Does Your Home Air Conditioner Start Cold Then Get Warm?

When the sweltering heat of summer hits, there’s nothing more frustrating than your air conditioner suddenly blowing warm air into your home. There are a few reasons why this happens, from simple adjustments to a more complex fault. Let’s explore each possible reason and learn some practical solutions to keep that refreshing cold air flowing.

Culprit #1: Fan Is Set to “On”

The most common reason your air conditioner starts cold but then blows warm air is because the fan setting on your thermostat is set to “On” instead of “Auto.” With this setting, the cooling cycle starts and stops as necessary, but the fans always stay running, potentially sending warm air through the vents.

It can be useful to set the fan to “On” sometimes, especially if you’re doing something that requires better air circulation and filtration. However, it’s best to keep the setting to “Auto” in most cases. When the fan only runs during the cooling cycle, cool air will be all you get.

Culprit #2: Incorrect Thermostat Settings and Malfunctions

Basic thermostats have auto, on, fan, cool, and heat settings. Newer thermostats give you the ability to schedule temperature settings in advance as the day progresses. 

If you’re getting warm airflow from your air conditioner, check your thermostat settings to be sure they’re as you intend. Change the batteries if they’re running low, and wipe the exterior so dust won’t collect inside. 

You can also check to make sure the thermostat is properly connecting. There are prongs on the back of the thermostat face, and if they’re missing or bent, it can interfere with normal operation. 

If all else fails, ask your HVAC technician if a new thermostat may be necessary.

Culprit #3: Dirty or Missing Air Filters

When the air filters get clogged, your system is not able to pull as much air into the air handler, and there’s more strain put on the blower and other essential parts of the air conditioner. This could be one of the reasons why your AC runs constantly. 

Missing filters are another significant concern, since their absence can cause the evaporator coils to get clogged up with dust, dirt, dander, and hair. Dirty coils can cause your air conditioner to malfunction or even freeze.

The EPA suggests that every homeowner change their air filters at a minimum every three months. However, not every homeowner should wait that long, and you should change your air filter as soon as you see that it is dirty.

Culprit #4: Low Refrigerant Levels

Refrigerant is the lifeblood of your air conditioner. If there’s a freon leak or you have low levels, your air conditioner will struggle to stay at the temperature you need it to.

You can prevent refrigerant issues by having your whole air conditioner inspected and tuned up a few times per year. Make sure your technician evaluates the condenser coils, outdoor and indoor units, ductwork, refrigerant levels, fan motors, and refrigerant lines.

Fixing refrigerant-related issues is not a DIY project due to the environmental and health risks involved. Instead, call an EPA Section 608-certified HVAC professional for the job.

Culprit #5: Dirty or Damaged Ductwork

Ductwork damage can cause hot air to blow out of your HVAC vents, decreases your air conditioner’s efficiency, and puts more strain on the components. 

To prevent ductwork issues, have your ducts professionally cleaned every few years or as needed. You should also consider sealing and insulating ducts if you haven’t already, especially in unconditioned spaces like attics and crawl spaces.

Culprit #6: Evaporator Coil Problems

The evaporator coil absorbs heat from indoor air — essentially, it’s where the magic happens in your air conditioner. Dirty coils won’t cool air efficiently, and frozen coils won’t cool air at all.

Look behind the air filter to check the coils for damage or ice. If you see any issues, turn your air conditioner off and call a professional for repair. You should also schedule professional coil cleaning as part of annual AC maintenance.

Culprit #7: Electrical Problems

If one of the components in your air conditioner isn’t getting enough power, it could lead to warm air blowing out of the vents. 

Check the circuit breaker by your outdoor AC unit, and reset it if you find it’s been tripped. Set the disconnect switch near the unit to the “On” position, and see what happens. 

If problems persist, you could have either a wiring issue or a faulty capacitor. At that point, there’s too much risk trying to fix the problem yourself, and you should call your HVAC company or a licensed electrician.