A heat pump that seems to blow cold air on a winter morning is often doing something normal: running a defrost cycle, or putting out air that’s warm but not as hot as a furnace’s. Sometimes it’s a real fault, and the thermostat usually tells you which.
Quick starting point: Steam from the outdoor unit and a fan that stops for a few minutes are normal. If the thermostat shows AUX and the vents blow cool air for longer than 15 minutes, or ice covers the outdoor unit and never clears, the heat pump needs a repair.
Why Is My Heat Pump Blowing Cold Air?
A heat pump blowing cool air in winter is in one of three situations: it’s in a defrost cycle, it’s working normally but its air feels cooler than a furnace’s, or something has failed. Defrost lasts a few minutes, and then warm air returns. Normal heat pump air leaves the vents at roughly 90 to 110 degrees, against about 150 off a gas furnace, so it can feel lukewarm on your hand even while the house is warming. If the air stays cool and the indoor temperature keeps falling for longer than a defrost cycle, the heat pump isn’t heating. Backup heat may be covering for it, or that may have failed too.
How Do I Know If My Heat Pump Is in Defrost Mode?
Steam rising from the outdoor unit, an outdoor fan that stops while the unit keeps running, and a whoosh when the system switches over are the signs of defrost mode. In cold, damp weather, frost builds on the outdoor coil. To melt it, the heat pump briefly runs in reverse, sends warm refrigerant to the outdoor coil, and switches back to heating once the coil is clear. A defrost cycle usually lasts 5 to 15 minutes, and most systems check for frost every 30 to 90 minutes. Charlotte’s damp winter mornings near freezing are prime defrost weather, and defrost can start with the air in the 40s, because the coil runs colder than the air around it. Many systems bring on AUX heat during defrost so the vents don’t blow cold, so a few minutes of AUX during defrost isn’t a problem by itself.
Is It Defrost or a Real No-Heat Problem?
Defrost is short and repeats on a pattern. A real no-heat problem doesn’t clear on its own.
| What you notice | Usually means | What to do |
|---|---|---|
| Steam from the outdoor unit for a few minutes | Normal defrost | Nothing |
| AUX for a few minutes on a cold, damp day | Defrost or a cold snap | Nothing |
| AUX on and cool air at the vents | A failed part in the heat pump, backup heat or controls | Call for repair |
| Ice covering the coil or the whole unit, not clearing | Defrost, airflow or refrigerant problem | Switch to EM heat and call |
| Outdoor fan not running, unit humming | Failed fan motor or capacitor | Switch to EM heat and call |
| Breaker trips again after one reset | Electrical fault | Leave it off and call |
| Burning smell or grinding noise | Motor or electrical failure | Turn the system off and call |
AUX plus cool air means something has failed, but not always the outdoor unit. In January 2025, a customer’s thermostats showed AUX heat on the coldest days with only cool air coming out. Our technician traced it to a failed zone control panel. In the customer’s words, “Once the panel was replaced, I was back with real heat,” adding that “that AUX heat cost me big time.”
What’s the Difference Between AUX Heat and Emergency Heat?
AUX heat comes on automatically to help a working heat pump, while emergency heat (EM HEAT) is a setting you choose when the heat pump itself has failed. Both use the same backup source, usually electric heat strips in the indoor air handler. With AUX, the thermostat adds the strips during defrost, on the coldest mornings, or when you raise the setting several degrees at once, then shuts them off when the heat pump catches up. With EM heat, the outdoor unit is locked out and the strips heat the whole house on their own. Leave the thermostat on HEAT, and switch to EM heat only when the outdoor unit is broken, iced over or making a bad noise, and switch back once it’s repaired.
| Setting | What runs | When to use it |
|---|---|---|
| HEAT | Heat pump, with AUX added automatically | All winter |
| EM HEAT | Backup heat strips only | Heat pump broken, until repaired |
In a dual-fuel home, the backup is a gas furnace instead of heat strips, and the same rules apply.
Why Does Emergency Heat Cost So Much to Run?
Emergency heat costs more because heat strips make heat from electricity, while a heat pump moves heat that’s already in the outdoor air. A heat strip turns each kilowatt-hour into one kilowatt-hour of heat and no more. A working heat pump delivers more heat than the electricity it uses, even on a cold morning. A house running on EM heat, or leaning on AUX because something has failed, uses noticeably more power, and it shows up on the next bill. A day or two on EM heat while you wait for a repair is fine. Leaving the thermostat there for weeks is the expensive mistake.
What Temperature Do Heat Pumps Stop Working?
Heat pumps don’t stop at a set temperature. They keep heating below freezing, but they gather less heat as the air gets colder, and the thermostat brings on AUX to make up the difference. On the coldest mornings it’s normal to see AUX more often and the heat pump running longer. Cold-climate models go further: Mitsubishi rates its Hyper-Heating (H2i) systems at 100% of rated capacity down to 5°F. Charlotte rarely gets near that. Our winters spend very few hours below the low 20s, so a correctly sized heat pump carries most of the season on its own. That’s why heavy AUX use on a mild day points to a problem rather than the weather.
Is It Normal for a Heat Pump to Run Constantly?
In cold weather, yes. A heat pump runs long, steady cycles by design, and on a morning in the 20s it may run for hours without stopping while the house holds its temperature. Running constantly is a problem when it’s mild out and the house still won’t reach the setting, or when the system starts and stops every few minutes. That stop-start pattern is short cycling. Common causes are a clogged filter, a thermostat mounted near a supply vent or a sunny window, or a refrigerant or control problem. Check the filter first, and call if that doesn’t fix it.
What Should You Check Before Calling?
Five checks clear up a good share of heat pump no-heat problems:
- Thermostat. Set it to HEAT, a few degrees above the room temperature. Avoid big jumps, which bring on AUX.
- Air filter. Hold it up to a light. If you can’t see light through it, replace it.
- Breakers. A heat pump usually has two, one for the outdoor unit and one for the indoor air handler. Reset a tripped breaker once. If it trips again, leave it off and call.
- Outdoor unit. Clear leaves, mulch and snow from around it. Don’t chip ice off the coil, because the fins and tubing puncture easily.
- Time. If you see steam or the outdoor fan has stopped, give it 15 minutes to finish defrosting before you turn anything off.
If it still isn’t heating, switch the thermostat to EM heat to keep the house warm and call for heat pump repair.
How Andersen Handles Heat Pump Repair in Charlotte
Andersen Heating & Cooling repairs heat pumps, air handlers and thermostats across the Charlotte area seven days a week from 7:30 a.m. to midnight, with no weekend or after-hours upcharge. We’re not a 24-hour service, so a no-heat call after midnight gets the first slot the next morning, and EM heat will keep the house warm until then. You get the price before any work starts. If a heat pump past 10 to 15 years needs a compressor or reversing valve, we’ll price the repair against heat pump replacement so you can decide.
Members of our HVAC maintenance program take 15% off any repair, and a fall tune-up checks airflow, refrigerant charge and defrost operation before the cold arrives. Andersen holds North Carolina HVAC license L.34538 and has served Charlotte homeowners since 1984.
Frequently Asked Questions
Why is there ice on my heat pump?
A light coat of frost on the outdoor coil is normal in damp winter weather, and the next defrost cycle clears it. Ice covering the whole coil or the top of the unit, or still there after a couple of defrost cycles, means defrost isn’t working or airflow is blocked. Switch to EM heat and call.
Should I switch to emergency heat when it’s cold?
No. Andersen Heating & Cooling recommends leaving the thermostat on HEAT and letting it bring on AUX when needed. EM heat shuts the heat pump off and costs more to run. Use it only when the heat pump has failed.
What if my heat pump stops working after midnight?
A call to Andersen Heating & Cooling after midnight gets the first slot the next morning. Overnight, switch the thermostat to EM heat if the backup heat works, keep doors to unused rooms closed, and use space heaters only as the manufacturer directs. Never run a generator, grill or camp heater indoors.
Get Your Heat Pump Heating Again
If you’ve worked through the checks and the heat pump still isn’t heating, call Andersen Heating & Cooling at (704) 596-2525 or request service online. Our furnace and heating repair page covers what a repair visit includes.