Heat Pump Acting Strange in Winter? Defrost, AUX & EM Heat Explained
Steam pouring off the outdoor unit. Cool air from the vents. An "AUX HEAT" light you've never noticed. Before you panic — almost all of it is normal. Learn the defrost cycle, the difference between auxiliary and emergency heat, and the warning signs that really do mean call a pro.
Quick Answer
- Steam off the outdoor unit is the normal defrost cycle melting frost — not smoke or a fault.
- Brief cool air from the vents during defrost is normal; backup heat usually offsets it.
- AUX (auxiliary) heat switches on automatically to help in cold weather or big temperature jumps — normal.
- EM (emergency) heat is a manual mode that runs backup only — use it just when the heat pump has failed.
- Heat-pump air feels cooler than a furnace (around 90–95°F) — that's by design, not a problem.
- Call a pro if AUX runs constantly in mild weather, the unit ices over and won't clear, or you get no heat.
Three things that look broken — but aren't
Heat pumps behave differently from the gas furnaces most people grew up with, and winter is when the differences show. Every cold season we get calls about three "problems" that are almost always completely normal: the outdoor unit steaming, the vents blowing cool for a few minutes, and a mysterious "AUX HEAT" indicator. Understanding each one will save you a worried phone call — and help you spot the rare times something genuinely is wrong.
Why is my heat pump steaming and blowing cool air in winter?
Both are signs of a normal defrost cycle. A heat pump pulls heat from outdoor air, and in cold, damp weather frost builds up on the outdoor coil. To clear it, the system briefly reverses into cooling mode, warming the outdoor coil to melt the frost — which produces visible steam or fog as the ice turns to vapor. During those few minutes the indoor vents may blow cooler air, so the system typically runs backup (auxiliary) heat at the same time to keep you comfortable. The cycle usually lasts just a few minutes and ends on its own. It's routine winter operation, not a malfunction.
The defrost cycle, explained
Because a heat pump extracts warmth from outside air, its outdoor coil gets cold — cold enough that moisture in the air freezes onto it. A frosted coil can't absorb heat well, so the system periodically runs a defrost cycle: it temporarily switches to cooling mode, sending warm refrigerant out to the coil to melt the frost, then switches back to heating.
What you'll notice during defrost:
- Steam or fog rising off the outdoor unit as ice melts and evaporates.
- The outdoor fan stops while the compressor keeps running.
- A "whoosh" or hissing sound as the reversing valve changes direction.
- Cooler air from the vents for a few minutes, often masked by backup heat.
How often it happens depends on how cold and humid it is — anywhere from occasionally to every 30–90 minutes in the worst conditions. It typically lasts only a few to about fifteen minutes and stops automatically.
AUX heat vs. EM heat — the big confusion
This is where most homeowners get tripped up. Your thermostat has two backup-heat concepts, and they are not the same thing.
❄️ AUX (Auxiliary) Heat
Automatic. The system turns on backup heat — electric strips, or the furnace in a dual-fuel setup — to help the heat pump when it can't keep up in cold weather, during defrost, or when you raise the thermostat several degrees at once. Seeing "AUX" in cold weather is normal.
🔶 EM (Emergency) Heat
Manual. You switch it on deliberately, and it bypasses the heat pump entirely, running only backup heat. It's meant for when the heat pump itself has failed. Because it's all resistance heat, it's expensive — never use it as your default setting.
The rule to remember: Let AUX do its thing automatically — it's the system being smart. Only flip on EM heat if your heat pump has clearly stopped working and you need heat until a technician arrives. Running EM heat by choice just to feel warmer air will send your electric bill soaring for no benefit.
Why the air feels cooler than a furnace
Even working perfectly, a heat pump delivers supply air around 90–95°F — warm, but below body temperature, so it can feel lukewarm on your hand compared to the hot blast of a gas furnace. This surprises people switching from a furnace, but it's completely normal and actually part of why heat pumps are so efficient: they deliver steady, gentle warmth over longer cycles rather than short bursts of very hot air. If the room reaches your set temperature, the system is doing its job — even if the air at the vent doesn't feel scorching.
When it really is a problem
Normal behavior has limits. These signs mean it's time to call a technician:
Call a pro if you see…
- AUX heat running constantly in mild weather (not just deep cold) — a sign of low refrigerant, a dirty coil, or an undersized or failing system.
- The outdoor unit encased in ice that never clears, or a defrost cycle that never seems to end — a stuck or failed defrost.
- No heat at all, or air that stays cold long after any defrost cycle should have finished.
- Constantly switching to EM/AUX and struggling to hold temperature.
- Loud grinding or banging (not the normal whoosh of the reversing valve).
If AUX or EM heat is carrying your home far more than it should, that's not just a comfort issue — it's an efficiency and cost problem worth diagnosing, and sometimes part of a bigger repair-or-replace conversation. For the fundamentals of how heat pumps perform in the cold, see our guide to heat pumps in winter.
"Most winter heat-pump 'emergencies' we hear about are the system working exactly as designed — defrosting, running a little backup heat, delivering gentler warm air. The one that actually matters is auxiliary heat running non-stop in mild weather. That's the system telling you something's wrong, and it's worth a look before it shows up on your bill."— Fairfax Mechanical Technical Team, Fairfax, VA
Heat pump leaning on backup more than it should?
Constant AUX heat in mild weather is worth diagnosing. Fairfax Mechanical offers same-day heat-pump service across Northern Virginia.
Book a Heat-Pump CheckHeat Pump Winter FAQ
Is it normal for my heat pump to blow cold air sometimes in winter?
Yes, briefly. During a defrost cycle the system reverses to melt frost off the outdoor coil, which can send cooler air through the vents for a few minutes. The system usually runs backup heat at the same time to offset it. This is normal and ends on its own. Cold air that persists well beyond a short defrost cycle, however, is not normal and should be checked by a technician.
What's the difference between AUX heat and emergency heat?
Auxiliary (AUX) heat turns on automatically to supplement the heat pump when it needs help — in cold weather, during defrost, or when you raise the thermostat sharply. Emergency (EM) heat is a manual setting you turn on yourself that bypasses the heat pump and runs only the backup heat, intended for when the heat pump has failed. AUX is normal and automatic; EM should be reserved for actual heat-pump failures because it's expensive to run.
Should I run my heat pump on emergency heat when it's very cold?
No. Emergency heat bypasses the efficient heat pump and runs only resistance backup heat, which costs much more to operate. Your heat pump is designed to run in cold weather and will call on auxiliary heat automatically when it needs help. Reserve emergency heat for situations where the heat pump itself has stopped working and you need heat until a technician can come.
Why does my heat pump steam or smoke outside?
What looks like smoke is almost always water vapor produced during the normal defrost cycle. As the system melts frost off the outdoor coil, the ice turns to steam that rises off the unit, especially in cold air. It's harmless and lasts only a few minutes. If you see actual dark smoke, smell burning, or hear grinding noises, shut the system off and call a technician, as those are not normal.
When should I call a technician about my heat pump in winter?
Call a pro if auxiliary heat runs constantly in mild weather, the outdoor unit stays encased in ice, a defrost cycle never ends, you get no heat, or the system can't hold temperature. These point to issues like low refrigerant, a dirty coil, a failed defrost control, or a system that's undersized or wearing out. Fairfax Mechanical is a licensed Virginia contractor serving Fairfax, Fair Oaks, Vienna, Brambleton, Bristow, and all of Northern Virginia — verify any contractor's license at DPOR, and confirm technicians are EPA-certified.
Know what's normal — and what's not
When your heat pump really is struggling, catching it early saves comfort and money. Fairfax Mechanical serves all of Northern Virginia with same-day service and licensed technicians.
Need help with your AC?
Free estimate for any HVAC service in Northern Virginia.