AC Heat Mode Not Working? 8 Causes & Fixes

By HVAC Boss

Having the option to use the same air conditioner unit for heating and cooling is one of the 21st century’s most underappreciated luxuries. However, what happens when you’re left without that option?

Whether you have a mini-split or more conventional AC system, the following article (written by a real, seasoned technician) will steer you in the right direction to getting your issue solved.

Your air conditioner’s heat mode may not be working because:
– There’s no heat mode function on the unit
– The settings are incorrect
– Ambient airflow is obstructed
– Filters are dirty
– It’s going into defrost cycle.
– Refrigerant is leaking from the unit or system
– Air is leaking from the ductwork
– The fan is broken or stuck
Luckily, these issues are easy to solve.

To put your mind at ease, I’m dedicating this article to explaining the potential issues one might run into with their air conditioner’s heat mode. I’ll also be sharing some simple tips and tricks on solving, or potentially even preventing, these problems. 

What Is the Heating Mode in an AC?

Whether the air is coming from the ducts, a wall unit, or an AC device installed on a window, air conditioners with a heating mode are something everyone on the market for a new AC should consider. 

There are many benefits to installing an air conditioner that can heat your home and cool it. On the other hand, there are a few drawbacks to these home appliances. 

Here’s a short list of the pros and cons of buying an AC unit that is capable of heating: 

Read: Why Does Air Conditioner Run At The Same Time As A Furnace?

Reasons AC Heat Mode Isn’t Working (and How To Fix It)

There’s nothing worse than sitting in a cold room, staring at an air conditioner that can’t do its job and get you warm. 

Before jumping into exploring the reasons behind the AC heating problem, I should note that every unit needs a few minutes to warm up before it starts to produce heat. Make sure you’re giving it enough time before panicking. 

While there might be other reasons why your AC unit can’t produce heat, the most common ones are: 

1. The Unit Has No Heating Option

There’s one solution to our problem of the day that seems basic but is commonly overlooked.

Readers who have just bought a new air conditioner and are experiencing problems with the heat mode might need to check out the user manual: 

Ensuring that your unit is equipped with a reversible air conditioning system should be the first thing you look at when the machine won’t start heating. AC buyers are commonly under the misconception that all of the units are the same. 

Prevent buying a non-heating AC unit by talking to the seller or checking the model’s specification list online. 

If you can’t find the info in the manual, having the heating mode icon displayed on the remote controller should clearly indicate whether you can use the AC for heating. 

2. You’re Using the Wrong Settings

Turning the air conditioner on just like you do during the summer isn’t going to cut it. For the heating mode to work, the settings need to be adequately adjusted. 

Here’s how you can make sure that your AC unit is in heating mode: 

• Set the mode to HEAT: Avoid putting the unit in Auto mode if you want to heat up the space. 
• Set the temperature higher than for cooling: For the heating function to work right, your air conditioner’s temperature needs to be set higher than usual. While the government-recommended setting for heating of 68°F (20°C) is excellent for saving on electricity, it won’t be enough to start the heat pump. Set the temperature to 75°F (24°C) or higher instead. 
• Adjust the fan speed: Once again, you don’t want to leave the AC unit in auto mode. Adjust the fan direction and speed according to your preference instead. 

Read: Why Does The Room Temperature Not Match The Thermostat Setting?

3. Air Flow Around the Unit Is Restricted

After adjusting the settings, the next step in solving why your air conditioner heat mode is not working is to check whether something is blocking the airflow: 

• Move any furniture that might be obstructing the flow of air. 
• With wall-mounted air conditioners, check if drapes are covering the unit. 
• Check if something is hindering the outside (condenser) unit. 

Air conditioner units commonly experience issues in heating/cooling due to insufficient air delivery, as the unit’s compressor can easily overload due to the lack of it. 

Read: How Many Watts Does A Heat Pump Use?

4. Clogged Filters

The lack of basic maintenance might be what’s causing the heating issue with your AC. Filthy and clogged filters are another one of the common causes for air conditioners to stop working as intended. 

The good news is that you can clean the air conditioner’s filter on your own in just a few easy steps provided by GE Appliances: 

• Vacuum the filter carefully every 30 days. 
• Soak dirtier filters in a mixture of mild dish soap and warm water. 
• Soak heavily soiled air conditioner filters in a baking soda and warm water bath to remove grime and odors. 
• Dry the filters thoroughly before reinstalling them. 

GE Appliances also underlines a couple things that you should never do with the AC filters. These are: 

• Never clean the AC filter in the dishwasher. 
• Never use the AC without reinstalling the filter. 

Read: Why Is My Air Conditioner Not Blowing Cold Air, But It’s Running?

5. AC Unit Going Into Defrost Cycle or Ice Buildup

When temperatures outside drop, ice can begin forming on your outdoor unit. To prevent this from becoming a problem, most reverse-cycle air conditioners have a built-in defrost cycle that kicks in automatically.

During defrost mode, the system temporarily reverses its operation — switching from heating to cooling — so it can push warm refrigerant through the outdoor coils and melt the ice.

This is completely normal. The cycle typically runs every 30-90 minutes in cold weather and lasts anywhere from 10-45 minutes depending on how much ice has accumulated.

You can tell your unit is in defrost mode when the indoor fan pauses, the compressor keeps running, and you notice steam or water pooling beneath the outdoor unit.

During this time the unit will blow cool or unheated air indoors — that’s expected and not a cause for concern.

The problem occurs when the defrost cycle either fails to initiate or gets stuck and won’t return to heating mode.

This can happen due to a faulty defrost thermostat, a bad defrost timer or control board, or a malfunctioning reversing valve.

When the defrost cycle fails entirely, ice continues to build up on the outdoor coils until airflow is completely blocked and the unit can no longer heat at all.

If your unit appears to be stuck in defrost mode for longer than 45 minutes, or if heavy ice buildup persists even after multiple defrost cycles, this is not a DIY fix.

Contact a licensed HVAC technician to diagnose the defrost control system.

6. Leaking Refrigerant

Refrigerant is the lifeblood of your air conditioner. Most people associate it only with cooling, but refrigerant is equally responsible for the heating function — it’s the substance that absorbs heat from the outside air and transfers it indoors.

Without enough of it, your system simply cannot heat effectively regardless of what settings you use.

Unlike a car that burns fuel, your AC doesn’t consume refrigerant — it circulates the same charge continuously.

This means if your refrigerant level is low, there’s a leak somewhere in the system. Low refrigerant doesn’t just happen on its own.

Signs you may have a refrigerant leak:

• The unit runs continuously but struggles to reach the set temperature
• You notice ice forming on the refrigerant lines or outdoor unit
• There’s a hissing or bubbling sound coming from the unit
• Your energy bills have increased without a change in usage
• The air coming from your vents is only slightly warm, not noticeably hot

What causes refrigerant leaks? The most common culprits are corrosion in the copper refrigerant lines, vibration loosening fittings and joints over time, or physical damage to the outdoor unit.
Older systems are particularly vulnerable as the refrigerant lines degrade with age.

What to do: Refrigerant handling is regulated by the EPA and requires a certified technician — this is not a DIY repair under any circumstances.
A technician will locate the leak using a detection tool, repair the source, and recharge the system to the correct level.
Simply topping off the refrigerant without fixing the leak is a temporary fix that will leave you with the same problem within months.

If your system is older and uses R-22 refrigerant (phased out in 2020), a significant leak may make repair cost-prohibitive and replacement the smarter financial decision.

Read: How Do I Get Rid Of The Smell In My Air Conditioner?

7. Leaking Ductwork

Sometimes the problem has nothing to do with your air conditioner itself. If your system is producing heat but certain rooms stay cold or your house just never seems to warm up, leaking ductwork could be the culprit.

Your duct system is responsible for carrying the warm air your AC produces and delivering it to every room in the house.

If there are gaps, cracks, or disconnected sections in that ductwork — particularly in unconditioned spaces like attics, crawl spaces, or basements — a significant portion of that warm air never makes it to where it’s needed.

According to Energy Star, the average home loses 20-30% of the air moving through the duct system due to leaks, holes, and poor connections.

Signs your ductwork may be leaking:

• Some rooms heat up normally while others stay consistently cold
• Your energy bills are higher than expected for the size of your home
• The system runs longer than usual to reach the set temperature
• You notice dustier-than-normal air in certain rooms
• There are visible gaps or disconnected sections in accessible ductwork

Common causes of duct leaks: Ductwork deteriorates over time. Joints and seams can separate due to temperature fluctuations causing the metal to expand and contract repeatedly over years.

Pest damage, poor original installation, and aging duct tape (which ironically is one of the worst materials for sealing ducts long-term) are also frequent causes.

What to do: Start by inspecting any accessible ductwork in your attic, basement, or crawl space. Look for obvious gaps, disconnected sections, or sections where the duct tape has dried out and peeled away.

These can be resealed with mastic sealant or with metal foil tape, both of which are available at any hardware store and are legitimate DIY fixes for accessible ducts.

For ductwork hidden inside walls or ceilings, or if you suspect widespread leakage throughout the system, a professional duct pressure test (called a duct blaster test) will pinpoint exactly where the losses are occurring.

An HVAC technician can then seal the system properly, which in many cases pays for itself quickly through reduced energy bills.

Note: We may earn a small commission on some purchases, at no extra cost to you.

Read: Why Window Air Conditioner Compressor Not Turning On?

8. Broken or Stuck Fan

Your air conditioner relies on two fans to heat your home effectively — the indoor blower fan that pushes warm air through your vents, and the outdoor fan that draws air across the condenser coils.

If either one fails, the heating process breaks down even if the rest of the system is working perfectly.

A broken indoor fan means the unit may be generating heat but has no way to distribute it.

A failed outdoor fan causes the system to overheat and shut down as a protective measure, or prevents the heat exchange process from happening altogether.

Signs of a fan problem:

• You can hear the unit running but little or no air is coming from the vents
• The outdoor unit is running but the fan blade isn’t spinning
• The system starts then quickly shuts off
• You hear grinding, rattling, or squealing coming from the unit

What causes fan failure: Worn out bearings, a burnt out motor, a capacitor that can no longer start the fan, or debris lodged in the outdoor unit blocking the blade are the most common culprits.

What to do: Clearing debris from around the outdoor unit is an easy first check.
Next, while the unit is trying to start, use a stick or long screwdriver to spin the fan blade to see if it will run.
If so, your start capacitor is probably shot. These are easy to test and replace. However, you’ll need a multimeter that reads capacitance. Follow these steps:

1. Turn off power and remove access cover on unit.
2. Visually inspect the capacitor for a bulge. The top of the capacitor (where the terminals are) should be completely flat. If there is a noticeable bulge, that is a dead giveaway. Replace it.
3. Warning: Before you touch it, deenergize it. Capacitors hold a charge that can be lethal. Hold a screwdriver across the “HERM” and “C” terminals. Do the same between the “FAN” and “C” terminals. This drains the capacitor.
4. Take a picture before detaching wire connections so you know how to hook up the new capacitor. Then disconnect the three wires.
5. If there is no bulge, test the capacitor with a multimeter. Test between the COMMON and HERM, and then between the COMMON and FAN. Compare readings to the info on the capacitor. Make sure it is within the allowable % range. If not, replace.
6. Replace with a capacitor that has the EXACT same specs (voltage, microfarads, etc.).

How To Prevent Air Conditioner Heat Mode From Failing?

While diagnosing and solving all of those problems is fairly straightforward, we should focus on their prevention. Following the simple tips from this article, you can prolong your air conditioner’s cooling and heating life and spare yourself a lot of stress and money. 

Here are a few things that AC users can do to ensure the heating function doesn’t leave them cold and stranded in the middle of the winter: 

Regular Service and Maintenance

Regular air conditioner maintenance includes: 

• Replacing or cleaning the filters. 
• Cleaning and inspecting the coils of both the outdoor and indoor units. 
• Checking for potential thermostat defects. 
• Inspecting the condensate pump and other vital elements of the system. 
• Cleaning and sanitizing the indoor unit. 

Every homeowner or a renter should do their best to service their air conditioning device at least once a year. This service interval ensures that the unit is safe for use and keeps running the way it should. 

While areas of AC maintenance, such as filter cleaning or replacement, can be done by anyone, it might be best to leave the more complex ones to professionals. 

Read: HVAC System Service – How It’s Done And How Much Does It Cost?

Using the AC Heating the Right Way

Although it might seem simple, effectively using the AC for heating requires some thought. 

The main thing to remember is not overloading the air conditioner for ductless systems, including the wall and window-mounted ones. This means starting the heating feature a bit sooner than you might need and gradually increasing the temperature as the weather outside gets colder. 

Avoiding the turbo mode that some devices have is also my recommendation. 

Refraining from constantly turning the device on and off is another way you can prevent putting too much stress on the air conditioner’s heater. 

Following this simple tip will not only save you money when the electricity bill arrives, but it will also ensure that the air conditioner lasts longer. 

Read: Why Does Air Conditioner Run At The Same Time As A Furnace?

Why is Cold Air Coming Out When My AC is in Heat Mode?

This is one of the most common and alarming complaints from AC users in heating mode — and in most cases, it’s not actually a problem.

The most likely explanation: your unit is in defrost mode.

As covered above, heat pumps and reverse-cycle ACs periodically reverse their operation to melt ice off the outdoor coils.

During this cycle — which can last anywhere from 10-45 minutes — the system blows cool or unheated air indoors.

The outdoor fan may also stop, and you might see steam rising from the outdoor unit. This is completely normal operation and will resolve on its own.

The second most likely explanation: the air feels cold, but it isn’t.

Unlike a gas furnace that produces air at 120-140°F, a heat pump in heating mode delivers air at around 85-95°F.

That’s well above room temperature and will heat your home — but it’s significantly cooler than your body temperature of 98.6°F, so it can feel cool or even cold coming out of the vents.

If you hold a thermometer up to the vent and it reads 85°F or above, your system is working exactly as it should.

When cold air is actually a problem:

If neither of the above explains what you’re experiencing, the issue is likely one of the following:

• A faulty reversing valve — this is the component that switches the system between heating and cooling mode. If it’s stuck or malfunctioning, the system may default to cooling even when set to heat. This requires a technician to diagnose and replace.
• A refrigerant leak — as covered in the section above, low refrigerant prevents effective heat transfer, resulting in air that never warms up properly.
• A failed defrost control — if the system is permanently stuck in defrost mode rather than cycling through it, cold air will blow continuously until the control board or defrost thermostat is replaced.

If your unit is blowing genuinely cold air for more than 45 minutes with no signs of returning to heat, contact a licensed HVAC technician — it’s unlikely to resolve on its own.

Conclusion

When your AC’s heat mode stops working, the cause is almost always one of the eight issues covered in this article — wrong settings, restricted airflow, clogged filters, a defrost cycle problem, a refrigerant leak, leaking ductwork, or a failed fan.

Start with the simple stuff first. Check your settings, make sure the unit has a heating option, clear any obstructions around the unit, and inspect your filters before assuming the worst. In many cases that’s all it takes.

If the basics check out, the next step is determining whether you’re dealing with a defrost cycle issue or something more serious like a refrigerant leak or fan failure. The symptoms outlined in each section above will help you narrow it down quickly.

One thing worth emphasizing: refrigerant leaks and fan motor failures are not DIY repairs. Attempting either without the proper tools, training, and EPA certification can cause further damage to the system and create safety hazards.

When in doubt, call a licensed HVAC technician — a proper diagnosis now is far less expensive than a compressor replacement later.

With regular maintenance — annual servicing, clean filters, and clear airflow around both the indoor and outdoor units — most of these problems are entirely preventable.

A well-maintained system should heat and cool reliably for 15-20 years.

If you’re still not sure what’s causing your heat mode to fail, use the chatbox above to speak with one of our HVAC technicians directly. No appointments, no in-home service calls.

Related: Do You Have To Vent a Portable AC In Heat Mode?