How Do I Fix My AC Not Heating? Steps That Work

To fix an AC not heating, start with the thermostat: set it to “heat” (not “cool” or “off”) with the target above room temperature, and give a heat pump a few minutes to switch over. If that’s correct, confirm the furnace or air handler has power and isn’t in lockout, then replace a dirty air filter, since restricted airflow is a top cause of weak heat. For heat-pump systems, the most common deeper culprits are low refrigerant from a leak, a stuck reversing valve, or a failed defrost cycle — all of which need a professional. A full system reset (off at the thermostat and breaker for a few minutes, then on) clears many minor faults. If your AC still won’t heat after these checks, call a licensed HVAC technician. Below we walk through each scenario, including heat pumps, central systems, apartments, and cars.

Why is my AC running but not heating?

If your system is a heat pump and it runs but won’t heat, the usual causes are a low refrigerant charge (almost always a leak), a stuck reversing valve that won’t flip the system into heating mode, or a defrost problem that leaves the outdoor coil iced over. Restricted airflow from a dirty filter or dirty coils can also leave the air feeling lukewarm.

Start with the free checks: confirm “heat” mode, replace a dirty filter, and make sure the outdoor unit is clear of ice, snow, and debris. If it still won’t heat, the refrigerant and reversing-valve issues require a professional with gauges — don’t attempt those yourself.

Why is my AC blowing cold air on heat?

A heat pump heats by reversing its refrigerant flow: instead of pulling heat out of your home, it pulls heat from the outdoor air and releases it inside. When it blows cold on “heat,” that process isn’t completing. The most common reasons:

1. Thermostat set wrong. If it’s on “cool,” the system blows cold regardless. Confirm “heat” mode first — it’s the most common mistake.
2. Defrost cycle. Heat pumps periodically run a brief defrost that blows cooler air for a few minutes. If it passes quickly, that’s normal.
3. Low refrigerant. A leak leaves too little refrigerant to move heat, so the air stays cool. This needs a pro.
4. Dirty air filter. Restricted airflow reduces heat transfer and can make the air feel cold. Replace it.
5. Stuck reversing valve. If the valve won’t switch to heating, the system effectively stays in cooling mode. This is a professional repair.

Top 5 AC Problems and How to Fix Them

Air conditioner not heating

When an air conditioner won’t heat, work from simple to complex. First confirm the unit is on and the thermostat is set to “heat” with an above-room target. Replace a dirty filter and clear the outdoor unit of ice or debris.

If the basics are fine, the problem is likely inside the system. Low refrigerant from a leak prevents effective heating and must be repaired and recharged by a professional. A compressor that isn’t working is a more serious failure that also requires a technician. Because both involve sealed-system and high-voltage components, these are not DIY fixes — confirm the diagnosis with an HVAC pro before any parts are replaced.

Air conditioning troubleshooting chart

Use this quick sequence to narrow down a heating or cooling fault:

• Thermostat: correct mode (heat/cool), correct temperature, fresh batteries.
• Power: unit plugged in or breaker on; check for a tripped breaker or blown fuse.
• Air filter: clean — a clogged filter restricts airflow and is the most common single problem.
• Outdoor unit: clear of dirt, leaves, snow, or ice so it can transfer heat.
• Evaporator coil: not dirty or frozen; ice blocks airflow.

If you’ve worked through all of these and the system still won’t heat or cool properly, it’s time to call a professional to diagnose the refrigerant charge and internal components.

How to fix AC in an apartment

In an apartment you may not control the central system, but you have options. If the unit is the landlord’s responsibility, report the problem in writing first. Beyond that, these steps help keep an apartment comfortable:

1. Add a portable air conditioner or heater you can move room to room.
2. Use ceiling or portable fans to circulate conditioned air and even out hot or cold spots.
3. Manage your windows — curtains and blinds block summer heat gain and help hold winter warmth.
4. Position fans wisely so they move air through living spaces rather than fighting your heating or cooling.
5. Keep the space and any unit filters clean so airflow stays unrestricted.

Air conditioning problems and solutions

For a printable-style checklist of common AC faults: start with power (plugged in, breaker on), then the thermostat (correct mode and setting). If it’s still not conditioning the air, suspect a low refrigerant charge — a clue is the refrigerant line feeling unusually cold or icing up — which calls for a professional, since refrigerant work requires certification.

Finally, check the air filter; cleaning or replacing a clogged filter resolves a surprising share of problems. If none of these fix it, have the system serviced by a technician.

How to fix AC in a house

If central air isn’t conditioning your home, check these five things before calling a tech. First, the air filter — a clogged filter is the most common cause of poor performance, because it restricts airflow over time. Second, the thermostat — confirm it’s in the right mode and the setting is correct.

Third, the blower fan, which moves conditioned air into the home; if it isn’t running properly the system can’t deliver heat or cool. Fourth, the evaporator coil, which can get dirty or freeze and block airflow. Fifth, the outdoor unit, which needs to be clean and unobstructed. If all five check out and it still won’t heat or cool, the refrigerant charge or compressor likely needs a professional.

More air conditioner problems and solutions

Many AC complaints trace back to three things: a dirty filter, dirty or frozen evaporator coils, or a refrigerant leak. Replace the filter first, then inspect the coils for dirt or ice and have them cleaned if needed.

If you suspect a refrigerant leak — weak performance plus icing or a hissing sound — contact a professional to find and repair the leak and recharge the system. Topping off refrigerant without fixing the leak only delays the next failure. Regular professional servicing prevents most of these problems.

How to fix AC in a car

For a car’s AC, start with the obvious: make sure it’s switched on and set correctly. If it’s running but the air isn’t cold (or warm, for the heater), the issue is likely in the system itself — often low refrigerant or a component fault — and you’ll need a mechanic to repair it. Car refrigerant work requires proper equipment, so leave charging and leak repair to a professional.

LG reverse cycle air conditioner not heating

A reverse-cycle unit (a heat pump) that won’t heat usually has one of three problems. First, confirm power and that the settings are correct for heating mode. Second, suspect low refrigerant from a leak — a frozen line or low-pressure port can be a sign, though confirming it safely requires gauges and a technician.

Third, if the compressor runs but no heat is produced, the compressor or the reversing valve may have failed, which needs professional repair. Because exact behavior and component access vary by model, check your unit’s manual and have a qualified technician confirm the diagnosis before replacing parts.

Heat pump vs. furnace: know which system you have

“AC not heating” means different things depending on your equipment, so identify your setup first. If your AC unit itself produces heat — meaning the outdoor unit runs in winter and you have no separate gas furnace — you have a heat pump. A heat pump reverses its refrigerant flow to pull heat from outdoor air and bring it inside, and it relies on backup electric “auxiliary” or “emergency” heat strips when it’s very cold outside.

If your home has a gas or oil furnace, the AC only cools and the furnace handles heat entirely. In that case, “no heat” is a furnace problem, not an AC one — check that the furnace has power, the gas supply is on, the pilot or igniter is working, and the furnace isn’t in a safety lockout. Knowing which system you have tells you where to look: a heat pump points you to refrigerant, the reversing valve, and defrost; a furnace points you to ignition, fuel, and the blower.

Understanding the defrost cycle on a heat pump

One behavior that worries heat-pump owners is completely normal: the defrost cycle. In cold, damp weather, frost builds on the outdoor coil and the system briefly reverses to melt it off. During defrost you may see steam rising from the outdoor unit, hear a whoosh as the reversing valve shifts, and feel cooler air indoors for a few minutes. This is the system protecting itself, not a fault.

Problems start when defrost fails. If the outdoor coil becomes a solid block of ice that never clears, the defrost control, sensor, or reversing valve may have failed, and the unit can’t produce heat efficiently. Never chip ice off the coil or pour hot water on it — you can damage the fins or refrigerant lines. Turn the system to “emergency heat” if available so the backup strips keep you warm, and call a technician to diagnose the defrost components.

Next steps

If your AC won’t heat, the fastest fixes are the simplest: set the thermostat to “heat,” replace a dirty filter, clear the outdoor unit, and try a full reset at the breaker. If you have a furnace, confirm it has power and isn’t locked out — a dead furnace means no heat regardless of the AC. If you have a heat pump and it still won’t heat, the likely causes are low refrigerant, a stuck reversing valve, or a defrost fault, all of which need a licensed HVAC technician. Don’t attempt refrigerant or sealed-system work yourself; get a professional diagnosis once you’ve ruled out the basics.

Frequently Asked Questions

Why does my heat pump blow cold air sometimes in heat mode?

Heat pumps run a periodic defrost cycle that briefly blows cooler air to melt frost off the outdoor coil. If it clears within a few minutes and warm air returns, that’s normal. If the air stays cold, check the mode and filter, then call a pro.

Can a dirty filter stop my AC from heating?

Yes. A clogged filter restricts airflow, reduces heat transfer, and can freeze the coil — leaving the air lukewarm or cold. Replacing the filter is the cheapest first fix to try.

When should I call a professional?

Call a technician if the system still won’t heat after you’ve checked the thermostat, filter, power, and outdoor unit. Low refrigerant, a faulty compressor, and a stuck reversing valve all require professional tools and certification.

What is “emergency heat” and when should I use it?

On a heat pump, “emergency heat” (or “aux heat”) switches the system to its backup electric heating elements instead of the heat pump itself. Use it when the heat pump or its outdoor unit has failed — for example, the coil is iced solid or the compressor won’t run — so you stay warm until a technician arrives. It costs more to run than normal heat-pump mode, so treat it as a temporary measure, not your everyday setting.

Why does my heat pump struggle on the coldest days?

Heat pumps pull heat from outdoor air, so the colder it gets, the less heat there is to move and the harder the system works. As temperatures drop, the backup heat strips kick in to keep up. If your home feels under-heated only during extreme cold, that may be normal for the equipment — but persistent weak heat, even in milder weather, points to a fault worth having checked.