Electric water heaters have two thermostats — upper and lower. Each controls one of the two heating elements. When a thermostat fails, the corresponding element either stops firing (cold water) or doesn't shut off (overheat trip). Thermostats are inexpensive ($25-50) and DIY-replaceable.
How they work
Each thermostat is a bimetallic switch with a manually-set temperature dial. The bimetal disc bends as the tank shell heats up; once it bends enough to break electrical contact, the corresponding element shuts off. When water cools, the disc reverses; element fires again.
The upper and lower thermostats wire together with the elements in a non-simultaneous control scheme:
- Cold water draw activates the upper thermostat
- Upper thermostat closes; upper element fires; top of tank heats
- Once upper is at setpoint, upper thermostat opens; transfers power to lower
- Lower thermostat closes if water is cool; lower element fires; rest of tank heats
- Both shut off at setpoint
The upper thermostat also includes a high-temperature cutoff (ECO — Energy Cut Off) — a separate safety device that trips at ~180°F to prevent runaway overheating. If the ECO trips, the unit is dead until you press the red reset button on the upper thermostat (after diagnosing why).
Failure symptoms
| Symptom | Likely failed thermostat |
|---|---|
| No hot water at all + ECO red button popped | Upper thermostat (or lower thermostat stuck closed) |
| No hot water at all + ECO not popped | Upper thermostat (or upper element) |
| Lukewarm water at sustained draw | Lower thermostat (or lower element) |
| Water too hot, scalding | Either thermostat stuck closed (element won't shut off) |
| ECO trips repeatedly after reset | Thermostat stuck closed — replace it AND the ECO portion |
How to test
- Turn off the breaker; verify with non-contact tester
- Remove the upper access panel
- Pull back insulation and plastic safety cover
- Press the red ECO reset button. If it clicks, the ECO had tripped — investigate why before just resetting
- With multimeter on Ohms: place probes on the thermostat's terminals
| Reading | Diagnosis |
|---|---|
| Continuity (close to 0 ohms) | Thermostat is closed — element should be firing |
| No continuity | Thermostat is open — element won't fire |
| Permanently closed when water is hot | Failed — replace |
| Permanently open when water is cold | Failed — replace |
The behavior should change as the tank heats/cools. A thermostat stuck in one position regardless of water temp is failed.
How to replace
Tools: Phillips screwdriver, flathead screwdriver (for the dial), multimeter, replacement thermostat ($25-50 — match the brand if possible, though most are cross-compatible).
- Shut off breaker; verify with voltage tester
- Remove access panel
- Pull back insulation and safety cover
- Photograph the wiring before disconnecting
- Loosen the thermostat retaining clip (or screws on older units)
- Remove the old thermostat
- Install the new thermostat; verify it's flush against the tank shell (good thermal contact is required for accurate sensing)
- Reconnect wires per photo
- Set the dial to 120°F
- Reattach safety cover, insulation, access panel
- Restore breaker
- Wait 60-90 minutes; test water temperature at a tap
Replacing upper vs lower
- Upper thermostat — typically more important; ECO reset button is part of the upper thermostat. Replace as a unit (thermostat + ECO)
- Lower thermostat — simpler unit, no ECO. Cheaper part ($15-30)
Setting the thermostat
The dial is calibrated in degrees F (or sometimes letters A/B/C). Set both upper and lower to the same temperature — 120°F is the recommended default. See temperature setting for the full discussion.
When the ECO trips
The Energy Cut Off (red button) trips when:
- Tank shell exceeds ~180°F
- Power surge damages the upper thermostat circuit
- A thermostat is stuck closed and the element won't shut off
If the ECO has tripped, press the red button. If it trips again within hours, the underlying thermostat or element is faulty — investigate, don't keep resetting.
Thermostat and warranty
Covered under parts warranty (5-10 years typical). DIY during warranty voids it; use an authorized installer if you want to keep warranty intact.
Related guides
Bottom line
Electric water heater thermostats are $25-50 and DIY-replaceable. Test with a multimeter before assuming failure. If the ECO has tripped, don't just reset — diagnose why. Replace upper thermostat as a thermostat+ECO unit when needed; lower is simpler.