Short definition

A thermostatic mixing valve at the water heater outlet — known in code circles as an ASSE 1017 valve or master mixing valve — installs on the hot-water outlet of the tank and mixes a small amount of cold water into the hot supply. The result: a consistent fixture temperature regardless of what the tank itself is set to. Standard pairing: tank at 140°F (Legionella suppression), valve set to deliver 120°F to fixtures (anti-scald).

What it is

The valve is a brass or bronze body, typically 3/4” residential, with hot, cold, and mixed-outlet ports. Inside, a thermostatic element expands and contracts as temperature changes, modulating the proportion of cold water added to the hot supply. The output stays close to the setpoint as long as the valve is sized for the load and the cold supply is available.

The performance standard is ASSE 1017 — Performance Requirements for Temperature Actuated Mixing Valves for Hot Water Distribution Systems. The standard tests stable mixed-water temperature under varying inlet pressures and conditions. ASSE 1017 valves are not the same as ASSE 1016 (in-fixture pressure-balance at tubs/showers) or ASSE 1070 (point-of-use limiter at sinks/lavs). Each standard covers a different role.

The 1017 valve is not code-required in standard residential installs. It becomes useful — sometimes effectively required — when the tank is run hotter than the desired delivery temperature for Legionella control or to extend usable tank capacity.

Why it matters to a homeowner

A hot-water system has two competing temperature requirements:

  • Hot enough to suppress Legionella. Legionella bacteria die quickly above 140°F and grow in the 77–113°F range. Tank temperatures of 140°F or higher are the durable answer where Legionella risk matters (households with vulnerable residents, low-use periods, recirculation systems).

  • Cool enough at the fixture to prevent scalding. Per the scald exposure curve, 140°F water burns through skin in under 5 seconds. 120°F gives 5 minutes — enough time for an able-bodied person to react.

A master TMV resolves the conflict. The tank holds water at 140°F (or higher). The valve mixes cold water into the hot supply at the outlet. The fixtures see 120°F (or whatever the valve is set to). One valve, both objectives.

A side benefit: the valve effectively expands tank capacity. By mixing cold into the hot supply at the outlet, you deliver more usable hot volume for the same tank size. A 40-gallon tank running with a master TMV behaves more like a 50-gallon tank for most use patterns.

When you’ll encounter this term

  • Setting tank to 140°F to suppress Legionella in a household with elderly residents
  • Recurring intermittent scald complaints not solved by replacing in-fixture valves — the master mix may be needed (or an existing failed master TMV may need replacement)
  • Hot-water demand exceeding tank capacity — TMV stretches usable hot volume
  • Recirculation pump install — pair with master TMV to avoid recirculating scalding water

Common variants and disambiguation

  • TMV (1017) at heater vs. TMV (1070) at fixture vs. anti-scald (1016) at tub/shower. Three distinct device families with different test standards and applications.
  • Master mixer vs. digital mixing valve. Newer digital units offer recirculation control plus multiple setpoints.
  • Tempering valve (less stringent) vs. mixing valve (ASSE 1017 tested). Only ASSE 1017 is code-acceptable in WA for this duty. A “tempering valve” without the 1017 listing is for different (less stringent) applications.

Common failure modes

  • Mineral buildup in valve seat. Drift toward delivering hot tank temperature; periodic flush or replacement.
  • Bypass set up wrong. Installer leaves bypass valves in the wrong position, defeating the TMV. Common service-call finding.
  • Setpoint mis-adjusted. Homeowner cranks for “more hot water,” gets scald risk.
  • Cold supply check valve missing. Required upstream to prevent reverse flow into cold supply.
  • Ignored end-of-life. 10–15 year typical service life; failed valve drifts cold (no hot water) or hot (scald).

Cost data

ItemCost
ASSE 1017 valve (3/4” residential)$150–$400
Install on existing tank$200–$500 labor
Combined retrofit (hardware + install)$400–$900 typical residential

Washington note

WA UPC (WAC 51-56) adopts ASSE 1017 as the reference standard for master mixing valves; AHJs may require an ASSE 1017 valve when the tank setpoint exceeds the maximum delivered fixture temperature limit. The valve is not required in standard residential installs unless the tank is run hot for Legionella suppression. Some commercial and multi-family contexts require it.

For a DIY-comfortable homeowner, adjusting the setpoint on an installed ASSE 1017 valve and verifying delivery temperature with a thermometer is fine. Installing the valve is permit territory in many WA jurisdictions and requires sweat or press fittings on the hot line plus a pressure test. The retrofit usually pairs with other water-heater service work.