There's nothing more unpleasant than the water in the shower suddenly changing temperature. However, it can be harder to determine just what, exactly, caused that sudden temperature change. Knowing how to identify why your shower's water temperature is suddenly fluctuating can help you fix the problem and get back to showering comfortably as quickly as possible.
Running Out of Hot Water
The most common reason why your shower may suddenly no longer have any hot water is because your water heater isn't functioning properly. This can happen over time, especially if your home's water supply contains hard water. Hard water has a large amount of dissolved minerals in it, which can cause a sediment to build up within your water heater's tank. While only a small amount of sediment will stick on at a time, over a long period this can turn into a mound that severely reduces the volume of your water heater's tank. If this is the case, you should call a plumber to have your water heater flushed and an anode rod installed. Anode rods are made out of specific materials that attract and dissolve the dissolved minerals in hard water, preventing sediment from settling in the tank.
Splashes of Hot and Cold Water
If you receive an unexpected splash of hot or cold water while in the shower, the likely reason is that another water appliance, like a toilet or a dishwasher, has started to use water. This issue lies with a pressure balancing valve, which is designed to keep water supplies to different water using fixtures separate. A faulty valve can allow a splash of cold water that was heading to the dishwasher to sneak into your shower, for example. If this is the case, you'll have to call a plumber to identify which pressure valve is at fault and replace it.
Unstable Water Temperatures
On the other hand, if your water temperature in the shower won't stop slightly fluctuation, the issue lies with your water heater. This will only happen if you have a tankless water heater, which heats water up rapidly as needed. A faulty burner or a unit that simply cannot keep up with your hot water demands will send water of varying temperature through your home's plumbing, which can be uncomfortable to shower in. If this is the case, you should contact a plumber about installing a mixing tank on your tankless water heater: these small tanks have a few liters of hot water stored in them, maintaining the small size of your tankless water heater but providing a more stable supply of hot water.