This author’s day started, like every other, with a relaxing shower. Then, right on schedule, the toilet flushed in the other room, a roommate yelled, “Sorry! Sorry! Sorry!” and the shower water turned scalding hot.

Finally I had to know. Why does flushing a toilet make the shower run hot? Luckily, a quick search turned up the explanation on Yahoo Answers: The same water supply feeds both the toilet and the shower. So when the toilet flushes, it uses all the cold water to refill its tank and leaves nothing for the shower. 


The problem can be caused by using cold water from any appliance. Kitchen sinks, washing machines, and sprinklers are all liable to steal away the cold water and leave you screaming.

So why do some people plan their mornings around this while other people get to shower free from fear? Better water flow. 

In apartments and houses with small, blocked-up pipes, and/or only one water source supplying many fixtures, there is so little water flow that a sink or toilet can steal the entire cold water supply. (The same logic explains why apartments with too little a flow of electricity through their wiring can’t run an AC unit and an electric stove at the same time.)

Newer, nicer places tend to have some combination of bigger pipes, multiple water sources, and regulating devices, which means that there is too much water flow for the toilet to steal it all away.

Luckily, if your shower scalds you every morning, you don’t have to move. It’s a quick fix:

A pressure-balanced shower valve is designed to compensate for changes in water pressure… It has a piston mechanism inside that moves with a change in water pressure to immediately balance the pressure of the hot- and cold-water inputs.

Any licensed plumber should be able to tackle the job. Repair for the single valve should take an hour or so once the replacement part is in hand. This is also something a willing do-it-yourselfer can tackle.

Mystery solved. All you have to do to keep from being burned in the shower every morning is convince your landlord to pay for a repair that he or she probably could care less about!

This post was written by Alex Mayyasi. Follow him on Twitter here or Google.