What temperature should a mixing valve be set at?

Skin burns depend on temperature and time. The U.S. Consumer Product Safety Commission (CPSC) puts numbers on it: "Most adults will suffer third-degree burns if exposed to 150 degree water for two seconds. Burns will also occur with a six-second exposure to 140 degree water or with a thirty second exposure to 130 degree water. Even if the temperature is 120 degrees, a five minute exposure could result in third-degree burns."

So 120 degrees is not burn-proof. It buys time. A child or older adult has minutes to react instead of seconds, which is why CPSC "urges all users to lower their water heaters to 120 degrees Fahrenheit."

The code builds on the same figure. The 2024 International Plumbing Code (IPC) Section 412.3 says individual shower and tub-shower valves "shall be equipped with a means to limit the maximum setting of the valve to 120°F (49°C), which shall be field adjusted in accordance with the manufacturer's instructions to provide water at a temperature not to exceed 120°F (49°C)." The same section says such valves "shall be installed at the point of use" and that "In-line thermostatic valves shall not be utilized for compliance with this section." Phoenix's published amendments to the 2024 IPC do not list Section 412 or Section 607, so those base-code sections are what the city is working from. Your city's adopted code is the final word, so confirm with your plan reviewer on a permitted job.

"Mixing valve" covers several different devices, and the set point depends on which one you have. ASSE International's scald task group sorts them by where they go:

• Shower or tub-shower valve (ASSE 1016). The temperature limit stop "must be set at the time of installation, and may need to be periodically adjusted for variations in water temperatures." The code ceiling for that limit stop is 120 degrees. Our page on pressure balancing versus thermostatic shower valves covers how these valves differ.
• Water temperature limiting device at a sink, lavatory, bidet or tub fitting (ASSE 1070). ASSE says the user control "allows the outlet temperature to be adjusted up to a maximum temperature of 120° F (49° C)." For tempered water, IPC Section 607.1.2 lists an ASSE 1070 limiting device "set to not greater than 110°F (43°C)" as one allowed control. Section 607.1 is where the code says nonresidential buildings may supply tempered water for bathing and washing.
• Master or tempering valve at the water heater (ASSE 1017). This one controls the temperature of the whole hot water distribution system. ASSE says these valves are installed "at or near the outlet of the hot water source ONLY" and are "NOT intended for point of use applications."

The master valve is the one people mean when they ask about a "tempering valve" on a water heater. Its set point is whatever distribution temperature the system is designed for, and then fixtures downstream still need their own protection. ASSE states that these devices "used alone do not provide thermal shock protection or adequate scald protection," and that "Further mixing downstream is allowed to provide final temperature control to protect against scalding."

Put simply, the master valve evens out the temperature in the pipes. The shower valve or fixture limiter is what protects the person.

If 120 degrees is safe, why would anyone heat water above it? Legionella. These bacteria grow in warm water, and the CDC's Legionella Control Toolkit, written for building water systems, advises: "Store hot water at temperatures above 140°F (60°C) and ensure hot water in circulation does not fall below 120°F (49°C)." It also says to keep cold water below "the favorable range for Legionella (77–113°F, 25–45°C)."

Water stored that hot will scald in seconds, so the CDC pairs the advice with a valve: "Install thermostatic mixing valves as close as possible to fixtures to prevent scalding while permitting circulating hot water temperatures above 120°F (49°C)." ASSE notes the same side benefit of a master valve, that it allows "the water to be stored at a higher temperature, extending the amount of hot water available, and also reducing the chance of legionella (ASHRAE Guideline 12)."

That is the tradeoff in one picture. Hot storage guards against bacteria. A mixing valve guards against burns. The valve's set point is where the two meet. For the building-level picture, see thermostatic mixing valves in commercial buildings and Legionella water management plans.

For a single-family home with a standard tank, you usually do not need a master valve at all. CPSC's advice is to set the water heater thermostat to 120 degrees, and that alone puts a ceiling on what reaches the tap. Our page on what temperature a water heater should be walks through that setting.

A mixing valve earns its place when you want the tank hotter, for example to store more usable hot water, which ASSE lists as a side benefit of a master valve. Then you set the valve outlet to 120 degrees or lower, and the shower valve limit stop stays at or under 120 as well. That's the fix.

Measure the water instead of trusting a dial.

1. 1Run the hot water at the tap or shower farthest from the valve until it stops climbing.
2. 2Measure the stream with a cooking or candy thermometer. CPSC suggests this same check for water heaters.
3. 3If the reading is above 120 degrees, adjust the valve or limit stop following the manufacturer's instructions, which is also what the code requires for shower valves.
4. 4Re-test after the valve settles. Check it again later, since ASSE notes the shower limit stop "may need to be periodically adjusted."

The CDC also lists thermostatic mixing valves among the water system components to clean and maintain regularly. A valve that drifts, sticks or lets the temperature swing can be worn out. If your shower temperature keeps changing, start with why shower temperature keeps changing. If the shower valve itself needs replacing, see how to replace a shower valve trim.

Adding or replacing a master valve means cutting into the hot water piping and getting the piping, check valves and any recirculation line right. That is licensed plumber work. HQ is licensed and insured and gives free estimates with upfront pricing, so you know the plan before we start.