Heat Pump Blowing Cold Air: Defrost, AUX and EM Heat, and What to Fix

Cool air from the registers with the heat on can be alarming in your first winter with a heat pump, and often nothing is wrong. A heat pump moves heat in from outdoors, so its air runs cooler than a furnace's, and in cold, damp weather it pauses to defrost its outdoor coil. When something is wrong, it is usually a thermostat setting, a dirty filter, an outdoor unit that is blocked or iced over, or backup heat that isn't coming on.

This page covers what is normal, including AUX and EM heat, then the causes from the easiest to check, what you can safely fix, and where the technician takes over. If your heat comes from a gas furnace instead, see furnace blowing cold air.

First, Check What Is Normal

Heat pump air is cooler than furnace air. Carrier puts heat pump supply air at roughly 85 to 100°F while heating, against roughly 120 to 140°F from a gas furnace, so air that is heating the house well can feel cool on your hand. The test is whether the house reaches the setting; long run times in the cold are normal if it keeps up, Bryant says.

Defrost comes with steam and noise. The outdoor coil runs 10 to 20°F colder than the outdoor air, Trane says, so frost can form even in the low 40s. To clear it, the heat pump reverses into cooling mode for a few minutes to warm the outdoor coil. The outdoor fan may stop, you may hear a whoosh or running water, and steam can rise from the unit, all normal per Trane, Bryant, and Daikin. A cycle typically lasts about 5 to 15 minutes (Lennox, Bryant) or 5 to 10 (Carrier), and Trane says its demand-defrost systems usually finish in 3 to 5.

AUX on the thermostat is usually normal. Most heat pumps have backup heat: electric heat strips in the indoor air handler or, in a dual-fuel system, a gas furnace. The thermostat runs it automatically, and shows AUX, during defrost, when it is too cold for the heat pump alone, and after you raise the setting several degrees at once. Trane notes that some systems, including dual-fuel systems that don't run the furnace and heat pump together, blow cool air during defrost, which is also normal.

EM heat is a manual switch for a breakdown. EM (emergency) heat shuts the heat pump off and runs the backup alone. Carrier and Trane say to use it only when the heat pump has failed, and to call for service when you do. On electric strips it uses more energy: Carrier calls strip heat 100 percent efficient against as much as 300 percent for a heat pump, and ENERGY STAR says a heat pump can deliver up to three times more heat energy than the electricity it uses.

Most Likely Causes

1. The thermostat is on COOL, AUTO, EM heat, or fan ON. COOL blows cold air, and a fan set to ON runs unheated air between heating cycles (Carrier, Lennox, Bryant); Bryant also warns that EM heat left on by mistake won't heat efficiently. Set HEAT, which ENERGY STAR prefers to AUTO so the system doesn't switch modes on an unusually warm day, and fan AUTO. If it started right after a new thermostat went in, a wrong O/B setting can swap heating and cooling; Google's fix is to switch O to B, or B to O, and retest (see thermostat not working).
2. A dirty filter. A clogged filter chokes airflow, so the heat pump can blow lukewarm air or shut down (Carrier, Bryant), and Carrier says it can push the system onto backup heat even in mild weather. Hold the filter to the light, open registers, and clear returns of furniture and rugs. Replace it, arrow toward the air handler, per our filter guide; ENERGY STAR says check it monthly and change it at least every three months.

3. The outdoor unit is blocked by snow, ice, leaves, or a cover. Snow, ice, leaves, or debris around the unit block the airflow it draws heat from (Lennox, DOE). Don't wrap it for winter: Lennox says heat pumps that run year-round should never be covered, Trane rules out solid covers, and Goodman warns that running an outdoor unit with a cover on can damage it. The most they allow is a top-only snow guard (Lennox) or a breathable cover made for the unit (Trane). A gutter dripping onto it adds ice (Lennox).
4. Ice that doesn't clear. Carrier calls frost under about 1/4 inch normal; thicker ice, or ice that stays after defrost, is not. Lennox lists the causes: a failed defrost control board, timer, or sensor; low refrigerant from a leak; restricted airflow; a clogged drain pan; and freezing rain that overwhelms the defrost. Trane says defrosts that come constantly, or run longer while ice builds, need a dealer. These are a technician's job, as is a stuck reversing valve, which Carrier says shows up as a heat pump that cools but won't heat.
5. The backup heat isn't coming on. If the air turns cold every defrost on a system with heat strips, or the house falls behind in deep cold, AUX may not be working. Carrier says the air handler, where the strips sit, has its own breaker, separate from the outdoor unit's, plus a power switch that looks like a light switch; check both. Switch a tripped breaker fully off, then on, once; if it trips again, leave it off and call (Bryant, Lennox). Our breaker page explains why. A dual-fuel backup is a furnace; see furnace blowing cold air.
6. Deep cold below the balance point, or an undersized system. The balance point is the outdoor temperature at which the heat pump's capacity equals the home's heating demand; below it, supplemental heat is needed, and Carrier says it varies by heat pump, home, and system design. ENERGY STAR certification tests cold-climate heat pumps down to 5°F; ENERGY STAR says they keep working below that but heat most efficiently with backup heat. An undersized system struggles; ENERGY STAR and the DOE say contractors should size it with a Manual J load calculation. If the house has fallen behind in every cold snap since installation, have the backup heat and sizing checked.

What You Can Fix Yourself

• Set HEAT, not AUTO, COOL, or EM heat, with the fan on AUTO, and raise the setting a degree or two at a time, as Trane suggests.
• Replace the filter and open every register and return.
• With the power off, gently sweep snow off the top with a broom and clear snow and leaves from the sides (Lennox, Bryant, Trane). Don't pile shoveled snow against it.
• Keep about two feet clear around it, the general guide from Carrier, Lennox, and Bryant; your model's manual sets the actual clearances.
• Never chip or pry ice with a shovel, an ice pick, or any hard or sharp tool, which can puncture the coil (Bryant) and damage fins and fan blades (Lennox). Never pour hot water on it; Lennox says the sudden temperature change can crack parts.
• Take off any wrap or tarp, redirect a gutter that drains onto the unit, and reset a tripped breaker once.

When to Call a Pro

Refrigerant, the reversing valve, the defrost board and sensors, and electrical parts belong to a licensed HVAC technician; EPA rules under Section 608 of the Clean Air Act require certified technicians for refrigerant work and prohibit venting it. Call when ice won't clear or returns quickly, ice coats the refrigerant lines, or the outdoor fan won't spin after the ice melts (Lennox); when a breaker trips again; when AUX runs for more than an hour or two above 40°F (Trane); or when EM heat is your only heat. If you'll be away while you wait, keep the heat no lower than 55°F, the Red Cross minimum for protecting pipes.

Prevent It

ENERGY STAR recommends an annual pre-season check-up, heating in the fall; Lennox and Bryant suggest twice a year for a heat pump. Bryant's list includes the refrigerant charge in both modes and the capacitors and contactors; Lennox's adds the defrost controls. Check the filter monthly, clear the outdoor unit after every snowfall, and keep plants trimmed back. If snow buries the unit, ask about raising it: Mitsubishi Electric says contractors should mount it above the snow line. And pick a temperature and leave it: ENERGY STAR says heat pumps don't save energy from being turned down, and Carrier notes deep setbacks bring on AUX during recovery. The winter guide schedules it all.

Frequently Asked Questions

Why is my heat pump blowing cold air in heat mode?

Check for HEAT with the fan on AUTO. A few minutes of cooler air with steam outside is defrost. If it stays cold, replace the filter, clear the outdoor unit, and check the air handler breaker.

Why is my heat pump steaming outside?

It is defrosting: frost is melting off the outdoor coil, often with the fan stopped and a whooshing sound, and it stops within minutes. Smoke is different. If you smell burning, turn the system off and call; if there is smoke or fire, get out and call 911.

What is the difference between AUX and EM heat?

AUX is automatic backup heat for defrost, deep cold, or a big setpoint jump. EM heat is a manual setting that turns the heat pump off and runs the backup alone, only for a failed heat pump, per Carrier and Trane.

How do I unfreeze my heat pump?

Lennox's steps: turn the heat pump off at the thermostat, clear around it (power off at the disconnect first, as above), check the filter, and let it thaw, about two to four hours above freezing, with EM heat keeping the house warm. Never chip it or pour hot water on it. If it hasn't thawed in six to eight hours or refreezes, call.

The Bottom Line

Lukewarm air, steam outside, a few minutes of cooler air during defrost, and AUX in the cold are normal. Set HEAT with the fan on AUTO, change the filter, keep the outdoor unit clear and unwrapped, reset a tripped breaker once, and never chip ice. Ice that won't clear, AUX running for hours in mild weather, or cold air that doesn't stop is a technician's call; if the heat pump has failed, EM heat can carry the house until they arrive.

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