If your heat pump is not heating, first confirm the thermostat is set to Heat, then check for a clogged air filter, an iced-over outdoor coil, or a defrost cycle that never ends. A brief pause with steam every 30 to 90 minutes in cold, damp weather is normal. A coil frozen solid is not.
Heat pumps behave differently from a furnace, and some of that behavior looks alarming when it is actually normal. The outdoor unit will steam, pause, and blow cooler air during defrost. The trick is separating that from a real fault. Work through the symptoms below before you call for service, because several of the common causes are things you can check or fix yourself.
Start here: fault or normal defrost?
In cold, damp conditions a heat pump periodically reverses itself for a few minutes to melt frost off the outdoor coil. During that window you will see steam, hear a whoosh as the valve switches, and feel cooler air indoors. A cycle of roughly 5 to 15 minutes, ending on its own, is expected. If the unit never leaves defrost, or the coil is buried in solid ice for hours, that is a fault.
Symptom, likely cause, and what to check
| Symptom | Likely cause | What to check first |
|---|
| Outdoor unit steams and pauses for a few minutes, then resumes heating | Normal defrost cycle | Time it. A short cycle every 30 to 90 minutes in cold, damp weather is expected. |
| Outdoor coil stays encased in solid ice for hours | Stuck defrost, failed sensor, or blocked drainage | Whether the fan and valve switch during defrost. Clear snow or leaves off the coil. |
| Weak airflow, rooms never reach setpoint | Dirty filter or blocked return air | Filter condition. Furniture, rugs, or closed vents choking supply and return. |
| Air from the vents is only lukewarm | Compressor not fully engaging | Thermostat mode, backup-heat lockout, possible low charge. |
| Only cold air, breaker recently tripped or reset | Wrong mode or thermostat fault | Thermostat set to Heat, not Cool or off. Reset a tripped breaker once. |
| Long runtimes, high bills, weak heat in deep cold | Low refrigerant or undersized system | Recent charge or leak history. Ice patterns. Whether backup heat is locked out. |
| Blows cold in Heat mode but warm in Cool mode | Reversing valve stuck | Professional diagnosis. Do not attempt a DIY fix on the sealed system. |
Normal defrost vs stuck in defrost
Normal defrost is short and self-ending, and heating resumes after it. Stuck-in-defrost means the unit reverses over and over, or sits in defrost so long the house cools down. Causes include a failed defrost sensor or control board, a low refrigerant charge that confuses the cycle, or an outdoor fan that is not running. Clearing snow off the coil is safe to try. A repeating or endless cycle needs a technician.
Iced-over outdoor coil
A light frost on the coil is normal in winter. A thick block of ice that survives the defrost cycle is not. Start with airflow: make sure nothing is stacked against the unit and that snow, leaves, or ice from a roof drip line is not smothering it. You can melt ice off gently with cool to lukewarm water. Never use boiling water, which can crack components, and never chip at ice with a tool. If the coil re-ices quickly after you clear it, the unit has a real fault, often low charge or a defrost failure.
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Dirty filter or blocked airflow
This is the single most common and most fixable cause of weak heat. A clogged filter starves the indoor coil of air, which cuts output and can trigger nuisance shutdowns. Check the filter and replace it if it is gray or matted. Then confirm return grilles are clear and that supply registers are not closed or blocked by furniture. Restricted airflow also makes the outdoor coil ice up faster, so a filter change sometimes fixes an apparent defrost problem too.
Thermostat set to the wrong mode
Confirm the thermostat is in Heat, not Cool, Auto, Off, or an Emergency Heat setting left on from a previous outage. On heat pump thermostats, Auto can call for cooling on a mild afternoon and leave the house cold at night. If the screen is blank, replace the batteries. Set the target several degrees above room temperature and wait a few minutes to confirm the system starts a call for heat.
Low refrigerant
Low charge, usually from a slow leak, shows up as weak heat, long runtimes, higher bills, and an outdoor coil that ices unusually. Refrigerant is not consumed, so a system that is low has a leak that needs finding and sealing. This is not a DIY or top-off job. It requires a licensed technician, both for the equipment and because handling refrigerant is regulated.
Aux or backup-heat lockout
Many systems lock out electric backup or auxiliary heat above a set outdoor temperature to save money, and some lock out the compressor below a set temperature to protect it. If the thermostat installer settings hold aux heat off during a cold snap, or force the system onto backup and lock out the heat pump, you can get weak or expensive heat. If you did not set these thresholds and the house cannot keep up, have the installer review the lockout settings.
Reversing valve
The reversing valve is what lets a heat pump switch between heating and cooling. If it sticks, the unit can blow cold air in Heat mode while still cooling normally in summer, or the reverse. That specific pattern points at the valve or its solenoid. It is a sealed-system component, so confirm the symptom, then book a technician rather than attempting a repair.
What you can check safely, and what needs a pro
Safe to do yourself: confirm the thermostat mode and batteries, replace the filter, clear snow and debris off the outdoor unit, gently melt coil ice with lukewarm water, and reset a tripped breaker one time. Call a professional for: repeating or endless defrost, a coil that re-ices after clearing, suspected low refrigerant, reversing-valve symptoms, a breaker that trips again, or any burning smell or repeated electrical fault.
Frequently Asked Questions
Why is my heat pump blowing cold air?
Usually one of three things: it is mid-defrost and will resume shortly, the thermostat is set to Cool or Auto, or the compressor is not engaging because of low charge or a stuck reversing valve. Check the mode and time the cold-air period first, since a normal defrost ends on its own within about 15 minutes.
How often should a heat pump defrost?
It varies with cold and humidity, not a fixed clock. In cold, damp weather a cycle every 30 to 90 minutes is common, and each one runs a few minutes. Constant defrosting, or cycles that never end, points to a sensor, control, airflow, or refrigerant problem worth a service call.
Should I pour hot water on a frozen heat pump?
Use cool to lukewarm water to melt ice gently. Never use boiling water, which can crack coils or plastic, and never chip ice off with a tool. Clearing ice once is fine, but if the coil freezes solid again soon after, the unit has a fault that needs a technician.
Why does my heat pump work in summer but not winter?
Cooling and heating share the same equipment but run the refrigerant in opposite directions through the reversing valve. If cooling is fine but heating is weak or cold, suspect a stuck reversing valve, backup heat locked out, low charge, or simply capacity derating in deep cold that leaves the system unable to keep up without backup.
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