Rheem Heat Pump Not Heating: Diagnosis and Fixes

Common Symptoms

• Rheem heat pump running in heat mode but supply air feels cool or cold
• Outdoor unit operating normally — fan spinning, compressor running — but house temperature dropping in cold weather
• Thermostat set to HEAT but system behavior mimics cooling mode
• Defrost cycle running constantly — outdoor coil steaming with no frost clearing
• Aux heat running non-stop without the heat pump contributing to heating output
• Outdoor coil completely encased in ice that does not clear after 45–60 minutes
• Heat pump short cycling on and off in heating mode every few minutes
• EcoNet app or thermostat showing a service alert or fault notification

Most Likely Causes

1. 1

   Reversing Valve Solenoid Coil Failure

   The reversing valve is a 4-way solenoid-operated valve that redirects refrigerant flow to switch between heating and cooling modes. On Rheem heat pumps, the O terminal (orange wire) energizes the solenoid during cooling — the solenoid is de-energized in heating mode and the valve spring-returns to heating position. When the solenoid coil fails open, 24VAC cannot energize the valve, and it stays in whatever position it was last in. If last in the cooling position, the Rheem heat pump runs in cooling mode even when the thermostat calls for heat — actively refrigerating the house in winter. The reversing valve solenoid coil on Rheem RP series outdoor units reads 8–15Ω when functional (same spec as Ruud units — components are interchangeable). OL (open circuit) on the multimeter confirms coil failure. Solenoid coil replacement does not require opening the refrigerant circuit — slide the coil off the valve stem, install the new coil, reconnect. If the coil tests in spec but the valve is mechanically stuck, a certified technician must evacuate and replace the full valve.

2. 2

   Defrost Board Fault — Stuck in Defrost or Not Initiating

   The defrost control board monitors the outdoor coil temperature thermistor and a time-based logic circuit to initiate defrost cycles every 30–90 minutes. When the board fails, it typically fails in one of two patterns: (1) stuck in defrost — the system runs continuously in reverse/cooling mode, the outdoor coil heats and steams without frost clearing, and supply air is cold; (2) defrost never initiates — the outdoor coil progressively builds frost until completely iced over, blocking airflow and halting heat absorption. Rheem RP15 and RP17 defrost boards share part numbers with Ruud UP15 and UP17 equivalent boards — if a Rheem-branded board is unavailable, the Ruud equivalent is a direct substitution. The defrost board is in the outdoor unit electrical compartment, typically with LED status indicators — a continuously lit defrost LED with no ice clearing confirms the board is stuck in defrost. Always test the coil temperature thermistor before replacing the full board.

3. 3

   Low Refrigerant — Schrader Valve Core Leak

   Low refrigerant in heating mode reduces heat absorption capacity from outdoor air: the suction line at the outdoor unit feels warm or hot (instead of cold and sweating as normal), because low charge causes suction superheat to spike. Rheem RP15 and RP17 units have a documented Schrader valve core leak failure pattern at the service ports — if the service port caps are missing or have been disturbed, inspect both high-side and low-side Schrader cores for oil staining or refrigerant frost/residue. On Rheem RP20 with EcoNet connectivity, a low-pressure alert may appear in the EcoNet fault history. On non-communicating RP15 units, inadequate heat with no codes is the only symptom. Low refrigerant requires EPA 608 certification for diagnosis and recharge. Rheem RP series heat pumps use R-410A; older RP units (pre-2009) may use R-22.

4. 4

   Failed Run Capacitor

   A failed run capacitor prevents the outdoor fan motor or compressor from starting, causing no-heat symptoms. Rheem RP outdoor units use a dual-run capacitor with HERM (compressor) and FAN sections. A failed FAN section allows the compressor to start but stops the outdoor fan motor, which eliminates airflow through the outdoor coil in heating mode — no airflow means no heat absorption. A failed HERM section prevents the compressor from starting, producing a humming outdoor unit with no compression. Test capacitance with a multimeter in capacitance mode: compare measured values to the label rating — a reading more than ±6% outside the rated value confirms failure. Common Rheem RP dual-run capacitor ratings: 45+5 μF or 40+5 μF at 370/440 VAC.

5. 5

   Outdoor Fan Motor Winding Failure

   The outdoor fan motor drives airflow through the outdoor coil during heating and cooling operation. A failed outdoor fan motor stops outdoor airflow entirely — in heating mode, this means zero heat absorption from outdoor air and rapidly rising high-side refrigerant pressure, causing the compressor to short cycle on its pressure protection. Test outdoor fan motor winding resistance: disconnect the motor harness, set multimeter to ohms, and measure resistance between winding terminals (typical Rheem outdoor fan motor winding resistance is 1–10Ω; OL indicates an open winding; near-0Ω indicates a shorted winding). Test each terminal to chassis ground — all should read OL; any reading less than OL indicates a grounded winding. Spin the fan blade by hand (with power off) — free rotation with no grinding confirms mechanical integrity.

6. 6

   Thermostat Wiring Error — O vs B Terminal

   Rheem heat pumps use the O terminal convention: 24VAC on the O wire (orange) energizes the reversing valve solenoid during cooling. If a replacement thermostat is configured for B-type operation — where 24VAC on B energizes the valve in heating — the heat pump operates in cooling mode when the thermostat calls for heat and heating mode when it calls for cool. The symptom is identical to a mechanically stuck reversing valve: the heat pump blows cold air in heat mode. Verify the thermostat O/B configuration in the installer setup menu. On EcoNet thermostats, this is set during the equipment configuration wizard. On standard thermostats, look for a DIP switch or configuration option labeled 'O/B' — it should be set to 'O' for Rheem heat pumps.

Quick DIY Checks

1. 1Step 1 — EcoNet fault check and thermostat mode verification: if the Rheem heat pump is connected to EcoNet, open the EcoNet app or check the EcoNet thermostat display for stored fault codes or alerts — note all active alerts before going outside. Confirm the thermostat is in HEAT mode, fan to AUTO, and the set point is at least 3–5°F above current indoor temperature. Verify the thermostat reversing valve configuration is set to O-type (not B-type) — this is found in installer setup or equipment configuration settings. On a standard non-communicating thermostat paired with an RP15, confirm mode is HEAT and the O/B selector is set to O. Emergency Heat mode on a Rheem system bypasses the compressor and runs electric strips only — if the air handler breaker has tripped, Emergency Heat produces little or no heat.
2. 2Step 2 — Inspect outdoor unit ice accumulation and defrost status: walk outside and visually inspect the outdoor coil. Some coil frost in cold, humid weather is normal and clears during scheduled defrost cycles. A completely ice-encased outdoor unit where no coil fins are visible indicates a failed defrost system. Do NOT chip the ice or pour hot water on the unit. Turn the system to fan-only mode or shut it off and allow 2–4 hours for natural melting. After the ice clears, restart in heating mode and observe whether the coil begins re-icing within 30–60 minutes. If it does, the defrost board or thermistor has failed. If the outdoor coil is continuously steaming without clearing frost, the defrost board may be stuck in defrost mode — open the outer electrical compartment and check the LED status indicator on the defrost board.
3. 3Step 3 — Test the reversing valve solenoid coil resistance: turn off the outdoor unit at the disconnect box. Locate the reversing valve on the refrigerant circuit — the cylindrical 4-port brass valve near the compressor, with a solenoid coil body on top. Disconnect the solenoid coil wire connector (one wire traces to the O terminal on the outdoor control board). Set your multimeter to ohms (Ω) and probe the two solenoid coil terminals. A functional Rheem RP series reversing valve solenoid reads 8–15Ω — the same specification as Ruud units, since components are identical. OL (open circuit) confirms coil failure. Solenoid coil replacement is DIY-accessible and does not require opening the refrigerant circuit. If coil resistance is within spec, restore power and test 24VAC at the solenoid connector with the thermostat in COOL mode — should read 24–28VAC. No voltage in cooling mode indicates a thermostat or O terminal wiring fault.

1. 4Step 4 — Test defrost board relay continuity and coil thermistor: with power off, locate the defrost thermistor — the small NTC sensor clipped to the outdoor coil fins, connected to the defrost board via a two-wire harness. Disconnect the thermistor from the defrost board. Set the multimeter to ohms and measure resistance: at ambient temperature (40–70°F), a functional Rheem defrost thermistor reads 10–50kΩ. OL indicates open-circuit failure; near-0Ω indicates a short. Replace a failed thermistor ($20–$40) before condemning the full defrost board — this resolves many apparent defrost board failures on Rheem RP units. Note that Rheem defrost boards are interchangeable with Ruud equivalent boards — if a Rheem-specific part is unavailable, the Ruud part number equivalent is a direct replacement. After replacing the thermistor, run in heating mode and verify the defrost cycle initiates and clears the coil within 90 minutes.
2. 5Step 5 — Check run capacitor capacitance: turn off the outdoor unit at the disconnect. Locate the dual-run capacitor inside the outdoor unit electrical compartment. Wait 5 minutes after disconnecting power (or discharge manually using a 10kΩ resistor across each set of terminals) before touching capacitor leads. Set the multimeter to capacitance mode (μF). Measure the FAN terminals and compare to the label rating; measure the HERM terminals and compare to the label rating. A reading more than ±6% outside the rated value confirms failure. A failed FAN section stops the outdoor fan motor; a failed HERM section prevents the compressor from starting. Common Rheem RP capacitor ratings: 45+5 μF or 40+5 μF at 370/440 VAC — match the replacement rating exactly.
3. 6Step 6 — Assess refrigerant charge and check Schrader valve cores: with the heat pump running in heat mode, touch the large suction line at the outdoor unit. It should feel cold and slightly moist — distinctly below ambient temperature. An unusually warm or hot suction line with the compressor running in heating mode indicates low refrigerant charge (high suction superheat). Touch the liquid line (small diameter copper tubing) — in heating mode it should feel slightly warm to warm. Inspect the Schrader valve service port caps on both high-side and low-side ports — missing or loose caps are a documented issue on Rheem RP15 and RP17 units. Oil staining or refrigerant residue at the valve core confirms a leak at the service port. Replace missing service port caps immediately — they are the last seal against refrigerant loss at the valve core. If refrigerant indicators are present, stop DIY and call an EPA 608-certified HVAC technician.

Repair vs Replace

Recommended Tools & Parts

• Rheem / Ruud Heat Pump Reversing Valve Solenoid Coil (24VAC)

  Replacement 24VAC reversing valve solenoid coil for Rheem RP15, RP17, RP20, and Ruud UP-series heat pump outdoor units (Rheem and Ruud components are interchangeable). Fixes heat pump stuck in cooling mode when the solenoid coil tests OL or out of the 8–15Ω specification. Does not require opening the refrigerant circuit — slides on and off the valve stem.

  $30–$80

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• Rheem / Ruud Heat Pump Defrost Control Board

  Replacement defrost control board for Rheem RP15, RP17, RP20, and Ruud UP-series heat pump outdoor units — Rheem and Ruud defrost boards are interchangeable. Fixes defrost stuck in continuous mode, defrost that never initiates, or a failed status LED. Match the part number from the existing board or from the outdoor unit wiring label.

  $80–$180

  Buy on Amazon →

• Klein Tools MM400 Digital Multimeter

  Auto-ranging multimeter with capacitance mode — essential for Rheem heat pump diagnosis. Tests reversing valve solenoid coil resistance (8–15Ω spec), capacitor MFD values, defrost thermistor NTC resistance (10–50kΩ range), 24VAC control circuit voltage, and fan motor winding resistance. The go-to field multimeter for HVAC technicians.

  $45–$60

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• Dual-Run Capacitor for Rheem RP Heat Pump

  Replacement dual-run capacitor for Rheem RP outdoor unit — fixes outdoor fan motor or compressor that fails to start or hums without running. Match the capacitance rating (μF) and voltage rating on the original capacitor exactly. Common Rheem RP ratings: 45+5 μF or 40+5 μF at 370/440 VAC.

  $20–$60

  Buy on Amazon →

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

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