Heat Pump Not Heating or Blowing Cold Air? Normal vs. Real Problems

Quick answer

A heat pump that feels cooler than a furnace is often working normally. Its air comes out warm, not hot (about 85 to 100°F, according to Carrier). It stops heating for a few minutes during defrost, and in a cold snap it runs for a long time and calls on aux heat. When there is a real problem, check the simple causes first: the thermostat set to Cool or the fan set to On, a clogged filter, a tripped breaker, or an outdoor unit blocked by snow or leaves. If the air stays cold with the right settings, ice won’t clear, or a breaker trips again, call a licensed HVAC technician.

Normal or a problem? Start with what you notice

Find the row that matches what you are seeing. The checks in the right column are explained in the sections below.

What you notice	Usually	What to check
Air from the vents feels warm, not hot, and the house stays near your setting	Normal	Nothing. Carrier puts heat pump air at about 85 to 100°F, versus about 120 to 140°F from a gas furnace.
Cool air for a few minutes, a cloud of steam over the outdoor unit, the outdoor fan stopped	Normal (defrost)	Wait. Lennox says a defrost cycle usually lasts 5 to 15 minutes.
“Aux heat” on the thermostat on a cold day, during defrost, or right after you turn the heat up	Normal	Nothing. Trane lists all three as normal reasons for aux heat.
Runs almost nonstop in a hard freeze, but the house holds its temperature	Normal	Nothing. Carrier says constant running can be completely normal in extreme cold.
Slow to warm up after a night or away setback	Usually normal	Keep a steady setting (see the cold weather section).
Thin frost on the outdoor coil that comes and goes	Normal	Carrier says frost under 1/4 inch that clears during defrost is normal.
Cool or room-temperature air all the time	Usually a setting	Thermostat on Heat, fan on Auto.
Nothing runs, or the thermostat screen is blank	Power	Thermostat batteries, a tripped breaker, the power switches near each unit.
Indoor fan runs, but the outdoor unit is silent	Likely a problem	The outdoor unit’s own breaker. If it is on, call for service.
Some rooms are cold, others are fine	Airflow	Registers and return grilles open and unblocked; possible duct leaks.
Runs nonstop in mild weather and never reaches the setting	Likely a problem	Filter, thermostat, a blocked outdoor unit. Then call for service.
Aux heat stays on for more than an hour or two when it is above 40°F	Likely a problem	Snow or debris on the outdoor unit. Then call for service.
Ice thicker than 1/4 inch, or ice that stays after defrost	Problem	Do not chip it. Look for a gutter dripping onto the unit. Call for service.
Cooled fine last summer, but won’t heat now	Likely a problem	Possibly a stuck reversing valve. Call for service.
Hissing, liquid pooling by the outdoor unit, grinding or banging, a burning smell	Problem	Call a licensed HVAC technician. For a burning smell or smoke, turn the system off first.

For central (ducted) heat pumps. Times and temperatures are general manufacturer guidance; your owner's manual may differ. Source: Carrier, Trane, Lennox, Google Nest, and ENERGY STAR pages listed in the sources

Why heat pump air feels cooler than furnace air

A heat pump does not burn fuel to make heat. It moves heat from the outdoor air into your house, which is why, according to ENERGY STAR, it can deliver up to three times more heat energy than the electricity it uses. The tradeoff is air that feels warm rather than hot. Trane says heat pump air is usually 85 to 93°F, and that it can feel cool on your hand because your body is about 98.6°F.

So the test is not how the air feels at the vent. The test is whether the house reaches your set temperature and stays there.

Safe checks, in order

1. Thermostat mode, fan, and schedule

Carrier, Trane, and Lennox all put the thermostat first.

• Mode: it should say Heat, not Cool. ENERGY STAR also suggests Heat rather than Auto, so the system doesn’t switch modes on unusually warm or cold days.
• Fan: set it to Auto. Trane explains that with the fan on On, the system blows air even when it is not heating, and that air feels cool.
• Schedule: on a programmable or smart thermostat, Carrier suggests making sure the schedule or settings haven’t been changed.
• Em Ht: leave emergency heat off unless you chose it. Honeywell Home explains that in the Em Ht position the compressor does not run; only the backup heat does. You still get heat, but it costs more.

Then try Lennox’s quick test: raise the setting at least 5 degrees and listen for the system to start. If nothing happens, replace the thermostat batteries if it uses them, or reset it as its manual describes. Set it back afterward. A big jump can bring on aux heat, which is normal.

If the room is consistently more than 2 to 3°F away from the setting, Carrier recommends having the thermostat and system checked.

2. The air filter

Carrier says a clogged filter can make the system blow lukewarm air or shut down. Trane adds that it can make the unit struggle, overheat, and shut off.

• Find it. It is usually in a return grille or a slot at the indoor unit. If you are not sure, Trane’s owner’s guide suggests asking your dealer, and ENERGY STAR says your contractor can show you.
• Check it. Lennox’s test: hold it up to the light. If you can’t see through it, replace it.
• Replace it correctly. Use the same size and type, with the arrows pointing in the direction of airflow, as Trane’s owner’s guide describes.
• How often. ENERGY STAR recommends inspecting it monthly. Carrier suggests checking every 60 to 90 days, and Trane says standard filters are changed every 1 to 3 months depending on the filter.

3. Power: thermostat, breakers, and switches

If nothing runs at all, look for a power problem. Look at switches; don’t open anything.

• Breakers. Carrier notes that the indoor and outdoor units are on separate breakers, and the outdoor unit is usually on a 220- or 240-volt breaker. If one has tripped, Lennox says to switch it fully off, then back on. If it trips again, stop and call a professional.
• Switches near the equipment. Carrier says the power disconnect near the outdoor unit and the switch at the indoor air handler (it looks like a light switch) must both be on. Check them by sight. Don’t open the disconnect box or any unit panel.
• Indoor unit door. Trane’s owner’s guide says the blower won’t run if its access door is removed or ajar. If it is ajar, close it securely.

If the indoor fan runs but the outdoor unit stays silent with its breaker on, call a technician. Carrier says a dealer can check the contactor, the capacitor, and the wiring.

4. The outdoor unit: snow, leaves, and ice

The outdoor unit needs air flowing freely through its coil. Snow, leaves, or anything stacked against it cuts its output.

• Clear it with the system off. Turn the thermostat off, then gently clear leaves, debris, and snow from around and on top of the unit, as Lennox and Trane describe. Trane’s owner’s guide says to clear snow from around the unit as soon as possible after it builds up. Don’t push anything into the fins or the fan opening.
• Keep space around it. Carrier’s general guideline is at least 24 inches of clear space; Lennox says two feet on all sides. Follow your model’s requirements. Trane’s owner’s guide adds: no shrubs right next to it, and nothing stacked against or draped over it.
• Light frost is normal; thick ice is not. Carrier says frost under 1/4 inch that clears during defrost is normal. Thicker ice, a solid coat, or ice that stays after defrost needs attention.
• Look up. Carrier notes that a leaking gutter can drip onto the coil and freeze. Fixing the drip removes the cause.
• Wait, then call. Trane suggests waiting a few hours to see if the ice melts on its own. If it doesn’t, call an HVAC professional.

5. Registers and return grilles

If some rooms are cold while others are fine, check that air can get in and out. When heating is weak, Trane’s owner’s guide says to check the supply registers and return grilles for blockage. Open closed registers and move furniture, rugs, or curtains that cover them.

Ducts can also be the cause. ENERGY STAR says a typical house loses about 20 to 30% of the air moving through its ducts to leaks, holes, and poor connections, and lists hard-to-heat rooms as a sign. Carrier notes that most ductwork is hidden and needs an HVAC contractor to diagnose.

Not keeping up in cold weather

A heat pump pulls heat out of outdoor air, and the colder it gets, the harder that is. Google Nest’s help pages note that heat pumps take longer to heat a home the colder it gets outside. How cold is too cold depends on the equipment:

• Standard heat pumps. Trane says today’s most efficient traditional models give full heating capacity down to around 32°F and may start losing efficiency below that, and most traditional heat pumps won’t run at all much below 0°F. Carrier says older heat pumps may struggle below 25 to 30°F and need supplemental heat.
• Cold-climate heat pumps. ENERGY STAR tests certified cold-climate models at 5°F. They keep working below 5°F, it says, but pairing them with backup heat is most efficient at even lower temperatures.

On the coldest days, long run times and aux heat are expected. These signs point to more than the weather:

• It can’t keep up in mild weather. Carrier links that to a dirty filter, a thermostat setting, or a dirty outdoor coil, and says a technician can check airflow, sizing, and refrigerant if it continues.
• It has never kept up. ENERGY STAR says a system that is too small will struggle, and that a contractor should size it with a calculation called Manual J.
• Aux heat runs for long stretches above 40°F. Trane says that can mean a problem with the heat pump or the thermostat.

Slow warm-up after a setback. ENERGY STAR says that, unlike a furnace, a heat pump does not save energy when you turn it down while you’re away or asleep. Trane explains that a jump of more than 3 to 4 degrees brings on aux heat, and suggests raising the setting a degree or two at a time. See our winter thermostat settings guide for more.

Defrost, aux heat, and emergency heat in brief

Defrost. Trane says the outdoor coil runs about 10 to 20°F colder than the outdoor air, so frost can form even in the low 40s. The heat pump briefly reverses to warm the outdoor coil. In most homes backup heat covers the gap and you may only see AUX on the thermostat; homes without backup heat may feel cool air, which Trane calls perfectly normal. Lennox says a cycle usually lasts 5 to 15 minutes. Trane recommends a service call if the unit seems to defrost constantly or the cycles keep getting longer while ice builds.

Aux heat. This is the backup heat, usually electric heat strips in the indoor unit or a gas furnace in a dual-fuel system, according to Trane. The thermostat runs it automatically. Google Nest estimates aux heat can cost about 2 to 5 times as much as running the heat pump.

Emergency heat. A manual setting that runs only the backup heat (Honeywell Home). Google Nest says it is for when the heat pump isn’t working or can’t keep the home warm, and that it is usually expensive and inefficient. Trane says to use it only in a true emergency and to call for service right away if you do.

More detail: the heat pump defrost cycle, aux heat vs. emergency heat, and a heat pump freezing up.

While you wait for a technician

• Emergency heat. Carrier says that if the outdoor unit isn’t working, you can switch the thermostat to emergency heat (if your system has it), but to use it sparingly because running only electric resistance heat can raise your bill significantly.
• Pipes. Trane’s owner’s guide reminds owners to take steps to keep water pipes from freezing if the heat isn’t working in cold weather, especially if the house will be empty.
• Space heaters. Read the safety rules in our space heater cost guide first.