Thermostatic mixing valves for water heaters, explained

What a mixing valve does

A. O. Smith describes thermostatic mixing valves as devices that automatically mix hot and cold water to limit the temperature at the tap. Its temperature guide adds that they keep the temperature constant and can increase the amount of usable hot water. The valve has a hot inlet, a cold inlet and a mixed outlet.

Why manufacturers recommend them

• Scald protection. A. O. Smith's manuals say that even at 120°F hot water can scald, and that if you choose a higher setting, mixing valves at each point of use are particularly important. Its burn table shows the stakes: more than 5 minutes for a serious burn at 120°F, under 5 seconds at 140°F.
• More hot water from the same tank. For a heater that runs out too fast but is in good condition, A. O. Smith suggests installing mixing valves at each point of use and then turning the thermostat up, so the stored water lasts longer once blended (see hot water runs out fast).
• Bacteria control. A. O. Smith says higher tank temperatures, around 140°F, kill the bacteria behind "smelly water" and can reduce levels of bacteria that cause waterborne diseases; a mixing valve lets you store water that hot and still deliver safe water (see water heater smells like rotten eggs).
• Stacking. A. O. Smith notes that repeated small draws can leave water at the top of a tank up to 30°F hotter than the setting. A mixing valve evens that out (see water heater too hot).

Point-of-use valves vs a valve at the heater

There are two common setups:

A. O. Smith's manuals recommend valves at each point of use. A single valve at the heater tempers the whole house at once, but every fixture then gets the same blended maximum. A plumber can advise which suits your plumbing and local code.

What the plumbing code says

• Bathtubs. International Residential Code section P2713.3 says bathtub valves must have or be supplied by a temperature-limiting device conforming to ASSE 1070, with its maximum set so water doesn't exceed 120°F (combination tub/showers are covered by the shower rule).
• Showers. Section P2708.4 requires pressure-balancing, thermostatic or combination shower valves meeting ASSE 1016, with a limit set so water doesn't exceed 120°F.
• The water heater thermostat doesn't count. Plumbing code section 607.1.1 says a water heater's thermostat can only serve as the temperature-limiting means if the heater itself meets ASSE 1082 or 1085.

Local codes vary in which edition they adopt, so check with your building department if you're adding or replacing one.

Setting and checking a mixing valve

Follow the valve maker's instructions. A. O. Smith's manuals say to adjust mixing valves to 120°F or lower, and if you've raised the tank temperature, to check water at several points of use, such as the tub faucet, shower and bathroom sink, because you could scald yourself if the valves aren't set right. Use a thermometer rather than your hand. If you aren't sure how to adjust the valves, or whether you have them, A. O. Smith says to contact a qualified person.

Signs a mixing valve is failing

A. O. Smith says many shower controls have built-in mixing valves, and when they fail they can reduce the hot water a shower or faucet delivers even though the tank is full. If one fixture is lukewarm while others are hot, suspect its valve before the heater (see hot water in the sink but not the shower and water heater not hot enough).

Is it worth adding one?

Consider a mixing valve if you keep the tank above 120°F, if young children or older adults live in the home (the CPSC says most tap water scald injuries and deaths are among older adults and children under five), if you want more usable hot water without a bigger tank, or if your heater is being replaced anyway. If you keep the tank at 120°F and your shower valves have working limit stops, you may not need one at the heater.