Carrier Heat Pump Not Heating: Diagnosis and Fix

Common Symptoms

• Heat pump running in heat mode but supply air is cold or only slightly warm
• Outdoor unit running, condenser fan spinning, but house temperature dropping
• Thermostat set to HEAT but the system behaves as if it is in COOL mode
• Ice covering the entire outdoor unit that does not clear after 30–60 minutes
• Aux heat or Emergency heat light on thermostat, but house still not warming
• Carrier Infinity or Performance thermostat displaying a fault code or service alert
• Heat pump runs but auxiliary heat strips never engage below 35°F

Most Likely Causes

1. 1

   Thermostat in Wrong Mode — Emergency Heat Locked Out (Check First)

   Before diagnosing any hardware, verify the thermostat is set to HEAT (not COOL, OFF, or Emergency Heat override). On Carrier Infinity thermostats, navigating to Emergency Heat mode locks out the compressor entirely and runs only the electric heat strips — if the strips are undersized or the breaker to the air handler has tripped, Emergency Heat mode produces little to no heat. On standard thermostats, also confirm the system mode is HEAT and the fan is on AUTO, not ON (which circulates unconditioned air continuously). A Carrier heat pump set to COOL with the outdoor unit running produces cold air in winter — exactly mimicking refrigerant or reversing valve failure. This takes 30 seconds to check and eliminates a $300+ service call.

2. 2

   Stuck Defrost Cycle (Ice-Covered Outdoor Unit)

   Heat pumps accumulate frost on the outdoor coil during normal heating operation in cold, humid weather. The defrost board monitors coil temperature and outdoor temperature, then initiates a timed defrost cycle — typically every 30, 60, or 90 minutes — to melt the frost. During defrost, the system temporarily reverses to cooling mode, the outdoor coil heats up and melts the frost, then the system returns to heating. If the defrost board fails, the defrost cycle either runs continuously (system stuck in reverse/cooling mode) or never runs (coil ices over completely). A completely ice-covered outdoor coil blocks airflow through the coil and prevents heat absorption — the system runs but delivers no useful heat. On Carrier 25HBB and 25HCC models, the defrost control board is mounted inside the outdoor unit electrical compartment. LED indicator lights on the board show defrost status — a continuously lit defrost LED with no heat output indicates the board is stuck in defrost mode.

3. 3

   Reversing Valve Failure — Stuck in Cooling Mode

   The reversing valve is a solenoid-operated 4-way refrigerant valve that redirects refrigerant flow direction, switching the system between heating and cooling modes. On Carrier heat pumps, the O terminal (orange wire) on the thermostat energizes the reversing valve solenoid during cooling — in heating mode, the solenoid is de-energized and the valve spring-returns to the heating position. If the solenoid coil fails open, 24VAC never reaches the solenoid and the valve stays in whatever position it was last in. If the valve spool mechanically sticks, the solenoid may energize correctly but the valve does not shift. A heat pump blowing cold air in heat mode with the compressor running and both lines cool at the service ports strongly suggests the reversing valve is stuck in cooling mode — the system is actively refrigerating the house while trying to heat it. The reversing valve solenoid coil on Carrier outdoor units reads 20–40 ohms when functional — OL reading means the coil has failed. Valve spool replacement requires evacuating and recharging the refrigerant circuit — EPA 608 certification required.

4. 4

   Low Refrigerant in Heating Mode

   Low refrigerant causes different symptoms in heating mode versus cooling mode. In heating mode, low refrigerant reduces the system's ability to absorb heat from the outdoor air — the suction line (larger diameter line) at the outdoor unit will be unusually warm (or even hot) rather than cold and sweating as it should be in normal heating mode, because the low refrigerant charge causes the suction superheat to spike. The liquid line (smaller diameter line) may feel barely cold or even ambient temperature. On Carrier Infinity series with communicating components, the system may display a low-pressure fault code in heating mode. Low refrigerant on a Carrier heat pump is almost always caused by a leak — common locations include the Schrader valve cores on the service ports, the brazed joints at the indoor coil, and the line set connections. Refrigerant charging and leak repair requires EPA 608 certification.

5. 5

   Auxiliary/Emergency Heat Strips Not Engaging

   Most Carrier heat pumps are installed with electric auxiliary heat strips in the air handler that supplement the heat pump output below the balance point (typically 35–40°F). If the heat pump is working correctly but the house can't maintain setpoint in cold weather, the problem may be that the aux heat strips are not engaging. Causes: a tripped breaker on the air handler circuit (aux strips run on 240V and can trip independently of the main panel breaker), a failed sequencer relay that staggers strip engagement, a failed strip element (check resistance — a 5kW strip reads approximately 9.6 ohms, a 10kW strip reads approximately 4.8 ohms), or an outdoor thermostat lockout setting on the thermostat that is set too low. On Carrier Infinity thermostats, the aux heat lockout temperature is adjustable — verify it is set above the current outdoor temperature for the aux strips to engage.

Quick DIY Checks

1. 1Check thermostat mode and settings first — this takes 30 seconds and eliminates the most common cause. On a Carrier Infinity thermostat: press the Mode button and confirm the display shows HEAT (not COOL, FAN ONLY, or EMERGENCY HEAT). Confirm the set temperature is at least 3–5°F above the current indoor temperature. Confirm the fan is set to AUTO (not ON). On a standard non-communicating thermostat, set to HEAT, auto fan, and raise the set point 5°F above room temp — then wait 5 minutes for the system to respond before proceeding. On Infinity and Performance thermostats, navigate to the diagnostics menu and check for stored fault codes — these identify whether the fault is in the outdoor unit (compressor, reversing valve circuit), indoor unit (air handler), or control wiring.
2. 2Check the outdoor unit for ice accumulation. Walk outside and look at the outdoor unit coil — some frost in cold weather is normal, but if the coil is completely encased in ice (no coil fins visible, the unit looks like an ice block), the defrost system has failed or is stuck. Do NOT pour hot water on the unit or chip the ice — turn the system to FAN ONLY mode or shut it off and allow the ice to melt naturally over 2–4 hours. Once the ice clears, restart in heating mode and watch whether the unit begins icing up again within 30–60 minutes. If it does, the defrost board is not initiating defrost cycles. On Carrier 25HBB and 25HCC models, the defrost board is in the outer electrical compartment — locate the LED status indicators on the board (check the unit wiring diagram inside the access panel for LED meanings). A continuously lit or rapidly blinking defrost LED indicates the board is stuck in defrost state.
3. 3Test the reversing valve solenoid with a multimeter. Turn off the outdoor unit at the disconnect box (pull the disconnect fuse block or flip the breaker). Locate the reversing valve — it is the cylindrical brass 4-port valve on the refrigerant circuit with a small cylindrical solenoid coil mounted on top. Disconnect the two-wire connector from the solenoid coil (one wire will trace back to the outdoor unit control board terminal, typically labeled O or RVS). Set your multimeter to ohms (Ω) and probe the two solenoid coil terminals. A functional reversing valve solenoid on Carrier outdoor units reads 20–40 ohms. OL (open circuit) means the coil has failed and needs replacement — a solenoid coil replacement is a DIY-accessible repair (remove one mounting screw and slide the coil off the valve stem) that does not require opening the refrigerant circuit. If coil resistance is in spec, restore power and test 24VAC at the solenoid connector terminals during a cooling call (thermostat set to COOL) — should read 24–28VAC. No voltage in cooling mode means the wiring or thermostat O terminal output has failed.

1. 4Check the auxiliary heat strips. Locate the air handler (indoor unit) — typically in a closet, attic, or basement. Check the breaker for the air handler in the main electrical panel — aux heat strips often trip their own breaker (usually a 30A, 40A, or 60A breaker labeled 'air handler' or 'AHU'). Reset any tripped breakers. Open the air handler access panel and locate the electric heat strips (look for a row of U-shaped wire elements mounted in the air stream). With power off, use a multimeter set to ohms to test each strip element across its terminals — a 5kW strip at 240V reads approximately 9.6 ohms, a 10kW strip reads approximately 4.8 ohms; OL reading means the element has burned out. Also locate the sequencer relays (small rectangular components near the strips) and test for continuity across the normally open relay contacts when 24VAC is applied — a failed sequencer means the strip gets power but doesn't engage on heat demand.
2. 5Diagnose defrost board operation on Carrier 25HBB and 25HCC models. With power on and the system running in heating mode, open the outdoor unit electrical compartment access panel. Locate the defrost control board — a small circuit board with LED status indicators and connector terminals. Identify the defrost sensing pins: most Carrier defrost boards use a coil temperature sensor (thermistor clipped to the outdoor coil) and time-based logic. Test the coil thermistor resistance at ambient temperature — most Carrier thermistors read 10–50kΩ at outdoor ambient; OL or 0 ohms means the sensor has failed and the board is flying blind on coil temperature. If the LED on the defrost board shows a continuous defrost indication (board stuck in defrost mode), the board itself has likely failed. Carrier defrost boards for 25HBB and 25HCC models are available through HVAC distributors — match the board part number from the outdoor unit wiring label.

Repair vs Replace

Recommended Tools & Parts

• Carrier Heat Pump Reversing Valve Solenoid Coil

  Replacement reversing valve solenoid coil for Carrier and Bryant heat pump outdoor units — fixes heat pump stuck in one mode when coil tests open (OL on multimeter). Slides on and off the valve stem with one mounting clip — does not require opening the refrigerant circuit. Match the voltage (24VAC) and connector type to your existing coil.

  $30–$80

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• Carrier Defrost Control Board

  Replacement defrost control board for Carrier 25HBB and 25HCC heat pump outdoor units — fixes defrost system that runs continuously, never runs, or has a failed status LED. Match the part number from the existing board or the outdoor unit wiring label.

  $60–$150

  Buy on Amazon →

• Carrier Heat Pump Coil Temperature Sensor (Defrost Thermistor)

  Replacement defrost thermistor/coil temperature sensor for Carrier outdoor units — fixes defrost board that cannot read coil temperature and either never initiates or continuously runs defrost. Clips onto the outdoor coil fins. Match resistance specifications to your existing sensor.

  $15–$45

  Buy on Amazon →

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

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