Short definition
A thermostatic mixing valve (TMV) uses a temperature-sensing cartridge to blend hot and cold water and hold a precise outlet setpoint. The most common residential application is at the water heater outlet — letting the homeowner store water at 130 to 140 degrees Fahrenheit (Legionella-suppressing) while delivering 120 degrees to fixtures. Standard names: ASSE 1017 at the heater, ASSE 1070 at the fixture, ASSE 1016 at the shower (covered separately).
What it is
Inside the valve body, a wax cartridge (or shape-memory alloy element) expands and contracts with outlet temperature. As the cartridge moves, it shifts a spool that proportions hot and cold flow. If the outlet is too cold, the spool opens hot more. If it’s too hot, more cold mixes in. The result is a stable outlet temperature that doesn’t drift when upstream pressure or temperature changes.
Three ASSE standards apply to different installations:
- ASSE 1017 — distribution-system mixing valve at the water heater outlet. Standard residential setpoint is 120 degrees. Lets you store at higher temp without scalding fixtures.
- ASSE 1070 — point-of-use temperature limiter at sinks, lavs, and tubs. Maximum setpoint 120 degrees. Must shut to less than 0.5 gpm within 5 seconds on cold-water loss.
- ASSE 1016 — point-of-use shower or tub-shower valve with combined scald and thermal-shock protection. Covered in the anti-scald-valve entry.
Why it matters to a homeowner
The Legionella problem: bacteria proliferate in tepid water — between roughly 77 and 113 degrees — and water heaters that store at the recommended-low 120 degrees can grow them. CDC and most public-health guidance recommends storage at 130 to 140 degrees to suppress growth. But hot tap water above 120 degrees burns: third-degree burns happen in seconds at 130-plus, faster for kids and seniors.
The TMV solves both at once. Set the heater to 140, install an ASSE 1017 valve on the outlet, set the valve to 120, and you have safe water at every fixture and a heater hot enough to kill the bacteria. It’s the standard recommendation for households with elderly residents, very young children, or any Legionella concern.
When a plumber says “tempering valve” or “hot water mixing valve” or “1017 valve,” that’s this part. Cost is $80 to $250 for the valve, $250 to $500 installed if it’s being added to an existing copper system.
Common failure modes
- Wax cartridge degrades. Output drifts hot or cold over months. Cartridge replacement or recalibration.
- Mineral scale lock-up. WA hard-water service makes the spool sluggish or stuck. Annual flush helps.
- Hot/cold reversed on install. Valve can’t sense properly. Won’t reach setpoint. Reverse the supplies.
- ASSE 1070 cold-loss test failure. Valve must shut down within 5 seconds — if it doesn’t, the standard isn’t being met.
Common variants
- ASSE 1017 vs. ASSE 1070 vs. ASSE 1016. Different applications, different test criteria. Don’t confuse them.
- TMV2 vs. TMV3 (UK). UK certification tiers — residential vs. healthcare. Not directly applicable in WA but informs vocabulary in international product literature.
- “Tempering valve” is colloquial — it can mean any of the three. Confirm which standard applies.
Washington note
WA adopts the 2021 UPC through WAC 51-56, which requires ASSE 1016 at every shower and tub-shower. ASSE 1017 at the water heater outlet is not universally required by code, but it’s strongly recommended whenever the heater is set above 120 degrees — which most plumbers and public-health guidance now advise. King County Public Health recommends ASSE 1017 plus 130-plus storage for licensed adult family homes and child-care facilities.
A practical pairing for older WA homes: a 130 to 140-degree water heater, an ASSE 1017 TMV at the heater outlet delivering 120 degrees to the distribution system, and ASSE 1016 valves at every shower. Spokane and Eastside hard-water service users should plan annual TMV maintenance — scale builds up faster than in soft Cedar/Tolt service.