Electric Rheem water heaters use two thermostats — an upper and a lower — that control the upper and lower heating elements respectively. When one fails, the symptom pattern is specific enough to diagnose without a multimeter. Replacement is a 45-minute DIY.
Symptoms by which thermostat failed
Upper thermostat failed: no hot water at all. The upper thermostat is the primary control — if it doesn't signal heat-on, neither element fires.
Lower thermostat failed: hot water briefly, then quickly turns lukewarm. The upper element fires (kept the top of the tank hot) but the lower can't keep up; as you draw water you pull cold up from the bottom faster than the upper element can heat.
High-limit (reset button) tripped: red button on the upper thermostat — manually reset once. If it trips again within 24 hours, the thermostat itself has failed, or there's a runaway-heating fault (often a stuck-closed element). Replace, then diagnose deeper if the new reset trips.
Rheem thermostat part numbers
- Upper thermostat (with reset/high-limit): AP12889B — most Performance, Performance Plus, Performance Platinum residential electric tanks
- Lower thermostat: AP12889C — same line as upper, but without the high-limit reset
- Marathon line: uses different sensor technology — verify with Marathon-specific parts
- ProTerra hybrid: AP18432M (specialized for HP control)
Universal Honeywell L4006 or similar thermostats work in Rheem residential electric tanks but verify the dial-temperature range matches your unit's spec before ordering.
Replacement procedure
- Shut off power at the breaker and verify with a voltage tester at the element terminals
- Remove the access panel covering the thermostat (top access for upper, bottom access for lower)
- Pull aside the insulation covering the thermostat
- Photograph the wiring before disconnecting — you'll need to put each wire back in its original terminal
- Disconnect all wires from the thermostat (usually 4 wires: 2 line-in, 2 element-out)
- Release the thermostat retaining clip — most Rheem thermostats are held by a single metal clip that springs open with a flathead screwdriver
- Pull the thermostat out — it sits against the side of the tank with insulation behind it
- Install the new thermostat, reattach the retaining clip — verify firm contact with the tank wall (this is how it senses water temperature)
- Reconnect wires following the photo you took
- Reset the high-limit (push the red button until you hear a click) on the upper thermostat
- Set the temperature — factory default is 120°F. Some Rheem dials use a numeric scale; calibrate after install.
- Restore power and verify the unit fires up. Allow 60+ minutes for full heat cycle.
Common mistakes
Loose contact with the tank wall: the thermostat senses water temperature through the tank wall. If the retaining clip doesn't hold firmly, the thermostat reads cooler than actual — it will keep firing and over-heat the tank, eventually tripping the high-limit reset.
Reversed wiring: swapping the line-in and element-out wires can short the circuit. Always photograph before disconnecting.
Forgetting to reset the high-limit: on a fresh thermostat, the high-limit button often needs to be pressed before the unit will fire.
When to replace both thermostats
If your unit is past year 8 and one thermostat has failed, consider replacing both during the same service visit. The labor cost is incremental (already have the access panels open) and the second thermostat is statistically likely to fail within 1–2 years anyway.
Temperature setting recommendations
Rheem ships factory-set at 120°F. This is the scald-safety standard recommended by the CPSC. Lowering to 110°F saves energy but may not reach the 120°F dishwasher input requirement. Raising to 130–140°F kills bacteria (some Legionella prevention guidance) but increases scald risk dramatically. See our temperature setting page for a full discussion.
Bottom line
Thermostat replacement is a $30–$45 part and 45-minute DIY. Diagnosing which one (upper vs lower) is symptom-driven and reliable. For element failures alongside, see our element replacement page. For full parts directory see the Rheem parts hub.