Mixing Valves
Updated February 20, 2026
Blend hot and cold water to deliver a safe, consistent temperature. Thermostatic models automatically adjust to prevent scalding.
Overview
Mixing valves blend hot and cold water to deliver a specific output temperature. Thermostatic mixing valves (TMVs) maintain a set temperature regardless of supply pressure changes. Anti-scald valves limit the maximum temperature to prevent burns. Installed at the water heater outlet (point-of-distribution) or at individual fixtures (point-of-use). Essential safety device in homes with children, elderly, or when the water heater is set above 120 degrees F for legionella prevention.
Pros & Cons
- Scald prevention
- Consistent temperature
- Required by code in many areas
- Adds installation complexity
- Can fail and affect temperature
Key Features
- Blends hot and cold to deliver a preset, consistent output temperature
- Thermostatic element reacts automatically to supply temperature and pressure changes
- Anti-scald protection: limits maximum output to 120 degrees F (adjustable)
- Point-of-distribution (at water heater) or point-of-use (at fixture) installation
- Allows the water heater to run at higher temperatures (140 degrees F) for legionella kill while delivering safe 120 degrees F to fixtures
Sizing & Selection
3/4 inch for point-of-distribution (water heater outlet) serving the whole house. 1/2 inch for point-of-use at individual fixtures. Flow rate must match demand -- a whole-house TMV typically handles 5-15 GPM. Check the valve rating against your peak hot water demand.
Installation
Point-of-distribution: install on the hot water outlet of the water heater with a cold water bypass connection. The valve mixes cold water into the hot before it enters the distribution system. Point-of-use: install under the sink or behind the shower valve. Both require a cold water supply tee. Follow manufacturer instructions for temperature calibration.
Maintenance
- Test the output temperature annually with a thermometer at the nearest fixture.
- The thermostatic element can wear or scale over time, causing temperature drift. Recalibrate or replace every 5-10 years.
- Descale the valve in hard water areas -- mineral buildup affects the thermostatic element response time.
- If the output temperature suddenly spikes or drops, the thermostatic element may have failed. Replace immediately.
Buying Tips
- Honeywell, Watts, and Leonard are the main residential TMV brands.
- A point-of-distribution TMV (-150) is the most cost-effective way to protect the entire house.
- Setting the water heater to 140 degrees F with a TMV at 120 degrees F gives you legionella protection AND scald protection simultaneously.
- ASSE 1017 rated valves are certified for point-of-use anti-scald protection. Look for this certification.
- If you have a recirculation system, the TMV must be installed after the recirculation loop -- not in it.