What Is an Anti-Scalding Valve on a Shower?

Function

• Anti-scald valves are installed behind bathroom walls near the shower controls, and they prevent water from getting hot enough to burn your skin. The valves also prevent thermal shock, which is a sudden change in water temperature that can cause you to move suddenly, slip and fall. Water temperature fluctuations may occur while you're showering if someone flushes a toilet or turns on the hot water at a sink. Cold water flows into a toilet bowl to refill it after a flush, which reduces pressure in water pipes. That reduced pressure means less cold water reaches the shower, so the water temperature increases. The opposite occurs when hot water is turned on at a sink.

Water Heaters

• Don Vandervort's HomeTips.com recommends turning down your water heater's thermostat to 120 degrees Fahrenheit to prevent scalding. However, the ASSE indicates that water heaters don't maintain a constant temperature. A thermostat is located at the bottom of a heater, so that its burner can cycle on and off as needed to heat incoming cold water. As a result, the water at the top of the heater is usually hotter than the thermostat reading, and it's commonly as hot as 155 degrees Fahrenheit, according to the ASSE. A water heater's irregular temperature can cause scalding water to spew from a showerhead without an anti-scald valve in place.

Pressure-Balance Valves

• A pressure-balance shower valve has a mechanism that senses changes in water pressure from the hot and cold waterlines. The valve responds to pressure changes by reducing the water flow from the hot or cold line as needed. It even shuts off water to prevent a scalding if it doesn’t sense pressure coming from the cold line. One drawback of a pressure-balance valve is that it only senses water-pressure changes, so it doesn't make adjustments for temperature variance.

Thermostatic Valves

• Some anti-scald valves detect changes in water pressure and temperature. For example, HomeTips.com notes that thermostatic valves adjust to pressure changes, and they can maintain a preset water temperature within 1 or 2 degrees Fahrenheit. However, the site indicates that thermostatic valves can cost more than four times as much as pressure-balance varieties, which start at about $100 at the time of publication.