Short definition
Tempered water is hot and cold water mechanically blended to a safe handwashing or bathing temperature, typically 100–110°F. The blend happens automatically in a thermostatic mixing valve (TMV). Tempered water lets you store hot water at higher temperatures for Legionella control while still delivering scald-safe water at the tap.
What it is
A thermostatic mixing valve (TMV) sits between the water heater output and the fixture. Hot enters one port, cold enters another, and a temperature-sensing element automatically adjusts the blend to keep the output at a fixed setpoint regardless of inlet pressure or temperature variations. Two listing standards govern residential TMV use:
- ASSE 1016. Anti-scald shower and tub valves. Limits delivery to 120°F maximum.
- ASSE 1070. Lavatory point-of-use TMV. Limits delivery to about 110°F at handwashing fixtures.
Tempered water shows up in three common contexts:
- Anti-scald shower / tub valve. Required by WA-amended UPC §409 / §413 at all new shower and tub installs. The valve produces tempered shower water at 120°F max.
- Master TMV at the heater outlet. Used when the tank is set to 140°F+ for Legionella control. Delivers tempered water house-wide at 110–120°F.
- Lavatory point-of-use TMV. Common in commercial and multi-family installs; less common in residential. Delivers tempered handwash water at 110°F.
Why it matters to a homeowner
If you’ve ever wondered why the hot side of your shower never gets above 120°F even with the heater turned up, you’re seeing the anti-scald shower valve doing its job. That’s tempered water by design.
The other place this term shows up: when a plumber recommends “tank at 140°F + TMV at the heater outlet” for a household with infants, elderly residents, or immune-compromised members. The TMV delivers tempered water at every fixture while the higher tank temperature keeps Legionella suppressed.
Common variants and what tempered is not
- Tempered vs. hot. Hot = direct heater output (130–140°F+). Tempered = post-mix output (~110°F).
- Tempered vs. cold. Distinct.
- Tempered water vs. anti-scald valve. The anti-scald valve produces tempered water; tempered water is the output.