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Why Is My Central Air Conditioner Blowing Hot Air?

If you live in Arizona, the last question you want to ask yourself during the summer is, “Why is my air conditioner blowing warm/hot air?”

Don’t worry, we have an answer for you.

Your air conditioner blowing hot air out the vents could be caused by a variety of problems, ranging from simple and cheap to more complex and expensive.

Some common reasons your AC is blowing warm air include:

Let’s go into each of these issues in more detail.

Rather have a pro take a look at your AC system and determine the issue quickly? We get it. We’re the most trusted HVAC professionals in the Phoenix area, and we’d be happy to take a look at your AC system.

Reason #1: Thermostat is on the wrong fan setting

Your thermostat’s fan setting controls the fan in your AC’s inside unit (called the air handler).

Now, this may sound counterintuitive, but to properly cool your home, your thermostat’s fan setting should be set to AUTO, not ON.

A thermostat showing both auto and on options for an AC fan

A thermostat set to "Auto".


Your AC system cools your home’s air in cycles. But, when your thermostat’s fan setting is set to “ON,” it means the fan is set to run 24/7—even when the air isn’t being cooled.

If you’re noticing that the air coming from your AC vents is only warm/hot sometimes, this is likely your issue.

Solution: Turn your fan setting to “AUTO,” this will ensure the fan automatically turns on only when your AC has cool air to blow into your home.

Reason #2: Dirty air filter

Your air filter traps dust and debris in the air so that the inner workings of your AC system (and the air you breathe) stays clean.

But when you leave your air filter in for too long, it gets clogged full of dirt and becomes worse than useless—it can do real damage to your AC.


A dirty air filter blocks air from reaching your indoor AC unit. If your AC isn’t supplied with enough air, it will struggle to cool your home properly (and could result in costly repairs).

Solution: Check your air filter at least once a month and change it if it’s dirty.

A clean air filter next to a dirty air filter of an air conditioner

Psst, here's a cheat sheet for you...if your air filter looks like the one on the right, it's time to change it.

Reason #3: Refrigerant leak

Your AC actually cools your home’s air by removing heat from the air inside and dumping that heat outside. It does this through a heat transfer liquid called refrigerant.

Heat is absorbed by refrigerant in your indoor AC unit and is then carried outside where it’s dumped into the outside air via your outdoor unit.

When your AC system is low on refrigerant, your AC isn’t able to transfer as much heat out of your home—especially on hot days. This means the air coming from your AC vents will be warmer than usual.

If you think your system may be low on refrigerant, but you’re not sure, look for these other signs:

  • Frozen refrigerant line (check your outside unit; you’ll quickly see it if it’s frozen)
  • AC blows hot air during the day, but cold air at night
  • You hear a hissing or bubbling noise near AC (this means the leak is significant)

Unfortunately, the only way you’ll have low refrigerant levels is if you have a leak (refrigerant isn’t used up like gas, it’s in a closed-loop system).

Solution: Schedule a professional AC repair if you think your system may be low on refrigerant. The technician will need to find the leak, repair it (if possible), and add refrigerant to your system.

Reason #4: Dirty outside AC unit

Over time, debris like leaves, sticks, dirt, etc. can coat your outside AC unit. When there is debris blocking the outside of your AC unit, your AC will struggle to cool your home’s air efficiently.

A dirty outdoor air conditioner unit

Example of a dirty outdoor AC unit.

Solution 1: Clean the outside unit at LEAST once a year using a garden hose on a gentle setting. DON’T turn your hose to full blast or you’ll bend the condenser coil fins, which could potentially affect airflow.

Solution 2: If you want to get your outside unit properly cleaned (and prevent AC breakdowns), you’ll need to invest in air conditioner maintenance by a professional.

AC maintenance is just like getting maintenance for your car (except instead of every 3,000-6,000 miles, it’s twice a year).

Learn more in our article, “Dirty AC Condenser Coils: Why You Need Them Cleaned in Phoenix.

Reason #5: Tripped breaker

Your air conditioning unit has two breakers: one for the outside unit, another for the inside.

If the outside unit’s breaker trips, then the inside unit will continue to blow air in your home, it just won’t be cool air.

Solution: Check your breaker box and see if the unit’s breaker has tripped (moved to neutral position). If so, reset the circuit breaker. If the breaker trips again, DON’T reset it. Doing so could damage your AC.

The next step will be to schedule an air conditioner repair and allow a trained professional to fix the issue.

Don’t think any of the issues above is your problem? Talk to us!

The above 5 issues are just a handful of problems that could cause your A/C to blow hot air. To find the true problem, you’ll need a professional A/C technician to check out your AC system.