Heat Pump Not Heating
Complete troubleshooting guide — from quick checks you can do right now to professional repairs. Diagnose why your heat pump is blowing cold air, running but not heating, or not heating enough.
Quick Check: Is Your Heat Pump in the Right Mode?
Before diving into complex diagnostics, run through these 60-second checks first — they solve roughly 30% of "no heat" calls.
-
1.
Thermostat mode: Confirm the thermostat is set to HEAT, not COOL or AUTO (some thermostats default back to cool). Set the temperature at least 3–4°F above current room temp to trigger the call for heat.
-
2.
Check the O/B reversing valve wire: On most heat pumps the O wire energizes the reversing valve in cooling mode (Carrier, Trane, Lennox). A few brands (Rheem, Ruud, some older units) use B wire and energize in heating. If the O/B setting in the thermostat is wrong, the system will cool when it should heat. Verify your thermostat's O/B setting matches your equipment manual.
-
3.
Air filter: Pull the filter and hold it up to light. If you cannot see light through it, replace it immediately. A clogged filter can reduce airflow enough that the heat pump cannot transfer heat effectively.
-
4.
Outdoor unit: Check that the outdoor unit is running. If the outdoor fan spins but no warm air rises from the top, the compressor may have failed or refrigerant is critically low.
-
5.
Is the outdoor unit covered in ice? Light frost is normal in cold weather. A solid block of ice is not — skip to the Defrost Cycle section below.
Common Causes & How to Diagnose Them
Thermostat Set Wrong or Auxiliary Heat Disabled
A surprisingly common culprit. If the thermostat O/B configuration is reversed, your heat pump will run in cooling mode even when heat is called. Similarly, if the auxiliary heat strips are disabled (breaker tripped, lockout set in thermostat), you'll have inadequate heating in cold weather.
- • Verify thermostat O/B setting in installer menu
- • Check auxiliary heat breaker in panel
- • Confirm thermostat auxiliary lockout temperature setting
- • Reset thermostat to factory defaults and reconfigure
- • Set to COOL mode — if outdoor unit heats up (warm exhaust), reversing valve is stuck in heat (O/B issue)
- • Set to EMERGENCY HEAT — if house warms up, compressor/refrigerant is the issue
Reversing Valve Stuck or Solenoid Failed
The reversing valve is a 4-way valve that switches refrigerant flow between heating and cooling modes. It is operated by an electromagnetic solenoid. If the valve body becomes mechanically stuck, or the solenoid coil fails, the system will operate only in one mode regardless of the thermostat setting.
- • Heat pump cools when set to heat (or heats when set to cool)
- • No audible "click" or hiss when switching modes at thermostat
- • Suction and discharge pressures don't change between modes
- • Outdoor coil acts as condenser (gets hot) in heating mode instead of evaporator (gets frosty)
Test solenoid with a multimeter — it should read 10–30 ohms. Open circuit = failed coil. Solenoid coils can be replaced without recovering refrigerant. Cost: $30–80 for the coil.
Mechanical valve failure requires full valve replacement with refrigerant recovery, brazing, evacuation and recharge. Typical cost: $400–900 parts + labor.
Defrost Board Malfunction (Ice Buildup → No Heat)
In cold weather, the outdoor coil accumulates frost as it absorbs heat from the cold outdoor air. The defrost control board periodically reverses the system to melt this frost (typically when outdoor temp drops below 40°F and coil temperature falls below a set threshold). If the defrost board, defrost thermostat/sensor, or defrost timer fails, ice builds up until the outdoor coil is completely blocked — dramatically reducing or eliminating heat output.
If outdoor unit is encased in ice, manually initiate a defrost by jumpering the defrost thermostat terminals on the control board. If ice melts and heat output resumes, the defrost thermostat is the likely culprit. Replacement cost: $15–40 for sensor, $80–200 for control board.
Low Refrigerant Charge
Refrigerant does not "wear out" or get consumed — if charge is low, there is a leak. Low charge reduces the amount of heat the system can move per cycle, resulting in weak heating or the unit running continuously without reaching setpoint. In severe cases, the suction pressure drops so low the compressor overheats and shuts down on thermal protection.
- • Lower-than-normal supply air temperature
- • Excessive frost/ice on outdoor coil
- • Low suction pressure (check with manifold gauges)
- • Higher-than-normal superheat at suction line
- • Compressor running hot
Values vary by OAT and equipment. Always check nameplate.
Handling refrigerants requires EPA 608 certification. A technician must find and repair the leak, pull a vacuum, and recharge to factory specifications. Never simply "top off" without fixing the leak — it will be low again within weeks.
Outdoor Unit Covered in Ice
Some frost on the outdoor coil is completely normal — this is how the heat pump absorbs heat from cold outdoor air. The system has a built-in defrost cycle to manage this. However, a solid block of ice encasing the entire unit indicates a problem that must be corrected.
- • Light frost or ice crystals on coil fins
- • Unit defrosts every 30–90 minutes and resumes heating
- • After defrost, steam rises from outdoor unit (normal)
- • Ice on coil only, not on cabinet or fan blade
- • Outdoor temp below 35°F (2°C)
- • Entire unit encased in solid ice
- • Ice on cabinet sides, top, and fan blade
- • No defrost cycle occurring (been iced for hours)
- • Ice building even in mild weather (>40°F / 4°C)
- • Unit still running but zero heat output
Turn the thermostat to FAN ONLY or EMERGENCY HEAT. Do NOT pour hot water over a frozen unit — rapid temperature changes can crack the coil. You can gently pour lukewarm (not hot) water or allow it to thaw naturally. Keep the area around the unit clear of snow drift blockage.
Failed Compressor
The compressor is the heart of the heat pump. A failed compressor means no refrigerant circulation and no heat transfer. Compressors can fail electrically (open, shorted, or grounded windings) or mechanically (seized, worn valves). A compressor that "trips" on thermal overload will restart after cooling but indicates an underlying problem.
- • Outdoor fan runs but no difference in suction/discharge pressures (pressures equalize)
- • Loud grinding, banging, or rattling from outdoor unit
- • Compressor hums but won't start (capacitor or motor winding failure)
- • Tripping the compressor circuit breaker repeatedly
- • Compressor draws locked rotor amps (LRA) continuously
Before condemning the compressor, test the start and run capacitors. A failed run capacitor is one of the most common causes of a compressor that hums but won't start, and costs $15–50 to replace vs. $1,000–3,000 for a compressor. Use a capacitor tester or multimeter in capacitance mode to verify.
Dirty Filters or Coils Reducing Capacity
Restricted airflow is among the most overlooked causes of poor heating. A dirty indoor air filter, clogged evaporator coil, or dirty outdoor coil fins all reduce the system's ability to exchange heat. The system may run fine in mild weather but fall short during cold weather when it needs to work at peak capacity.
Below Balance Point — Aux/Emergency Heat Needed
This is not a malfunction — it is by design. When outdoor temperatures drop below your system's balance point, the heat pump's output becomes insufficient to maintain setpoint. Auxiliary (supplemental) electric resistance heat strips automatically activate to make up the difference.
The balance point is the outdoor temperature at which your heat pump's heating output exactly equals your home's heat loss rate. Below this temperature, the heat pump still runs but cannot maintain the setpoint alone — auxiliary heat must supplement it.
If your thermostat has the auxiliary heat lockout set too high (e.g., locked out below 50°F), your system won't call for aux heat when it needs it, resulting in an underheated home in cold weather. Adjust the lockout temperature in the thermostat installer settings.
Heat Pumps in Cold Weather
Heat pumps move heat rather than generate it, making them 200–400% efficient under normal conditions. However, as outdoor temperature drops, the heat available in the outdoor air decreases, and so does the heat pump's output capacity and efficiency (COP). This is a fundamental thermodynamic limitation, not a malfunction.
| Outdoor Temp | Typical HP Capacity | Approximate COP | Aux Heat? |
|---|---|---|---|
| 47°F (8°C) | 100% (rated) | 3.0–4.0 | Not needed |
| 35°F (2°C) | ~70–80% | 2.0–3.0 | Maybe needed |
| 17°F (-8°C) | ~50–60% | 1.5–2.0 | Usually needed |
| 0°F (-18°C) | ~25–40%* | 1.0–1.5 | Always needed |
*Cold climate inverter heat pumps maintain higher capacity at low temperatures. Standard single-stage heat pumps are not rated below 0°F.
These are different operating modes. Aux Heat runs automatically alongside the heat pump when extra capacity is needed — this is normal and efficient. Emergency Heat shuts off the heat pump entirely and runs only the electric strips — use this only when the heat pump is broken. Running emergency heat long-term is very expensive.
Normal vs. Abnormal Defrost Cycle
Understanding the defrost cycle prevents unnecessary service calls. During a normal defrost, the heat pump temporarily switches to cooling mode to heat the outdoor coil and melt accumulated frost. This is why you may briefly feel cool air from the vents.
Normal Defrost Cycle
- ✓ Occurs every 30–90 minutes in cold weather
- ✓ Lasts 5–15 minutes then returns to heating
- ✓ Outdoor fan stops during defrost
- ✓ Steam or vapor rises from outdoor unit
- ✓ Aux heat strips activate during defrost
- ✓ Supply air feels warm again after defrost ends
- ✓ "Defrost" or "Wait" indicator on thermostat
Abnormal — Service Needed
- ✗ No defrost occurring — unit iced solid for hours
- ✗ Defrost initiates but never terminates (runs 20+ min)
- ✗ Unit defrosts but re-ices within minutes
- ✗ Ice forming even when OAT is above 40°F
- ✗ Outdoor fan continues running during defrost
- ✗ Supply air stays cold even after defrost cycle
- ✗ Ice on cabinet top, sides, and fan guard
DIY Fixes vs. Professional Required
DIY — Safe to Do Yourself
- Replace air filter
- Check and correct thermostat settings and O/B configuration
- Clear snow and debris from outdoor unit (min 18" clearance)
- Reset tripped circuit breaker (once — if it trips again, call a tech)
- Rinse outdoor coil fins with garden hose
- Replace run/start capacitor (with proper safety discharge procedure)
- Replace defrost thermostat sensor
- Replace defrost control board
- Replace reversing valve solenoid coil (not the valve itself)
Professional Required
- Refrigerant leak detection and repair (EPA 608 required)
- Refrigerant recharge (requires recovery, vacuum, recharge equipment)
- Reversing valve replacement (brazing, refrigerant system work)
- Compressor replacement
- Metering device (TXV/EEV) replacement
- Checking and adjusting refrigerant charge with manifold gauges
- Indoor evaporator coil cleaning (requires coil access)
- Electrical diagnosis beyond thermostat wiring
Frequently Asked Questions
The most common reasons a heat pump blows cold air in heat mode are: the reversing valve is stuck in cooling position, the system is in active defrost cycle (which temporarily blows cool air for 5–15 minutes), low refrigerant charge, or the outdoor temperature has dropped below the system's balance point so the compressor alone cannot keep up. Check that the thermostat is set to HEAT and the reversing valve O/B wire configuration is correct for your unit.
A heat pump that runs continuously without adequately heating usually indicates: refrigerant is low, reducing heat transfer capacity; the outdoor unit coil is heavily iced over due to a defrost board failure; the compressor is worn or failed; or outdoor temperatures are below the balance point (typically 30–40°F / -1–4°C) and auxiliary heat is disabled or undersized. Check airflow first — dirty filters and coils dramatically cut heating capacity.
A stuck reversing valve causes the system to operate in the wrong mode — cooling when set to heat, or vice versa. Test: set thermostat to HEAT and feel supply air. If air is cool or cold, check outdoor unit — if the outdoor coil is acting as the condenser (hot) rather than evaporator, the reversing valve is likely stuck. You can also hear/feel the valve shift when switching modes. A stuck valve usually requires professional replacement.
Yes — during the defrost cycle (typically every 30–90 minutes in cold weather) the heat pump temporarily reverses to melt ice off the outdoor coil. This lasts 5–15 minutes and supply air feels cool or lukewarm. The system may also switch on the auxiliary electric strips during defrost. This is completely normal. Abnormal is cold air blowing continuously for more than 20 minutes outside of a defrost cycle.
Most standard heat pumps lose efficiency significantly below 35–40°F (2–4°C) outdoor temperature. Cold-climate heat pumps (e.g., Mitsubishi Hyper Heat, Bosch IDS 2.0) can operate down to -13°F (-25°C). The "balance point" is the outdoor temperature at which the heat pump output exactly matches home heat loss — below this point auxiliary or emergency heat must supplement. Balance points typically fall between 25–40°F (-4 to 4°C) depending on the system and home.
Technically yes, but it is very expensive. Emergency heat mode bypasses the heat pump entirely and runs only the auxiliary electric resistance strips, which are 2–3x less efficient than the heat pump. Your electricity bill can increase 50–200% depending on climate. Emergency heat should only be used temporarily while waiting for repair. Fix the underlying heat pump issue as soon as possible.