Your heat pump is running. The thermostat says HEAT. But the air coming out of the vents feels cool — maybe just lukewarm — and the house isn't warming up. So you're wondering if the system is broken.
It probably isn't. Heat pumps are responsible for more unnecessary service calls in winter than almost any other HVAC equipment, because they behave differently from gas furnaces in ways that look like malfunctions but aren't. Before you call for service, spend 10 minutes with this guide.
Why Heat Pumps Feel Like They're Not Heating (When They Are)
The single biggest source of "my heat pump isn't working" calls is a misunderstanding about supply air temperature.
A gas furnace heats air to around 120–140°F before pushing it through your vents. Put your hand in front of a furnace supply register and the air feels hot — noticeably, obviously hot.
A heat pump delivers air at 85–95°F. Put your hand in front of the same register and the air feels warm — sometimes barely warm on a very cold day. That's not a malfunction. That's how heat pumps work.
The physics: a heat pump doesn't burn fuel to create heat. It moves heat from the outdoor air into your home using refrigerant. The efficiency of that process depends on how large the temperature differential is between outside and inside. The colder it is outside, the harder the system works to extract heat, and the cooler the supply air feels. The house still warms up — it just does it gradually, not in blasts.
If you upgraded from a furnace to a heat pump and the supply air feels different, that's expected. See our heat pump vs. furnace comparison for more detail on how these systems differ in feel and cost to operate.
Normal Heat Pump Behavior That Looks Like a Problem
These four things alarm a lot of homeowners. All four are normal.
The Outdoor Unit Is Steaming
On cold, humid days, the outdoor unit periodically enters a defrost cycle — typically lasting 5 to 15 minutes. During defrost, the system temporarily reverses refrigerant flow to melt frost off the outdoor coil. You'll see steam rising from the unit and feel the supply air get cooler or stop briefly. Some systems slow or pause the indoor fan during this period.
When defrost ends, the unit switches back to heating mode and the supply air resumes.
This is normal. How often it happens depends on outdoor temperature and humidity. In damp, cold conditions — 30s and 40s°F with high humidity — defrost might cycle once every 60 to 90 minutes. If it's running every 20 to 30 minutes, that's worth a tech visit. Once or twice an hour on a wet winter day is within spec.
The Auxiliary Heat Light Is On
Most heat pump thermostats have an "AUX HEAT" indicator. When it lights up, backup heating elements (or a backup furnace in a dual-fuel system) are supplementing or replacing the heat pump.
This kicks in automatically when outdoor temperatures drop below what the heat pump can efficiently handle on its own — a threshold called the balance point, typically somewhere between 25°F and 40°F depending on your system and home. The thermostat manages this automatically. AUX HEAT during a cold snap is expected behavior, not a sign of failure.
If AUX HEAT is running constantly and it's 45°F outside, that's a different story — something isn't right.
The System Pauses and Restarts
During defrost or when switching between heating stages, a heat pump may pause for 30 to 60 seconds. This is a normal part of compressor protection logic. Systems have a short delay built in to prevent rapid short-cycling, which damages the compressor.
The House Warms Up Slowly
This is the furnace comparison problem again. A gas furnace blasts 130°F air and raises room temperature quickly. A heat pump delivers 90°F air steadily over a longer period. The total heat output can be similar, but the pattern feels different.
On days when you've turned the heat up by 5 or more degrees, give the system 45 to 60 minutes before concluding it isn't working.
DIY Checks Before You Call a Tech
If you've confirmed the above are normal and the system still isn't maintaining temperature, work through these before calling for service.
1. Thermostat Settings
Check that the system is set to HEAT (not COOL, not FAN ONLY). If you share a thermostat with another household member, it's not unheard of for someone to have switched it. Also confirm the set temperature is meaningfully above current room temperature — if the room is 67°F and the thermostat is set to 68°F, the system may not call for heat at all.
Make sure the fan is on AUTO, not ON. The ON setting runs the fan continuously regardless of whether heating is active, which pushes room-temperature air through the vents constantly — the definition of "blowing cool air."
2. The Outdoor Unit — Snow, Ice, and Debris
Go outside and look at the outdoor unit. In winter, the most common physical problems are:
- Snow and ice accumulation: An inch of snow on top of the unit is fine. A unit buried under drifted snow or encased in thick ice is not — the fan can't exhaust air and the system will struggle or shut down on a safety cutout. Brush snow off the top and sides. If the unit is iced up beyond what a defrost cycle can handle, that needs a tech.
- Blocked airflow: The unit needs 18–24 inches of clearance on all sides. Winter debris, stacked firewood, or holiday storage nearby will reduce efficiency.
- Unobstructed fan: The fan on top of the unit should spin when the system is running in heating mode. If it's not, the system has likely shut off on a safety.
Don't pour hot water on an iced-over unit. Let the defrost cycle handle it, or if ice buildup is severe, call for service.
3. Air Filter
A clogged air filter restricts airflow across the indoor coil and reduces heating capacity. If you can't remember the last time you replaced it, check it now. Hold it up to light — if you can't see through it, replace it. Standard 1-inch filters should go every 30 to 90 days. This is a $5–$25 fix that takes two minutes and is the cause behind a lot of "heat pump not working" calls.
4. Circuit Breakers
Heat pump systems use two breakers — one for the outdoor unit and one for the indoor air handler. Both can trip independently. Check your panel for any breaker that isn't fully in the ON position. Flip tripped breakers fully to OFF, then back to ON.
If a breaker trips again within a few hours, don't keep resetting it. A breaker that trips repeatedly under load indicates an electrical problem that needs diagnosis.
5. Emergency Heat Mode Setting
Check your thermostat. If it's been switched to EMERGENCY HEAT mode, the heat pump compressor is fully bypassed and you're running on backup heat strips only. That's expensive (2–5x the operating cost of the heat pump itself) and wasn't designed for extended use.
Emergency Heat mode should be set only when the heat pump itself is confirmed broken and you're waiting for service. If it got switched accidentally, switch it back to normal HEAT mode.
When Emergency Heat Is and Isn't Appropriate
Normal operation: The heat pump runs as the primary heating source. AUX HEAT supplements automatically at low temperatures. You never need to manually select emergency mode.
Appropriate emergency heat use: You've confirmed the outdoor unit isn't running (compressor failure, refrigerant leak, reversing valve stuck in cooling mode). The house needs heat while you wait for a technician. Turn on Emergency Heat temporarily so the backup strips maintain temperature.
Inappropriate: Switching to Emergency Heat because the system "doesn't seem to be heating well." If the AUX HEAT light is on during cold weather, that's normal. Running on full emergency mode for weeks because the heat pump seems slow is burning money unnecessarily — electric resistance strips use 2 to 5 times more electricity than the heat pump.
If you feel the heat pump isn't keeping up and conditions aren't extreme, that's worth a service call to diagnose — not a reason to switch to emergency mode long-term.
Actual Problems That Need a Technician
These are the issues where DIY ends and professional diagnosis begins.
Refrigerant leak: Low refrigerant from a leak causes the system to lose heating capacity progressively. Signs include the outdoor unit running but the indoor temperature never catching up, ice forming on the refrigerant lines, and a gradual decline over weeks or months rather than a sudden failure. Refrigerant requires EPA 608 certification to handle — this isn't a DIY repair.
Stuck reversing valve: The reversing valve is what allows a heat pump to switch between heating and cooling modes. If it sticks in the cooling position, the system blows cool air in heating mode — but the compressor runs fine. This is a common misdiagnosis ("I think my heat pump is running but blowing cold air"). The fix is valve replacement. Cost: $450–$1,200 depending on parts and labor rates in your area.
Failed defrost control board: If the defrost cycle never initiates, the outdoor coil ices over completely and heating capacity drops to zero. Symptoms: outdoor unit with excessive ice buildup that doesn't clear even on mild days, declining output, system shutting down. Defrost board replacement runs $200–$500 installed.
Compressor issues: The compressor is the heart of the system. A failing compressor may run intermittently, make grinding or clanking noises, or draw excessive current and trip the breaker. Compressor replacement is the most expensive single heat pump repair at $800–$2,800, and on older systems, it often makes more financial sense to replace the system than the compressor.
Blower motor failure: If the indoor fan isn't running, no heated air gets distributed regardless of what the heat pump is doing. You'll hear the outdoor unit running but feel no airflow from the vents. Blower motor replacement: $300–$700.
For any of these, turn the system off at the thermostat before calling for service. Running a heat pump with a refrigerant leak or compressor problem accelerates damage and makes repairs more expensive. See our HVAC repair cost guide for a full breakdown on what common repairs run by region.
Cold-Climate Heat Pumps: What's Possible Now
Standard heat pumps were designed with a practical lower limit around 20–25°F — below that, efficiency dropped sharply and backup heat carried most of the load. That ceiling has moved significantly in the past decade.
Current cold-climate models from Mitsubishi (H2i Hyper-Heating), Daikin, and Carrier operate down to -13°F and in some configurations to -22°F. Mitsubishi's H2i units are notable for maintaining 100% of rated heating capacity at 5°F — a threshold where older equipment was already struggling.
This matters if you're in a climate where temperatures routinely drop below 20°F and your current heat pump is leaning heavily on aux heat all winter. A cold-climate replacement unit may reduce backup heat usage substantially, which cuts operating costs.
If you're considering an upgrade, federal tax credits and utility rebates currently apply to qualifying heat pumps — check our HVAC rebates guide for 2026 for what's available before you start getting quotes.
What to Tell the Technician
When you do call for service, give them specifics:
- System brand, model, and approximate age (labels on outdoor unit and air handler)
- Outdoor temperature range during the past few days
- What you've already checked (thermostat settings, filter, breakers, outdoor unit)
- Whether the AUX HEAT or EMERGENCY HEAT light is on
- Whether the outdoor unit is running (fan spinning, compressor humming)
- Any unusual sounds — grinding, hissing, clicking, clanking
The more specific you are, the faster diagnosis goes. A tech who knows the outdoor unit is running but the indoor unit isn't distributing air will bring different parts than one who arrives cold.
To find a licensed HVAC technician in your area, use HVACJobs.IO's technician finder or read our guide on how to evaluate HVAC contractors before booking.
Frequently Asked Questions
Why is my heat pump blowing cool air in winter?
Usually one of three things: the system is in defrost mode (temporary, resolves in 5–15 minutes), the fan is set to ON instead of AUTO (running continuously regardless of heating cycle), or the reversing valve is stuck in cooling position (needs a technician). Start by checking the thermostat fan setting.
Is it normal for a heat pump to run constantly in cold weather?
Yes. Unlike a furnace that cycles on hard and off quickly, heat pumps are designed to run at lower capacity for longer periods. In cold weather, your heat pump may run almost continuously — that's efficiency by design, not a problem. What's not normal is running continuously and still losing ground on indoor temperature.
At what temperature does a heat pump stop working?
Standard heat pumps lose significant efficiency below 25–30°F and may not keep up without backup heat. Cold-climate models (Mitsubishi Hyper-Heating, Daikin cold-climate, Carrier Greenspeed) are rated to -13°F and below. The specific rating is on your equipment's data plate or installation manual.
Should I switch to emergency heat when it's very cold?
Not unless the heat pump itself has confirmed failed. Emergency Heat bypasses the compressor entirely and runs on electric resistance strips, which cost 2–5 times more to operate. The AUX HEAT light during cold weather is normal — that's the system supplementing the heat pump automatically. Emergency Heat is a repair-period fallback, not a routine cold-weather setting.
Why does my heat pump outdoor unit have ice on it?
Light frost on the coil fins is normal and is cleared by the automatic defrost cycle. A unit encased in thick ice that doesn't clear after several hours indicates a defrost system problem — failed defrost sensor, board, or a refrigerant issue causing abnormally cold coil temperatures. That needs a technician.
How long do heat pump defrost cycles last?
Typically 5 to 15 minutes. During defrost, the system reverses refrigerant flow to melt the coil frost, the outdoor fan stops, and you may see steam. The indoor fan may slow or pause. When defrost ends, normal heating resumes automatically. If defrost cycles seem to run longer than 20 minutes or the unit never fully clears, the defrost control board may need replacement.
What does a heat pump reversing valve replacement cost?
Expect $450 to $1,200 depending on your equipment brand, the local labor rate, and refrigerant handling involved. Get two quotes — this repair varies considerably by region and contractor.
When does it make more sense to replace a heat pump than repair it?
The standard guidance: if the system is more than 12–15 years old and the repair exceeds 50% of replacement cost, replacement usually wins financially — especially given the efficiency improvements in current equipment and available tax credits. A compressor replacement on a 14-year-old system is a case where that math almost always favors replacement.