If your heat pump is blowing cold air in heat mode, the most common causes are a thermostat setting, a normal defrost cycle, or a dirty filter — all things you can check yourself in 10 minutes. If those aren’t it, the problem is usually the outdoor unit: it’s blocked, iced over, has lost power, or is low on refrigerant. Those need a technician.
Start with these quick checks, in this order:
- Thermostat is set to Heat (not Cool or Auto), the fan is on Auto (not On), and the setpoint is above room temperature.
- Wait 15 minutes. If the outdoor unit is steaming or quiet, it may be in a defrost cycle.
- Pull the air filter. If you can’t see light through it, replace it.
- Go outside. Is the outdoor unit running, and is it free of snow, ice and leaves?
- Check the breaker panel and the outdoor disconnect box for a tripped switch.
First: is the air actually cold?
A heat pump doesn’t make air as hot as a furnace. A gas furnace pushes out air around 120–140°F, while a heat pump typically delivers about 85–100°F, and less on very cold days. Because that’s close to your skin temperature (about 90°F), it can feel cool on your hand even when the system is working fine.
Test it: hold a kitchen thermometer in a supply vent for a few minutes. If the air is at least 15–20°F warmer than the room and the house is slowly warming, your heat pump is likely working normally. If the vent air is within a few degrees of room temperature, read on.
Causes you can usually fix yourself
1. The thermostat is set wrong
Signs: cold air from the moment the system turns on; or air that blows constantly, alternating between warm and cool.
The most common culprits:
- Mode set to Cool or Auto. In Auto, some thermostats switch to cooling if the house briefly overshoots the setpoint.
- Fan set to On instead of Auto. With the fan on, the blower keeps running between heating cycles, pushing room-temperature air through the vents that feels cold.
- Setpoint below room temperature, so the heat never calls.
- New thermostat wired or configured wrong. Heat pumps use a reversing-valve wire (O or B). If it’s set to the wrong one, the system cools when you ask for heat and heats when you ask for cool.
Fix: set mode to Heat, fan to Auto, and the setpoint 3–4°F above room temperature. If you recently installed a new thermostat, check its setup menu for “heat pump” and “O/B reversing valve” settings, and try switching O to B (or vice versa). If the system then heats in Heat mode, that was the problem.
2. The heat pump is in a defrost cycle (normal)
Signs: cool air for 5–15 minutes, then warm air returns; the outdoor fan stops; steam or a whooshing sound comes from the outdoor unit. Most common when it’s between about 20°F and 40°F and humid.
In winter, frost builds up on the outdoor coil. To melt it, the heat pump briefly reverses into cooling mode, which can send cooler air indoors. Most systems turn on backup heat during defrost to prevent this, but not all do.
Fix: nothing — this is normal. Wait it out. It becomes a problem only if defrost happens much more than once an hour, or the outdoor unit stays caked in ice (see cause 5).
3. It’s lukewarm air, not cold air
Signs: the house warms slowly, the vent air feels cool to your hand but measures 85°F or more, and it’s especially noticeable on very cold days.
As it gets colder outside, an air-source heat pump has less heat to pull from the air, so supply air gets cooler and run times get longer. Standard (non-cold-climate) models lose a lot of capacity below about 25–30°F.
Fix: if the house holds its temperature, nothing is wrong. Avoid big setbacks at night — heat pumps recover slowly and may trigger expensive backup heat. Close curtains at night, and check that supply vents aren’t blocked by furniture or rugs. If this happens every cold snap and the house can’t keep up, your system may be undersized for your climate.
4. A dirty air filter
Signs: weak airflow from vents, a system that runs constantly, rising energy bills, or ice forming on the indoor coil or refrigerant lines.
A clogged filter starves the indoor coil of airflow, so the system can’t move heat into the house properly. In severe cases it can trip high-pressure safety switches or damage the compressor.
Fix: replace it. Most 1-inch filters need changing every 1–3 months during heating season; 4–5-inch media filters every 6–12 months. Use the MERV rating your system’s manual recommends (usually MERV 8–11). Very high-MERV filters can restrict airflow on some systems.
Outdoor-unit causes
5. The outdoor unit is blocked or iced over
Signs: a solid shell of ice on the coil and fan that doesn’t clear after a defrost cycle; snow drifts, leaves or debris packed around the unit; water dripping from a gutter onto it.
A light layer of frost is normal. A thick block of ice is not — it smothers the coil so the unit can’t absorb heat from outside air.
Fix:
- Turn the system off at the thermostat first.
- Clear snow and leaves to at least 18–24 inches around the unit.
- Melt ice with warm (not boiling) water from a watering can or garden hose. Never chip ice with a screwdriver, shovel or ice pick — you can puncture the coil and cause a refrigerant leak.
- Fix the cause: redirect gutters dripping onto the unit, and raise the unit on a stand or riser if it sits in snow.
- If it ices up again within a day, the defrost control, a sensor, or low refrigerant is likely to blame. Call a technician.
6. The outdoor unit has no power
Signs: the indoor blower runs but the outdoor unit is silent — no fan, no compressor hum.
With the outdoor unit off, the indoor fan simply circulates room-temperature air (or the system falls back to backup heat only).
Fix: check the breaker panel for a tripped double-pole breaker labeled for the heat pump or condenser, and check the disconnect box mounted near the outdoor unit. Reset a tripped breaker once. If it trips again, stop and call a technician; repeated tripping points to an electrical fault. If power is on but the unit hums without starting, a failed capacitor or contactor is the likely cause — a common, relatively cheap repair, but one for a pro because capacitors can hold a dangerous charge.
7. Low refrigerant (a leak)
Signs: air that’s only slightly warm in heat mode and slightly cool in AC mode; frequent icing; a hissing or bubbling sound; ice on the copper refrigerant lines; steadily rising bills.
Heat pumps are sealed systems, so low refrigerant always means a leak. Without enough refrigerant, the system can’t carry enough heat indoors.
Fix: this needs a licensed technician. They’ll find and repair the leak, then recharge the system. Topping up without fixing the leak just postpones the problem. Note that older systems use R-410A, which is getting more expensive as production winds down; newer 2025+ systems use R-454B or R-32.
8. A stuck or failed reversing valve
Signs: the system blows cold air in heat mode and works fine in cooling mode (or vice versa), even after the thermostat settings are confirmed correct.
The reversing valve switches the direction refrigerant flows, which is what lets one machine both heat and cool. If its solenoid fails or the valve sticks, the system can get stuck in cooling mode.
Fix: a technician will test the solenoid coil first (a cheap fix) before replacing the valve itself (an expensive one). Sometimes a stuck valve frees up after cycling the system between heat and cool a few times, but if it recurs, have it repaired.
9. Backup heat isn’t working in very cold weather
Signs: the system was fine until a cold snap, then started blowing cool air; the thermostat shows “Aux heat” or “Emergency heat” but the air stays lukewarm.
Many heat pumps, especially older or non-cold-climate models, depend on electric heat strips (or a furnace in dual-fuel systems) when it drops below about 25–35°F. If a heat strip element, its breaker, or its relay fails, the heat pump alone can’t keep up.
Fix: check the separate breaker(s) for the air handler’s heat strips. In dual-fuel systems, make sure the furnace is getting gas and power. If the breakers are fine, call a technician to test the heat strips and sequencers.
When to call a technician
Call a licensed HVAC technician if:
- You’ve checked the thermostat, filter, breakers and outdoor unit, and the air is still cold
- A breaker trips more than once
- The outdoor unit hums but won’t start, or makes grinding, screeching or loud clanking noises
- Ice keeps coming back within a day of clearing it
- You hear hissing or see oily residue on the refrigerant lines
- You smell burning or electrical odors (turn the system off right away)
While you wait, switch the thermostat to Emergency Heat (“EM Heat”) if you have it. This runs your backup heat only. It’s more expensive to run but keeps the house warm and protects the heat pump from further damage.
Typical repair costs
Approximate 2026 US prices, including parts and labor. Expect higher prices for nights, weekends and holidays.
| Repair | Typical cost |
|---|---|
| Service call / diagnostic visit | $100–$250 |
| Capacitor replacement | $150–$450 |
| Contactor replacement | $150–$400 |
| Defrost control board or sensor | $250–$700 |
| Reversing valve solenoid coil | $150–$450 |
| Reversing valve replacement | $1,000–$2,500 |
| Refrigerant leak repair + recharge | $400–$1,500+ |
| Heat strip element or sequencer | $200–$700 |
| Compressor replacement | $2,000–$4,500 |
Repair or replace? If your heat pump is over 12–15 years old and the repair is a major one (compressor, reversing valve or a large refrigerant leak), compare the repair price with a new system. A common rule of thumb: if the repair cost times the system’s age is more than about $5,000, replacement is usually the better value. See our guide to heat pump installation costs to compare.
How to keep it from happening again
- Change the filter on schedule. Set a phone reminder for every 1–3 months in winter.
- Keep 2 feet of clearance around the outdoor unit, and clear snow after storms.
- Raise the unit on a riser or wall bracket if you get regular snow, and keep gutters from dripping onto it.
- Get a tune-up every fall. A technician checks refrigerant levels, electrical parts, the defrost control and the reversing valve before the cold arrives. Most cost $100–$200.
- Skip big thermostat setbacks. Drops of more than 2–3°F at night make the system run its backup heat to recover in the morning.
- Keep supply and return vents open and unblocked, even in unused rooms.
- Use a thermostat built for heat pumps, so it controls auxiliary heat and defrost properly.
Frequently asked questions
Why does my heat pump blow cold air for a few minutes then warm up?
That’s almost always a normal defrost cycle. The system briefly reverses to melt frost from the outdoor coil, then returns to heating. Cycles typically last 5–15 minutes.
Why is my heat pump blowing cold air when it’s below freezing?
It’s either lukewarm air that feels cool (normal as outdoor temperatures drop), a defrost cycle, or backup heat that isn’t kicking in. If vent air is close to room temperature for more than 20 minutes, check the outdoor unit for ice and the breakers for the heat strips.
Should I turn on emergency heat if my heat pump is blowing cold air?
Use emergency heat only when the heat pump itself is broken or iced over and you’re waiting for a repair. It’s reliable but costs two to three times more to run, so switch back to normal Heat once the system is fixed.
Can low refrigerant make a heat pump blow cold air?
Yes. Low refrigerant means a leak, and the system can’t move enough heat indoors. You’ll often see ice on the coil or refrigerant lines too. This requires a licensed technician.
How long should a heat pump run in winter?
Heat pumps run longer than furnaces by design, and on very cold days it’s normal for them to run almost continuously. Constant running is a problem only if the house can’t hold its temperature or bills spike suddenly.
How do I reset my heat pump?
Turn the thermostat off, switch off the breakers for both the indoor and outdoor units, wait 5 minutes, then turn the breakers back on and set the thermostat to Heat. Some systems need up to 5 minutes before the compressor restarts. If the problem returns, a reset won’t fix it; call a technician.
Related guides
- Heat Pump Frozen Outside: How to Defrost It Safely
- Heat Pump Defrost Cycle: What Is Normal and What Is Not
- Emergency Heat on a Heat Pump: When to Use It and What It Costs
- Heat Pump Short Cycling: Causes, Risks and Fixes
- How Often to Change Heat Pump Filters (and Which MERV Rating)
- Repair or Replace a 12-Year-Old Heat Pump? Cost Breakdown and Rule of Thumb
This guide is general information. Turn off power before inspecting equipment, and leave electrical, refrigerant and internal component work to a licensed HVAC technician. Repair prices are approximate 2026 national ranges.