Lennox Heat Pump Not Heating: Diagnosis and Fixes

Common Symptoms

• Lennox heat pump running in heat mode but supply air is cool or barely warm
• Outdoor unit running normally — fan spinning, compressor operating — but house temperature dropping
• Thermostat set to HEAT but the system behavior mimics cooling mode
• Defrost cycle running constantly — outdoor coil steaming continuously without clearing frost
• Aux heat running non-stop with the heat pump appearing to contribute nothing
• Ice completely encasing the outdoor coil that does not clear after 45–60 minutes of normal operation
• Heat pump short cycling — compressor starts and stops every few minutes in heat mode
• iComfort S30 or E30 thermostat displaying a fault alert or service notification in heating mode

Most Likely Causes

1. 1

   Reversing Valve Solenoid Coil Failure — Most Common Lennox XP No-Heat Cause

   The reversing valve is a 4-way solenoid-operated refrigerant valve that switches the system between heating and cooling modes by redirecting refrigerant flow. On Lennox heat pumps, the O terminal (orange wire) energizes the reversing valve solenoid during cooling — in heating mode, the solenoid is de-energized and the valve spring-returns to heating position. When the solenoid coil fails open, 24VAC cannot energize it and the valve stays in whatever position it was last in. If last in the cooling position, the system actively refrigerates the house while the thermostat calls for heat. The Lennox XP series solenoid coil reads 8–15Ω when functional — notably tighter than the 20–40Ω range on Carrier or Goodman units. An OL reading confirms coil failure. Solenoid coil replacement does not require opening the refrigerant circuit — remove the mounting clip, slide the old coil off the valve stem, slide the new coil on, and reconnect the wires. If the coil tests in spec but the valve spool is mechanically stuck, a certified technician must evacuate and replace the full valve.

2. 2

   Defrost Board Fault — Stuck in Defrost or Never Initiating

   The defrost control board monitors the outdoor coil temperature thermistor and a time-based logic circuit to initiate defrost cycles. When the board fails, it typically fails in one of two modes: (1) stuck in defrost — the reversing valve is energized continuously, the system runs in reverse/cooling mode, the outdoor coil heats and steams without clearing frost, and supply air is cold; or (2) defrost never initiates — the outdoor coil progressively builds frost until it becomes a solid ice block, blocking all airflow and halting heat absorption. Both conditions produce little to no useful heat. Lennox XP16 and XP20 defrost boards use a coil temperature thermistor (typically rated 10–50kΩ at ambient) and have LED status indicators on the board. Common Lennox defrost board part numbers referenced in XP16 service bulletins include the 100176-12 and 100176-17 board families — always verify against the wiring label inside the outdoor unit access panel. On XP25 variable-capacity units, the variable-speed drive board integrates defrost logic and communicates faults to the iComfort thermostat.

3. 3

   Low Refrigerant Charge in Heating Mode

   Low refrigerant affects heat pump performance differently in heating mode versus cooling mode. In heating mode, the outdoor coil is the evaporator absorbing heat from outdoor air. Low refrigerant charge reduces heat absorption capacity: the suction line (large diameter line) at the outdoor unit feels unusually warm or hot — instead of cold and sweating as it should be during normal heating operation — because low charge causes suction superheat to spike. The liquid line (small diameter line) may feel barely cool or ambient temperature. On Lennox XP25 and XP20 with iComfort thermostats, a low-pressure alert may appear in the fault history. On non-communicating XP16 units, the symptom is inadequate heat with no fault codes. Low refrigerant on a Lennox XP heat pump is almost always due to a leak — common locations include the Schrader valve cores on the service ports, the brazed connections at the indoor coil (TXV or EXV connections), and the line set flare fittings. Refrigerant recovery and recharge requires EPA 608 certification.

4. 4

   Failed Capacitor — Outdoor Fan Motor Not Starting

   The run capacitor on the Lennox XP outdoor unit provides the phase-shifted voltage needed to start and run the outdoor fan motor and compressor. A failed capacitor can prevent the outdoor fan motor from starting, which stops airflow through the outdoor coil — in heating mode, zero airflow means zero heat absorption from outdoor air. The system may attempt to run with no fan, causing the compressor to overheat and shut down on its thermal protector, producing a short-cycling pattern. Lennox XP16 and XP20 units typically use a dual-run capacitor (labeled HERM and FAN) — the FAN section failure stops the outdoor fan while the HERM section failure prevents the compressor from starting. Measure capacitance with a multimeter in capacitance mode: compare the measured value to the capacitor label rating — a reading more than ±6% outside the rated value confirms failure.

5. 5

   Outdoor Fan Motor Failure

   The outdoor fan motor draws heat exchange air through the outdoor coil during both heating and cooling operation. A failed outdoor fan motor — burned windings, seized bearings, or failed run capacitor — stops outdoor airflow entirely. In heating mode, no airflow through the outdoor coil means the heat pump cannot absorb heat from outdoor air, and the compressor is forced to work against increasing pressure differentials. Test the outdoor fan motor winding resistance with a multimeter: measure resistance between each motor winding terminal and ground (should read OL — open circuit to ground) and between the winding terminals (typical fan motor winding resistance is 1–10Ω; OL indicates an open winding, near-0Ω indicates a shorted winding). Also spin the fan blade by hand (with power off) — it should rotate freely with no grinding or resistance.

6. 6

   Thermostat Wiring Error — O vs B Terminal Reversed

   Lennox heat pumps use the O terminal convention: 24VAC on the O wire (orange) energizes the reversing valve solenoid during cooling, and the valve is de-energized in heating mode. Some thermostat brands (notably older Honeywell models) use the B terminal convention instead — where 24VAC on B energizes the valve in heating mode. If a technician replaced a thermostat without checking the O/B setting, the system may be configured for B operation on a Lennox heat pump that uses O — meaning the reversing valve is energized in heating and de-energized in cooling: the opposite of correct operation. The heat pump will cool when it should heat and heat when it should cool. Verify the thermostat O/B configuration in the installer settings menu. On iComfort thermostats, this is configurable in the advanced setup — verify the reversing valve setting matches O-type operation.

Quick DIY Checks

1. 1Step 1 — Thermostat mode and iComfort fault check: confirm the thermostat is set to HEAT mode, fan to AUTO, and the set point is at least 3–5°F above current indoor temperature. On a Lennox iComfort S30 or E30 thermostat, navigate to the menu icon, select 'Diagnostics' or 'Service Menu,' and review stored fault codes with timestamps — note any codes before going outside. Verify the thermostat reversing valve setting is configured for O (not B). On a standard non-communicating thermostat paired with an XP16, confirm mode is HEAT and the O/B terminal jumper or dip switch is set for O-type reversing valve operation. A thermostat set to Emergency Heat mode on a Lennox system bypasses the compressor entirely and runs electric strips only — if the air handler breaker has tripped, Emergency Heat produces no heat.
2. 2Step 2 — Inspect outdoor unit for ice and defrost status: walk outside and visually inspect the outdoor coil. Some frost on the coil fins in cold, humid weather is completely normal — the heat pump runs scheduled defrost cycles every 30–90 minutes to clear it. A completely ice-encased outdoor unit (no coil fins visible, the cabinet looks like an ice block) indicates the defrost system has failed. Do NOT chip the ice or pour hot water on the unit. Set the thermostat to fan-only mode or shut the system off and let the ice melt naturally over 2–4 hours. After the ice clears, restart in heating mode and watch whether the coil re-ices within 30–60 minutes. If it does, the defrost board or coil temperature thermistor has failed. If the outdoor coil is steaming continuously with no frost clearing, the defrost board may be stuck in defrost mode — open the outer electrical compartment and check the defrost board LED status indicator (refer to the wiring diagram inside the access panel for LED meaning).
3. 3Step 3 — Test the reversing valve solenoid coil resistance: turn off the outdoor unit at the disconnect box (pull the fuse block or flip the disconnect breaker). Locate the reversing valve on the refrigerant circuit — it is the cylindrical 4-port brass valve, typically mounted near the compressor, with a solenoid coil body on top. Disconnect the two-wire connector from the solenoid coil (one wire traces back to the O terminal on the outdoor control board). Set your multimeter to ohms (Ω) and probe the two solenoid coil terminals. A functional Lennox XP series reversing valve solenoid reads 8–15Ω. OL (open circuit) confirms the coil has failed. Solenoid coil replacement is DIY-accessible: slide the coil off the valve stem after removing the mounting clip, install the new coil, reconnect the wires. This does not require opening the refrigerant circuit. If coil resistance is within spec, restore power and test 24VAC at the solenoid connector terminals with the thermostat in COOL mode — should read 24–28VAC. No voltage in cooling mode indicates a wiring or thermostat O terminal fault.

1. 4Step 4 — Test the defrost board relay continuity and thermistor: with power off, locate the defrost control board inside the outdoor unit electrical compartment (a small circuit board with LED indicators and wire harness connectors). Locate the defrost thermistor — the small NTC sensor clipped to the outdoor coil fins, connected via a two-wire harness. Disconnect the thermistor from the defrost board. Set your multimeter to ohms and measure thermistor resistance: at ambient temperature (40–70°F), a functional Lennox defrost thermistor reads 10–50kΩ. OL indicates open-circuit failure; near 0Ω indicates a short. A failed thermistor ($20–$40 part) is often the root cause of apparent defrost board failures — replace it before condemning the full board. To test the defrost board relay: with power on and the board in defrost mode (or initiate a test defrost cycle by jumping the board test pins if accessible per the wiring diagram), use a multimeter set to AC volts and verify 24VAC is present at the reversing valve solenoid connector — absence of voltage during commanded defrost confirms relay failure on the board.
2. 5Step 5 — Check the run capacitor with a multimeter: turn off the outdoor unit at the disconnect. Locate the capacitor — a cylindrical or oval metal can mounted on the interior of the outdoor unit electrical compartment. WAIT 5 MINUTES after disconnecting power to allow the capacitor to discharge through the unit's bleed resistor, or discharge it manually using a 10kΩ resistor across the terminals. Set your multimeter to capacitance mode (symbol: —|(— or μF). Probe the FAN terminals and record the reading; probe the HERM terminals and record the reading. Compare each to the capacitor label rating (e.g., 45+5 μF ±6%). 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 — both cause no-heat symptoms.
3. 6Step 6 — Test outdoor fan motor winding resistance: with power off and the capacitor discharged, locate the outdoor fan motor. The motor is mounted in the top of the outdoor unit and connects to the control board via a 3-wire or 4-wire harness (typically brown = common, black = high-speed winding, blue or purple = low-speed winding if two-speed). Disconnect the motor wire harness. Set your multimeter to ohms. First, test each motor terminal to ground (chassis metal): all should read OL (open circuit) — any reading less than OL indicates a grounded winding (motor must be replaced). Next, measure resistance between winding terminals: typical Lennox outdoor fan motor winding resistance is 1–10Ω; OL indicates an open winding (motor failed); near-0Ω indicates a shorted winding. Also spin the motor fan blade by hand — it should rotate freely with no grinding, binding, or scraping sounds. Resistance from stiff bearings confirms a mechanically failed motor.

Repair vs Replace

Recommended Tools & Parts

• Lennox Heat Pump Reversing Valve Solenoid Coil (24VAC)

  Replacement 24VAC reversing valve solenoid coil for Lennox XP16, XP20, and XP25 heat pump outdoor units. Fixes heat pump stuck in cooling mode when the solenoid coil tests open (OL) or out of range (should read 8–15Ω on Lennox XP units). Does not require opening the refrigerant circuit — slides on and off the valve stem with one mounting clip. Match the connector type to your existing coil.

  $30–$80

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• Lennox Heat Pump Defrost Control Board

  Replacement defrost control board for Lennox XP16 and XP20 heat pump outdoor units — fixes defrost stuck in continuous mode, defrost that never initiates, or failed LED indicator. Match the board part number from the outdoor unit wiring label inside the access panel (common Lennox defrost board families: 100176-12, 100176-17 series).

  $80–$200

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• Klein Tools MM400 Digital Multimeter

  Auto-ranging multimeter with capacitance measurement mode — essential for Lennox heat pump diagnosis. Tests reversing valve solenoid coil resistance (8–15Ω spec), capacitor MFD (±6% tolerance check), fan motor winding resistance, 24VAC control circuit voltage, and defrost thermistor NTC resistance (10–50kΩ range).

  $45–$60

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• Dual-Run Capacitor for Lennox XP Heat Pump

  Replacement dual-run capacitor for Lennox XP outdoor unit — fixes outdoor fan motor or compressor that fails to start or runs sluggishly. Match the capacitance rating (μF) and voltage rating printed on the original capacitor exactly. Common Lennox XP ratings: 45+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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