A heat pump that runs but does not heat is almost never broken in the way a furnace is broken. A furnace either burns fuel and produces heat, or it does not. A heat pump moves heat — it extracts thermal energy from the outdoor air and transfers it inside. The outdoor air always contains some heat, even at 0 degrees Fahrenheit. The problem is almost never that the heat pump cannot find heat to move. It is that something is preventing the refrigeration cycle from completing: a dirty air filter that has choked airflow to zero, a frozen outdoor coil that has insulated itself from the air it is supposed to extract heat from, a reversing valve stuck in cooling mode, or a refrigerant charge too low to sustain the pressure differential the cycle depends on.
The five DIY checks below take 15 minutes and solve roughly 40% of “heat pump not heating” service calls without a technician. The four professional-level causes below those require an HVAC technician and cost $200 to $2,500 depending on which component has failed. The difference between the two categories is visible in the symptoms if you know what to look for. A heat pump that runs constantly but produces lukewarm air at the register — air that feels cool to your hand but is actually 85 to 95 degrees — may be working correctly and struggling against extreme cold. A heat pump that runs briefly, shuts off, and repeats is short-cycling and likely has a refrigerant or compressor problem. The checklist below maps each symptom to its most likely cause.
5 DIY Checks — 15 Minutes, No Tools, Solves 40% of Calls
1. Thermostat settings. The thermostat must be set to “heat,” not “cool,” not “off,” not “emergency heat.” The fan setting must be “auto,” not “on.” A fan set to “on” runs the blower continuously, including between heating cycles when the indoor coil is cold, blowing unheated air through the registers. The setpoint temperature must be at least 2 degrees above the current room temperature or the system will not call for heat. A programmable thermostat with an incorrect schedule — set to 62 degrees during the day when the house is occupied, will maintain 62 degrees while you wonder why the heat pump is not heating the house to 70. Check the schedule before calling a technician.
2. Air filter. A clogged air filter is the single most common cause of a heat pump that is not heating. The filter restricts airflow across the indoor coil. When airflow drops below the minimum required for heat transfer, the coil temperature drops until ice forms on the coil surface. The ice further restricts airflow. The system eventually shuts down on a safety limit switch. A $5 filter replaced every 1 to 3 months prevents a $200 service call. If the filter is visibly dirty, gray or brown, not white, replace it and run the fan on “on” for 2 hours to defrost the coil before switching back to heat.
3. Outdoor unit clearance. The outdoor unit needs at least 2 feet of clearance on all sides and 4 to 5 feet above the fan discharge. Snow, leaves, grass clippings, or a fence panel leaning against the unit restricts the airflow the system needs to extract heat from the outdoor air. Clear debris from around the unit. If snow has drifted against the coil, brush it off gently, do not chip ice off the coil fins with a sharp tool. Bent fins restrict airflow permanently and require a fin comb to straighten.
4. Circuit breakers. A heat pump has two separate circuits, the indoor air handler and the outdoor unit. Both must have power. Check the electrical panel for a tripped breaker. Reset it once. If it trips again, there is an electrical fault in the system, do not reset it repeatedly. Call a technician. The outdoor unit also has a disconnect switch, a small box mounted on the wall next to the unit with a pull-out handle or a switch. Verify it is in the “on” position. A disconnect that was pulled for service and never pushed back in is the simplest possible cause of a heat pump that will not run.
5. Emergency heat test. Switch the thermostat to “emergency heat.” This bypasses the heat pump entirely and activates the electric resistance heat strips in the air handler. If the emergency heat works and produces warm air from the registers, the problem is in the heat pump itself, the outdoor unit, the refrigerant circuit, or the reversing valve. If the emergency heat does not work, the problem is in the air handler, the thermostat wiring, or the electrical service. The emergency heat test narrows the diagnosis from the entire system to one half of it. This single piece of information saves the technician 30 minutes of diagnostic time and the homeowner a diagnostic fee that measures the wrong thing first. Tell the technician the result of the emergency heat test when you call. They will know exactly where to start.
According to ENERGY STAR, a heat pump moves heat rather than generating it, making it inherently more efficient than electric resistance or gas combustion heating, but that efficiency depends on the refrigeration cycle operating at the correct refrigerant charge and with unrestricted airflow on both the indoor and outdoor coils.
4 Causes That Require a Technician, $200 to $2,500
Low refrigerant charge. A heat pump is a closed system. Refrigerant does not get consumed. If the charge is low, there is a leak. The leak may be in the evaporator coil, the condenser coil, the service valves, or any of the braze joints connecting the copper tubing. A low charge reduces the system’s heating capacity, less refrigerant moving through the cycle means less heat transferred from outdoors to indoors. The compressor runs longer, the discharge air temperature drops, and eventually the low-pressure safety switch cuts power to the compressor to prevent it from running without adequate lubricating oil return. A technician recovers the remaining refrigerant, pressurizes the system with nitrogen to locate the leak, repairs the leak, evacuates the system, and recharges it with the correct weight of refrigerant. The repair costs $500 to $1,500 depending on the location and difficulty of the leak repair. This is not a DIY repair. Refrigerant handling requires EPA Section 608 certification.
Faulty reversing valve. The reversing valve is the component that switches the heat pump between heating and cooling modes by changing the direction of refrigerant flow through the system. In heating mode, the hot gas from the compressor is directed to the indoor coil. In cooling mode, it is directed to the outdoor coil. A reversing valve stuck in cooling mode means the system is air conditioning the house when it should be heating it, the indoor coil is cold and the outdoor coil is hot. The symptom is a heat pump blowing cold air that is unmistakably cold, not lukewarm. Reversing valve replacement costs $500 to $1,200 including labor and refrigerant recovery and recharge. The valve itself costs $100 to $200. The labor to access it, unbraze the copper connections, install the new valve, and recharge the system accounts for the rest.
Failed auxiliary heat strips. When the outdoor temperature drops below the heat pump’s balance point, typically 25 to 35 degrees Fahrenheit depending on the system, the heat pump alone cannot keep up with the building’s heat loss. The auxiliary electric resistance heat strips in the air handler energize to supplement the heat pump’s output. If the heat strips have failed, a burned-out element, a tripped thermal limit switch, a failed sequencer relay, the heat pump runs continuously and the house temperature drops because the heat pump alone cannot overcome the outdoor temperature deficit. Auxiliary heat strip repair costs $200 to $600 for a typical single-element failure. This is a common failure in older heat pump systems where the heat strips have cycled thousands of times over 10 to 15 years.
Failed compressor. A compressor that hums but does not start, or starts and immediately trips the breaker, has either a failed start capacitor, a seized internal mechanism, or shorted motor windings. The start capacitor is a $150 to $300 repair. The compressor itself is a $1,500 to $2,500 repair that is usually not economically justified on a system more than 10 years old. When the compressor fails on an older system, replacement of the entire outdoor unit or the complete heat pump system is typically the better financial decision. A new compressor in a 12-year-old system is a $2,000 repair on a system that may fail again in another component within 2 to 3 years. System replacement costs $4,000 to $8,000 for a mid-range heat pump and delivers a new warranty and 15 to 20 years of service life.
| Cause | Symptoms | DIY Check? | Repair Cost |
| Thermostat settings | No heat, wrong mode | Yes, check heat/auto | $0 |
| Dirty air filter | Low airflow, frozen coil | Yes, replace filter | $5–$20 |
| Outdoor unit blocked | Runs constantly, low heat | Yes, clear debris | $0 |
| Tripped breaker | Unit won’t start at all | Yes, reset once | $0 |
| Low refrigerant charge | Lukewarm air, frozen coil, short cycling | No, EPA certification | $500–$1,500 |
| Faulty reversing valve | Blowing cold air in heat mode | No | $500–$1,200 |
| Failed aux heat strips | Heat pump runs, house still cold below 35°F | Test emerg heat | $200–$600 |
| Failed compressor | Buzz, tripped breaker, no start | No | $1,500–$2,500 |
Frequently Asked Questions
Why does my heat pump blow air that doesn’t feel hot?
This is normal. A heat pump supplies air at 85 to 95 degrees Fahrenheit, which feels cool to human skin (98 degrees) but is warmer than the room air it is heating. A gas furnace supplies air at 120 to 140 degrees and feels unmistakably hot. The heat pump air feels cool not because the system is failing but because the temperature difference between the supply air and your skin is smaller. The heat pump is still heating the house, it just does not feel like it when you put your hand over the register. If the air temperature at the nearest register to the indoor unit is 15 to 20 degrees warmer than the return air temperature, the heat pump is working correctly. If the temperature difference is less than 10 degrees, there is a problem, most likely low refrigerant or a dirty coil.
Why is my outdoor unit covered in ice in winter?
Some frost on the outdoor coil is normal, the coil temperature drops below freezing as the refrigerant extracts heat from the outdoor air, and moisture in the air condenses and freezes on the coil surface. The heat pump periodically enters defrost mode, briefly switching to cooling mode to pump hot gas through the outdoor coil and melt the frost. A complete defrost cycle takes 5 to 10 minutes, during which the indoor blower may run at low speed or shut off. If the outdoor unit is encased in thick ice that does not melt between defrost cycles, the defrost control board or the defrost temperature sensor has likely failed. Call a technician. Do not chip the ice off with a hard tool, the refrigerant lines and coil fins are easily punctured.