want to hear when you're in the shower, it's a toilet flushing. That whoosh sound will have you bending and contorting your body however you have to in order to dodge as much water as you can. But, why does your shower get scalding hot when someone flushes the toilet?
When hot water exits your water heater, it travels down one big pipe called a trunk line. From here, it branches off to the various fixtures that need hot water, including faucets, your washing machine, the dishwasher, and of course, your shower. The cold water does the same thing, usually running close to the hot water lines. This design is 3/4 of the problem.
Because the toilet in your bathroom shares the same branch as the shower, there is only a finite supply of cold water that travels to both. So when the toilet needs filling, it grabs a large chunk of that cold water, leaving your warm shower with a ratio of hot-to-cold that is now heavily weighted to the hot side, resulting in you dancing your way out of drops of liquid magma.
Can You Stop It?
There are several ways to stop your shower from being afflicted whenever someone flushes. The best way is to install a thermostatic valve (TMV). These valves maintain an exact water temperature for your shower and protect you from the fluctuations in water supply caused by people flushing or running water. The beauty of a TMV is that once you set the temperature, you never have to worry about being frozen or scalded ever again.
If getting a TMV isn't an option, restricting the water flow to the toilet will reduce the amount of cold water being siphoned away from your shower. This minimizes how much your hot/cold shower water mix is affected.
A more involved solution involves renovating the plumbing in your house; you remove the trunk and branch lines and essentially run a dedicated line to each fixture in your home. A manifold controls how the water is distributed, ensuring that your hot/cold mixture is never impacted by much.
Just remember that when you shower, you're trying to maintain a perfect ratio of hot and cold water. There's only so much of that that is available to your house, so any other use will impact your mix. Getting that perfect shower means that you either eliminate other draws on your water supply or you increase it to accommodate other uses. For more information, contact companies like The King's Helper.