Heat Pump Water Heater Not Working — Error Codes, Mode Confusion & Compressor Fixes

Common Symptoms

• No hot water or lukewarm water with heat pump mode selected — compressor fan not running
• Error code displayed on control panel (E1, E2, E3, E5, E6 on Rheem; 01, 02, 03 on AO Smith Voltex)
• Unit runs continuously in electric resistance mode but never switches to heat pump mode
• Excessive noise — loud rattling or vibration from compressor area (normal: 50–65 dB; abnormal: grinding or screeching)
• Water pooling on floor near unit — condensate drain line clogged or not sloped
• Cold air blowing from the unit into the installation space with no temperature rise in tank
• App shows fault or unit offline (Rheem EcoNet or iCOMM connectivity)

Most Likely Causes

1. 1

   Wrong Operating Mode — Electric Only or Vacation Mode Selected (Most Common)

   Rheem ProTerra and AO Smith Voltex have four operating modes: Heat Pump Only (most efficient, heat pump only — may not keep up with high demand), Hybrid (default and recommended — heat pump + electric backup), High Demand (aggressive electric backup + heat pump for peak usage), and Electric Only (100% resistance element, no heat pump — used during HVAC system servicing or in spaces below 40°F). If the unit was accidentally set to Electric Only, it functions but at 3–4x the energy cost and users may still perceive insufficient performance. Also check that the unit wasn't left in Vacation Mode — it will heat to only 60°F in that mode. Navigate to the control panel or EcoNet/iCOMM app to confirm the mode is set to Hybrid.

2. 2

   Ambient Temperature Below 40°F — Compressor Lockout

   Heat pump water heaters extract heat from the surrounding air. When the installation space drops below 40°F (minimum: 40°F–109°F operating range), the heat pump compressor locks out automatically as a protection measure — the refrigerant cannot absorb sufficient heat energy to complete the cycle. The unit falls back to electric resistance only. Common in garages in cold climates during winter. Solutions: insulate the installation space, add a mini-split to maintain temperature, or accept reduced efficiency during cold months. Also verify the installation space meets the minimum 1,000 cubic feet (10 ft × 10 ft × 10 ft) requirement — too small a space cools down too fast and keeps the unit in lockout.

3. 3

   E1 / E2 Temperature Sensor Fault — Rheem ProTerra (Inlet/Outlet NTC)

   Rheem ProTerra error codes: E1 = inlet water temperature sensor fault (open or short circuit in NTC thermistor), E2 = outlet water temperature sensor fault. These sensors are 10kΩ NTC thermistors that monitor tank temperature for the control board. Test with a multimeter in resistance mode at room temperature — a functional 10kΩ NTC reads approximately 10kΩ at 77°F. An OL or near-zero reading indicates a broken wire or failed sensor. Rheem evap sensor part number: AP21306-1. The control board for ProTerra PROPH80 T2 is part number RH375-30.

4. 4

   E3 Compressor Fault / AO Smith Voltex Error 01 High Limit

   Rheem ProTerra E3 indicates a compressor protection fault — triggered by compressor overtemperature, refrigerant pressure fault, or locked rotor. AO Smith Voltex PHPT-80 error code 01 indicates the high-limit thermostat has tripped (tank exceeded safe temperature, or the high-limit sensor has failed). For E3: power cycle the unit by turning off the circuit breaker for 5 minutes — if it returns, the compressor needs professional diagnosis. For Voltex 01: check the tank temperature setpoint and reset the high-limit by pressing the button behind the upper access panel. If 01 repeats within two heating cycles, test the high-limit sensor with a multimeter.

5. 5

   E5 / E6 Evaporator or Ambient Temperature Sensor Fault

   Rheem ProTerra E5 = evaporator temperature sensor fault (evap coil NTC thermistor), E6 = ambient air temperature sensor fault. These sensors determine when the compressor is safe to run — a failed ambient sensor may prevent the compressor from starting at all (the control board assumes the ambient is below lockout threshold) or allow it to run in dangerously cold conditions. Test the sensor resistance at a known temperature. If confirmed bad, replace the evap sensor (Rheem #AP21306-1). AO Smith Voltex error 02 = inlet sensor, error 03 = outlet sensor — same diagnostic approach.

6. 6

   Clogged Condensate Drain Line

   Heat pump water heaters remove moisture from the air as a byproduct of heat extraction — producing 0.5–2 gallons of condensate per day in humid climates. The condensate drain line (typically 3/4-inch PVC or flexible vinyl) must slope continuously to a floor drain, condensate pump, or utility sink. If the line develops an upward sag, algae/slime build up, or the floor drain clogs, condensate backs up into the unit's drain pan and then overflows onto the floor. Clear the drain line with a wet-dry vacuum, then flush with a 50/50 bleach-water solution to kill algae. If no gravity drain is available, install a Little Giant VCMA-20ULS condensate pump.

7. 7

   Depleted Anode Rod — Accelerated Tank Corrosion

   HPWHs use the same sacrificial anode rod system as conventional tanks. Rheem ProTerra ships with a magnesium anode rod; some AO Smith Voltex and Bradford White Aerotherm models use a hybrid anode (combination anode + hot water outlet fitting). In hard water areas, anode rods deplete faster — inspect every 3 years (not 6 like a conventional tank, because the heat pump cycles the water more frequently, accelerating mineral precipitation). A depleted rod leads to rust-colored water and accelerated tank failure. Replace with a Camco 11562 magnesium rod or a Corro-Protec CP-R-MG powered anode.

Quick DIY Checks

1. 1Step 1 — Verify operating mode and check for error codes: on the control panel, navigate to Mode (Rheem ProTerra: press Mode button; AO Smith Voltex: press the Mode touchpad). Confirm the mode is set to Hybrid (recommended for most households). If the unit is in Electric Only or Vacation mode, change to Hybrid and allow 2–3 hours for recovery. For EcoNet-connected ProTerra units or iCOMM-connected Voltex units, open the app and check the fault log before proceeding. Note any error code on the display. Rheem ProTerra codes: E1=inlet sensor, E2=outlet sensor, E3=compressor fault, E5=evap sensor, E6=ambient sensor. AO Smith Voltex codes: 01=high limit, 02=inlet sensor, 03=outlet sensor. A power cycle (breaker off 5 minutes, back on) often clears transient faults.
2. 2Step 2 — Check ambient temperature and installation space: measure the air temperature in the installation space with a thermometer. The unit's operating range is 40°F–109°F ambient — below 40°F the heat pump will not run. Confirm the space is at least 1,000 cubic feet (10 ft × 10 ft × 10 ft room, or a larger open space). If the unit is in a closet smaller than 1,000 cu ft, it will repeatedly cool the air below the lockout threshold and run inefficiently. Also confirm the unit has at least 7 inches of clearance above the air inlet and exhaust vents are not blocked by stored items. In cold climates, add insulation to the garage or utility room walls if ambient regularly drops below 50°F in winter.
3. 3Step 3 — Test temperature sensors for E1, E2, E5, E6 faults: turn off the circuit breaker. Remove the upper access panel (typically 2–4 screws). Locate the NTC temperature sensor connector — it is a small 2-pin connector near the control board. Disconnect the sensor and set your multimeter to resistance (Ω). At room temperature (77°F), a functional 10kΩ NTC should read approximately 10kΩ ± 10%. An OL reading = open circuit (broken wire or failed sensor), a reading near 0 = short circuit. For Rheem ProTerra evap sensor: Rheem part #AP21306-1. Replace any sensor that fails the resistance test. The control board (Rheem RH375-30 for ProTerra PROPH80 T2) can also fail — if all sensors test good but codes persist after replacement, the board is suspect.

1. 4Step 4 — Inspect and clear the condensate drain line: locate the condensate drain port at the base of the unit — it typically accepts 3/4-inch PVC or 5/8-inch vinyl tubing. Disconnect the drain line and blow through it — it should be clear. A blocked line feels like blowing through a straw filled with water. Use a wet-dry vacuum to suck the clog from the drain end. Flush the line with a solution of 50% bleach and 50% water (pour through the drain port with the tubing disconnected at the far end) to kill algae and slime. Reconnect the line and verify it slopes continuously downward without any low spots that trap condensate. If no floor drain exists, install a Little Giant VCMA-20ULS condensate pump (115V, 1/30 HP, 20 ft vertical lift, 3/8-inch OD tubing) below the drain port level.
2. 5Step 5 — Inspect the anode rod: turn off the cold water supply. Relieve pressure by opening a hot tap. Locate the anode rod hex port on top of the unit (may be labeled, or may be integrated with the hot water outlet on hybrid anode models). Use a 1-1/16-inch socket and breaker bar — torque is typically 40–60 ft-lbs. Remove the rod and inspect: if corroded to the steel core wire, coated in calcium nodules, or shorter than 1/2 its original diameter, replace immediately. Apply 5 wraps of PTFE tape on replacement rod threads before reinstalling. Torque to 40–60 ft-lbs. In hard water areas (>11 grains/gallon hardness), inspect every 2–3 years rather than the standard 6-year interval.
3. 6Step 6 — Test compressor operation and diagnose E3: with the unit powered on and set to Heat Pump Only mode, stand near the unit and listen for the compressor and fan to start within 1–3 minutes of a call for heat. Normal compressor operation produces a low hum plus fan noise at 50–65 dB — louder than a conventional water heater but not dramatically loud. Grinding, screeching, or clicking on startup indicates a compressor issue. An E3 code after a power cycle with ambient temperature in range (40°F–109°F) and all sensors testing good requires professional refrigerant-circuit diagnosis — do not attempt to open refrigerant lines without an EPA 608 certification. If the unit is under the 10-year warranty period (register at rheem.com or aosmith.com within 90 days of installation), the compressor is covered.
4. 7Step 7 — Vibration reduction and noise diagnosis: if the unit produces excessive vibration noise (rattling, buzzing from the base), install an anti-vibration pad under the unit — Rheem insulation pad #SP20337 reduces transmitted vibration to the floor by 60–80%. Also check that the condensate drain line is not rigid-connected in a way that transmits vibration. Verify all access panels are securely fastened — a loose panel rattles against the cabinet at compressor frequency. If noise is a high-pitched screeching or metal-on-metal grinding, this indicates a failing compressor bearing or fan motor bearing — do not continue operation and contact the manufacturer for warranty service.
5. 8Step 8 — Register warranty and log installation data: Rheem ProTerra 10-year tank and parts warranty requires registration at rheem.com within 90 days of installation. AO Smith Voltex 10-year warranty registers at aosmith.com. Without registration, the warranty may default to 6 years. After registration, note the serial number, model number, and installation date in a permanent location (inside the access panel door is common). Schedule the first anode rod inspection for 3 years post-installation. In hard water areas (>11 GPG), also plan to flush the tank annually using the drain valve to remove sediment before it accumulates on the lower heating element.

Repair vs Replace

Recommended Tools & Parts

• Rheem AP21306-1 Evaporator Temperature Sensor

  OEM NTC thermistor evaporator sensor for Rheem ProTerra heat pump water heaters. Resolves E5 (evap temperature sensor fault) error codes. 10kΩ at 77°F. Includes wire harness connector.

  $20–$35

  Buy on Amazon →

• Rheem RH375-30 Control Board (ProTerra PROPH80 T2)

  OEM main control board for Rheem ProTerra PROPH80 T2 heat pump water heater. Controls operating mode, compressor, heating elements, and sensor inputs. Replace when sensor codes persist after confirmed-good sensors.

  $120–$200

  Buy on Amazon →

• Little Giant VCMA-20ULS Condensate Pump

  115V condensate pump for installations without a gravity drain. 20 ft vertical lift, 3/8-inch OD discharge tubing, built-in safety float switch (normally-closed) that wires to the HVAC Y-terminal to shut off the system if the reservoir overflows. Required when no floor drain is available near the HPWH.

  $50–$80

  Buy on Amazon →

• Rheem SP20337 Anti-Vibration Pad

  OEM-spec insulation and vibration-damping pad for heat pump water heater installations. Reduces transmitted floor vibration from the compressor by 60–80%. Particularly effective on slab-on-grade or wood-framed floors that amplify low-frequency compressor noise.

  $20–$35

  Buy on Amazon →

• Corro-Protec CP-R-MG Powered Anode Rod

  Impressed-current titanium anode rod compatible with Rheem ProTerra, AO Smith Voltex, and most HPWHs. Eliminates rotten-egg odor in softened water homes. Includes 6-foot power cord. Replaces sacrificial magnesium rod with active electrochemical protection that does not deplete.

  $45–$70

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

Related Repairs