Heat pump not heating: runs but blows cold, or does not run at all

Three things to check at the outdoor unit first

Walk to the outdoor unit while the thermostat is calling for heat. What you see at the cabinet decides which step below to start with.

• Compressor humming, fan spinning, but the air at the vents is cool. The system is energized and pumping refrigerant. The problem is in heat-mode operation: reversing valve, defrost cycle, refrigerant charge, or aux heat not engaging. Jump to step 4.
• Compressor humming, fan not spinning. Fan capacitor or fan motor. The compressor will overheat and trip on internal protector within minutes. Shut the system off, jump to step 7.
• Nothing running outside at all. The 24-volt call never reached the contactor. Could be the thermostat, the control wires, the contactor itself, or a tripped breaker. Jump to step 2.

One more pattern that shows up most after a snowstorm: outdoor unit fully encased in ice or buried in a drift so the fan cannot turn. That is a defrost-cycle failure, not a refrigerant fault, covered in step 4. Do not chip the ice. Clear the perimeter, switch to emergency heat for a few hours, and let it thaw on its own.

Step 1: Thermostat and the emergency heat trap (2 minutes, $0)

Confirm the thermostat is in HEAT mode with the setpoint above room temperature, fan set to AUTO not ON, and batteries showing fresh. Then look for an EM HEAT or EMERGENCY HEAT indicator. This is the single most common false alarm on a heat pump no-heat call. A homeowner flipped emergency heat on during a cold snap last winter, forgot to turn it back, and the compressor has been locked out ever since while electric strips do all the work at two to five times the operating cost.

The distinction matters: auxiliary heat is automatic and supplements the compressor when the outdoor temperature drops below the balance point. Emergency heat is manual and shuts the compressor off so the strips run alone. Homeowners often use the words interchangeably; the thermostat does not. If the display shows EM HEAT, AUX ONLY, or EMERGENCY, switch back to plain HEAT and wait 5 minutes for the compressor to come on. Heating bills drop the same week.

On smart thermostats (Ecobee, Nest, Honeywell), check one more thing: a setting that tells the thermostat to stop using the compressor when it gets cold outside. The Ecobee app ships with this set to 35 degrees by default, which silently shuts a perfectly good heat pump off any time the temperature dips and runs the expensive electric strips instead. Nest has the same setting (called Heat Pump Balance) buried in the heat pump menu. If your heat pump is newer than about 2018, it can probably handle outdoor temps far below 35 degrees. The cold-climate heat pump guide explains how to look up what yours is actually rated for so you can set the threshold correctly.

Step 2: Both breakers and the control wire (5 minutes, $0)

Heat pumps run on two separate breakers in most installs: one for the outdoor condenser (usually 30 or 40 amps, double-pole 240 volts) and one for the indoor air handler with the electric heat strips (often 60 amps or larger, double-pole 240 volts). A single tripped breaker disables one half and leaves a confusing intermediate state where the strips work but the heat pump does not, or vice versa. Reset both. If either trips again immediately, do not keep flipping it; the breaker-tripping diagnostic covers locked rotors and capacitor failures that draw the kind of current that pops breakers on call.

If both breakers are on and the indoor blower runs but nothing happens outside, the signal from the thermostat is not making it to the outdoor unit. The most common cause is a small fuse on the indoor air handler's control board that has blown, usually because a wire got pinched or a critter chewed the thermostat cable outside. Other common causes: weed trimmers cutting the wire where it enters the outdoor unit, or a tired transformer on the air handler. A tech can usually trace and fix this in under an hour. Total bill: $150 to $300, mostly labor.

Step 3: Filter and indoor coil (5 minutes, $0 to $30)

In heat mode the indoor coil is the condenser. Hot refrigerant gives up heat to the return airstream there. If the filter is choked or the blower is weak, that airflow drops, head pressure climbs, and the high-pressure safety switch trips the compressor. The symptom on the wall thermostat is the heat pump running for a few minutes then stopping, then waiting 5 minutes, then trying again. The compressor sounds like it is short-cycling because it is.

Pull the filter, hold it to a light, and replace it if you cannot see the bulb through it. While you are there, listen at a supply register: a healthy blower produces a steady stream of air you can feel from 2 feet away. A weak blower delivers a noticeable but soft draft. The static pressure calculator gives the duct-and-coil restriction your blower is designed to handle; a service tech with a manometer can read what it actually is.

The indoor coil can also frost in heat mode, which is rare but happens on systems that have been short of charge for a long time. Frost forms first on the suction line leaving the indoor coil (the smaller of the two copper lines in heating mode). Suction-line frost indoors means the coil is operating below freezing because there is not enough refrigerant in the loop to absorb the building heat into the cycle. That is a refrigerant call; skip to step 11.

Step 4: Outdoor unit visual and defrost cycle (5 minutes, $0)

A normal heat pump in cold weather develops a thin layer of frost on the outdoor coil over 30 to 90 minutes of operation, then runs a defrost cycle for 5 to 15 minutes. During defrost the reversing valve flips to cooling mode, the outdoor fan stops, hot refrigerant heats the outdoor coil, and steam comes off the unit. The indoor blower keeps running and pushes cool air at the vents during this window. This is design behavior, not a fault, and the furnace blowing cold air page has more on the normal defrost timing if the cool-air complaint is intermittent.

What is not normal: the outdoor coil glazed in solid ice from top to bottom, the fan blade frozen to the shroud, or a snow drift packed against the entire unit. Solid icing means the defrost cycle is not running at all. Three things break defrost: the defrost control board has failed (older units use a timer, newer use demand-defrost logic), the defrost thermistor or temperature sensor on the outdoor coil has failed or come loose, or the reversing valve is not switching during the defrost call.

A snow drift that buries the bottom of the outdoor unit is its own problem. The fan pulls air in through the sides and bottom of the coil; if the bottom is plugged with snow or ice, the unit pulls air up through itself, which is the recipe for it freezing solid. Maintain 18 to 24 inches of clearance around all sides of the outdoor unit and 4 feet above. After a heavy snowfall, walk out and clear the perimeter; do not pour warm water (it refreezes), do not chip with metal tools (you will bend coil fins), and do not run a hair dryer (it does not have the wattage to matter). Switch to emergency heat, give the unit 6 hours, and let the sun and ambient air do the work.

To distinguish a frozen-solid unit from one that simply has not defrosted in the last hour, look at the bottom of the cabinet on the basepan. A healthy unit shows water stains and ice patches around the basepan drain holes (defrost meltwater that ran off as designed). A unit that has not been defrosting at all shows clean dry pan with a uniform ice block above it; the meltwater never ran.

Step 5: Reversing valve stuck in cool position (5 minutes, $40 to $1,200)

With the thermostat calling for heat and the compressor running, put on an insulated glove and touch the thicker insulated copper line at the outdoor unit (the discharge line). It should be 160 to 200 degrees, too hot to hold for more than a tap. If it is cool or only warm, the reversing valve is in the cooling position and the system is actually trying to air condition the house.

Here is the thing most homeowners (and some techs) do not know: a reversing valve has two parts that fail separately, and one of them is cheap. The small electromagnet that bolts to the top of the valve (called the solenoid coil) fails most of the time; swapping it is a 10-minute job that runs $100 to $350 installed. The brass valve body itself is the expensive part, $450 to $1,200 installed, because the tech has to drain the refrigerant first. Roughly one in three reversing-valve complaints is just the coil. If a contractor quotes $1,200 without first checking whether the coil alone fixes it, get a second opinion. The free quote tool will get you two or three competing numbers in writing.

Step 6: Outdoor fan stopped, compressor humming ($150 to $800)

The fan on top of the outdoor unit shares a single small electrical part (called a capacitor) with the compressor. When that capacitor weakens, the fan stops turning even though you can still hear the compressor running inside. This is the most common AC repair in the country.

Do not open the unit yourself. Capacitors hold enough stored charge to knock you across the yard even with the breaker off. What you can do is shut the system off right away. The compressor will overheat and burn out in 10 to 30 minutes if it keeps running without the fan moving air through the outdoor coil. A tech can discharge the old capacitor safely and swap it in 15 minutes; total bill is $150 to $400. If the new capacitor does not bring the fan back, the fan motor itself is done, and that runs $400 to $800 installed. The capacitor size calculator confirms the right rating for your unit if a contractor proposes a substitute part.

Step 7: Compressor humming hard, never starts ($200 to $5,000)

If you can hear the compressor humming loud for several seconds, then it goes quiet for five minutes, then tries again, the compressor is fighting to start and losing. This is one of three things, in order of how cheap they are to fix:

• Weak capacitor. See step 6. $150 to $400 installed and the compressor starts again.
• Tired compressor that needs help starting. Techs add a small part called a hard-start kit that gives the compressor an extra kick on startup. It is a legitimate fix for a compressor that runs fine once it gets going but cannot get past the first half-second. $200 to $400 installed. Buys you one to three more years on equipment that would otherwise need full replacement.
• The compressor itself is failing. If a new capacitor and a hard-start kit do not solve it, the motor inside the compressor is mechanically stuck or electrically shorted. Compressor replacement runs $1,500 to $5,000 installed. On a heat pump over 10 years old, the replace-vs-repair calculator will almost always say put the money toward a new system instead.

Step 8: Aux heat strips not engaging (15 minutes, $125 to $2,000)

Below the balance point (typically 30 to 40 degrees outdoor), most heat pumps cannot keep up with the house heat load on their own. Electric resistance strips in the air handler engage to make up the gap. Symptom of failed aux: the heat pump runs forever, the supply air at the vents is 80 to 90 degrees instead of 95 to 110, the thermostat never reaches setpoint, and on the wall display "AUX HEAT" either shows steady or never illuminates at all when it should.

Aux strips are wired to a sequencer (a small box with heat-activated bimetallic switches that bring the strips on in stages so the entire bank does not energize at once). Failed sequencers, broken strip elements (a strip element looks like a giant coiled toaster filament), or a tripped strip-kit breaker all kill aux. Strip-kit breakers are separate from the air-handler control breaker; they sit in the panel labeled something like "FURNACE 1" or "ELEC HEAT" and run 30 to 60 amps each.

Cost: a single 5-kilowatt strip element runs $125 to $300 part, $300 to $600 installed. A full aux heat kit replacement (multiple strips plus sequencer) is $500 to $2,000 depending on kilowatt rating. Sequencers alone are $80 to $200 part. Confirm at the air handler that you can hear sequencer clicks during a Y+W call; silence means the sequencer is not getting the W2 signal, which sometimes traces back to a broken control wire between the air handler and the outdoor unit rather than the strips themselves.

Step 9: The refrigerant check (tech only, $200 to $400 just to diagnose)

A heat pump that runs constantly with only lukewarm air at the vents (75 to 85 degrees instead of the normal 95 to 110) is usually short on refrigerant. The signs are simple: the system never reaches setpoint even on a mild day, run times keep getting longer, and the heating bill jumped without you changing anything.

Diagnosing refrigerant is tech-only work. EPA rules make handling refrigerant without certification a federal violation, and the gauges run a few hundred dollars on their own. What a tech will do: hook up gauges to read the system pressures, figure out whether the system is low, look for the leak (electronic detector or nitrogen pressure test), repair it, and then refill to the right amount. Total bill for a leak find, repair, and refill: $300 to $1,500, with the refrigerant type making most of the swing (see the next section).

One thing worth knowing if you are getting quotes: most leak-and-recharge work in heating mode is verified by switching the system over to cooling briefly to confirm the charge is right, because heat-mode readings are harder to interpret. Any tech who recharges your heat pump in winter without doing a quick cooling-mode check is skipping a step. The refrigerant charge calculator shows the target amount your specific unit should hold, so you can spot a recharge quote that does not match.

What a refrigerant recharge costs by type

The AIM Act ended manufacture of R-410A residential systems on January 1, 2025. Replacement refrigerants are R-454B (GWP 466) and R-32 (GWP 675). Existing R-410A systems are grandfathered and can still be serviced, but supply is on a multi-year phasedown and the per-pound price has roughly doubled in the last two years.

• R-410A (systems 2010 through 2024): $40 to $75 per pound installed today. A typical 3-ton residential heat pump holds 6 to 9 pounds, so a full recharge after a leak repair runs $240 to $675 in refrigerant alone, plus diagnostic and labor.
• R-454B and R-32 (systems 2025 forward): $50 to $90 per pound installed. Both are A2L (mildly flammable) refrigerants, which adds installation requirements but does not change recharge mechanics for a certified tech.
• R-22 (systems pre-2010): $90 to $250 per pound installed, sometimes over $400 when supply is dry. R-22 production ended in 2020 under the Montreal Protocol; all R-22 on the market today is reclaimed. A 3-ton recharge can cost $700 to $2,000 in refrigerant alone, which usually makes the replace-vs-repair math obvious.

Why the refrigerant work always has to go through a licensed tech: handling refrigerant without an EPA certification is a federal offense, and the gauges plus leak-detector cost a few thousand dollars on their own. Even on a small system this is not DIY territory.

What heat pump repairs actually cost

Cheapest to most expensive. Most no-heat calls turn out to be one of the top five items on this list, not the bottom ones.

• Thermostat reset, EM HEAT off, lockout setting corrected: $0. Resolves about 15 to 20 percent of no-heat calls outright.
• Filter replacement: $8 to $30. Resolves another 10 percent.
• Snow or ice clearing around outdoor unit: $0. Seasonal, but accounts for 5 to 10 percent of winter calls in snow climates.
• Control board fuse (3-amp glass): $1 part, $150 to $250 service-call labor.
• Capacitor replacement: $150 to $400 installed.
• Contactor replacement: $150 to $300 installed.
• Hard-start kit add: $200 to $400 installed.
• Reversing valve solenoid coil only: $100 to $350 installed.
• Defrost sensor or thermistor: $150 to $300 installed.
• Aux heat strip element: $300 to $600 installed.
• Sequencer replacement: $200 to $400 installed.
• Defrost control board: $300 to $650 installed.
• Outdoor fan motor: $400 to $800 installed.
• Reversing valve body (full): $450 to $1,200 installed.
• Refrigerant leak find, repair, and recharge: $300 to $1,500.
• Full aux heat kit: $500 to $2,000 installed.
• Compressor replacement: $1,500 to $5,000 installed.
• Full system replacement: $7,000 to $15,000 installed for a 3-ton heat pump. Use the replacement cost calculator for your zip code and tonnage.

Numbers above are current installed-quote ranges across the major US markets. Two rules of thumb worth knowing before you sign anything: on a heat pump older than 10 years, a single repair over $1,500 is worth a second opinion, and a repair over $2,500 almost always loses to a full system replacement on lifetime cost. The replace-vs-repair calculator runs your specific numbers against both paths.

Common questions about a heat pump that will not heat

Why is my heat pump running but not heating the house?

Most common cause: the reversing valve is stuck in cooling position or the outdoor unit is iced over and the defrost cycle is not running. Second most common: low refrigerant charge from a leak, where the compressor still runs but the heat capacity drops 20 to 50 percent. Third: aux heat strips are not engaging, so the heat pump alone cannot keep up below the balance point. Touch the discharge line at the outdoor unit (insulated glove) during a heat call; hot means flow direction is right and the fault is charge or aux, cool means the reversing valve is the problem.

At what temperature does a heat pump stop heating?

Depends on how old it is. Heat pumps installed before about 2015 lose most of their punch around 25 to 30 degrees outside and lean on backup electric strips below that. Newer cold-climate models keep working all the way down to 0 degrees, and the best ones down to -15. There is no single cutoff. The cold-climate heat pump guide walks you through finding what your specific unit is rated for.

How do I reset a heat pump?

Turn the thermostat off. Switch both AC breakers (indoor air handler and outdoor unit) off at the panel, wait 5 minutes, then turn them back on. Set the thermostat back to heat. If the power was out for more than 30 minutes in cold weather, switch to emergency heat for the first 6 to 8 hours after restoring power. The compressor needs that time to warm up before it runs, and skipping the wait can damage it permanently.

Why is my heat pump iced over outside?

Light frost on the outdoor coil is normal during operation; the defrost cycle should clear it every 30 to 90 minutes. Heavy ice that does not clear means the defrost cycle has failed: defrost control board, defrost thermistor, or a reversing valve that does not switch when called. Snow buildup that buries the unit is a site-maintenance problem; clear 18 to 24 inches of perimeter and let it thaw on emergency heat. Never chip with metal tools or pour hot water; both make it worse.

Should I switch to emergency heat when it is cold?

No, not unless the heat pump is actually broken. Emergency heat disables the compressor and runs the electric strips alone, which costs two to five times more per BTU of heat delivered. Use emergency heat only as a temporary measure while you wait for a service call, or during the cold-restart window after a long power outage. Auxiliary heat, which the thermostat brings on automatically when the heat pump cannot keep up alone, is the right setting in cold weather.

Can low refrigerant cause a heat pump to blow cold air?

Yes, and it is the second most common refrigerant-related no-heat call after the reversing valve. As charge drops, the heat capacity falls faster than the cooling capacity does, so a slow leak shows up in winter before the next summer. The supply air temperature drops from a normal 95 to 110 degrees down to 75 to 85 degrees, the system runs constantly, and the thermostat never reaches setpoint. The fix is finding the leak first, then recharging to the unit's nameplate spec or to the calculated subcooling target.

For a system that does heat correctly but cannot keep up below freezing, the issue is sizing or backup-heat strategy rather than a fault. The heat pump sizing calculator confirms whether the unit was correctly sized to the heat load, and the heat pump vs mini-split comparison covers the case where a ductless system would better match the building.