So you’re taking a shower, just like you do all the time. The water is the perfect temperature, hot but not scalding, and you take a moment after shampooing to just soak it all in and relax. As you close your eyes, you hear the familiar sound of a toilet flushing. Suddenly the relaxation is gone, your eyes are wide and you only have a split second to respond before you are being burned alive by scalding hot water. You start preparing the angry words in your mind to deliver to the responsible parties once you’re out of the shower, but in truth they are only partly responsible. Part of the responsibility lies with the plumbing itself and the way it is arranged, for it is this arrangement that can turn a shower into a modern day torture device.
Local plumbing codes determined the way plumbing is set up in your home or business. These codes are in place to reduce the danger of flooding, damages and injury, and included in them are provisions about how many water lines can run to a certain number of plumbing fixtures. Roto-Rooter and other plumbers must follow the plumbing codes when they perform any plumbing work or installations. When you have a bathroom with a shower, tub and toilet, there is likely only a single cold water line that runs to all three. The fewer water lines you have, the less you have to worry about corrosion, leaks and other issues, and this is a major advantage. However, this can lead to the hot shower problem previously mentioned.
When you turn on the shower, you adjust the temperature to somewhere between the hottest and the coldest settings. The temperature adjusts by changing the amount of water being supplied from the hot and cold lines, so if the knob is set right in the middle, roughly equal amounts of hot and cold water are being sent to the shower. Even when you are taking a pretty hot shower, there is still a bit of cold water there to prevent you from getting burned (unless your water heater doesn’t produce water that hot).
When the toilet flushes while you’re showering, the toilet demands a load of cold water, and because it shares a cold water line with the shower, the shower temporarily loses pressure from the cold water line. Without the cold water to temper the hot, the shower can become uncomfortably hot.
The easiest solution to this problem would be to just avoid flushing the toilet while the shower is in use. Newer shower valves are pressure-balanced, so installing new valves or retrofit kits can solve the problem as well.