Tank-set temperature (120–130°F)

Short definition

The tank-set temperature is the thermostat setpoint on your water heater. WA requires new heaters to be preset at 120°F. That setting is a deliberate trade-off: hot enough for daily use, cool enough to prevent scalding, but cool enough that Legionella bacteria can survive in the tank. The optimal setup for most households is 120°F. For households with vulnerable residents, 140°F + a thermostatic mixing valve.

What it is

Every storage water heater has at least one adjustable thermostat. The dial or screw lets you choose a setpoint between roughly 90°F and 150°F. The thermostat opens and closes the heating circuit (electric) or the gas valve as the tank crosses that setpoint.

The tradeoff at each end of the dial:

• 120°F (WA preset). Scald-safe — a 5+ minute exposure before second-degree burn risk. Legionella-permissive — bacteria can multiply slowly. Modest energy use. Modest scale formation.
• 130°F. Faster recovery (more usable hot water from a given tank), shorter scald-safety margin (~30 seconds to second-degree burn), still inside Legionella growth range.
• 140°F. Faster recovery still, scald-dangerous (~5 seconds to second-degree burn), kills 90% of Legionella in about 32 minutes. Standard for hospitals and long-term care.
• 150°F. Manufacturer max on most residential. Used for thermal disinfection.

Most plumbers recommend 120°F for households with children or elderly residents, 125°F as a comfortable middle ground for adult-only households, and 140°F + a thermostatic mixing valve (TMV) at the heater outlet for households with immune-compromised residents.

A TMV at the heater output mixes hot tank water with cold to deliver a fixed temperature (typically 110–120°F) house-wide. That gives you Legionella control in the tank and scald safety at the fixture.

Why it matters to a homeowner

This is one of the most-googled water-heater questions (“what temperature should my water heater be set to?”) and the answer in WA is straightforward: 120°F unless you have a specific reason to go higher and have installed a TMV.

The energy math: every 10°F reduction saves roughly 3–5% on water-heating energy. Going from 140°F to 120°F saves about 6–10%, which is meaningful but not life-changing.

The safety math: pediatric burn rate at 130°F+ tap water is the reason WA mandates the 120°F preset. Children, elderly, and people with mobility limitations don’t react fast enough at 130°F-plus. 120°F gives ~5 minutes before second-degree burn — enough time to react and pull away.

When a contractor sets a new water heater to 140°F without installing a TMV, that technically violates RCW 19.27A.060 — a common WA contractor mistake worth checking after install.

When you’ll encounter this term

• A new water-heater install asks what temperature to set.
• A scalding incident in the household triggers a temperature review.
• An immune-compromised family member’s care plan recommends a higher setpoint with a TMV.
• A vacation home pre-occupancy procedure includes raising temperature for thermal disinfection.

Common variants and what tank temperature is not

• Setpoint vs. delivered temperature. Setpoint is at the tank thermostat. Delivered is at the tap, reduced by mixing in any TMV or fixture mixing valve.
• 120°F vs. 140°F. 120°F is WA default — scald-safe but Legionella-permissive. 140°F is Legionella-safe but scald-risky and requires a TMV at fixtures.
• Tank set vs. distribution loop temperature. In a recirculation loop, return temperature matters for Legionella control as much as the tank setpoint.

Washington note

WA RCW 19.27A.060 requires new water heaters be preset at 120°F or the lowest setting if the heater can’t go that low. The exception is explicit: higher reservoir temperature is allowed if a thermostatic mixing valve or scald-prevention design restricts delivered temperature to 120°F.

WAC 51-56 also pulls in the UPC anti-scald rules at fixtures (UPC §409 / §413) — pressure-balance or thermostatic mixing valves are required at all new shower and tub installs in WA. So even at higher tank setpoints, the fixture protection is mandatory.

The recommended WA setup for most households:

1. Tank at 120°F for child-safe and elderly-safe daily use.
2. No TMV required at fixtures (each shower/tub has anti-scald valve).

For households with immune-compromised residents or persistent Legionella concerns:

1. Tank at 140°F.
2. TMV at the heater outlet delivering 120°F house-wide.
3. Or TMV at each handwash lavatory delivering ~110°F.

For a vacation home that will sit unused for months:

1. Set to vacation mode when leaving.
2. Before re-occupancy, raise to 150°F for several hours.
3. Run every hot fixture for 5+ minutes to flush lines.
4. Return to normal setpoint.

FAQ

What temperature should I set my water heater?

For most WA homes, 120°F — scald-safe, energy-efficient, and the WA-required preset. Adult-only households can comfortably run 125°F. Households with infants, elderly, or mobility-limited residents should stay at 120°F. Households with immune-compromised residents should set 140°F and install a TMV at the heater outlet to deliver 120°F at fixtures.

Is 120°F too cold for my water heater?

Not for most uses. 120°F is hot enough for showers, dishwashers (which boost their own water), and laundry on warm settings. The trade-off is slightly slower hot-water delivery to fixtures and modest Legionella growth potential in the stored water. The setpoint is a balance, not a compromise.

Can I set my water heater to 140°F to save on operating cost?

No — higher setpoints cost more to operate, not less. Every 10°F reduction saves 3–5% on water-heating energy. You’d raise temperature for Legionella control, faster recovery, or higher delivered temperature, not for savings. And a 140°F setpoint without a TMV is a scald risk and a WA RCW violation.