What Temperature Air Should My Central Air Conditioner Be Putting Out?

Written By Lester Mclaughlin
Updated On

Are you wondering what temperature your air conditioner should be putting out?

You’ve come to the right place!

In this Blue National HVAC guide, you’ll learn:

  • How central air conditioner systems work
  • What supply and return temperatures are
  • How cold the air should be coming from your AC unit

And much more!

Temperature Central AC Putting Out

So, if you’re looking for answers on what temperatures your central AC unit should be outputting, keep reading our detailed guide below to get answers to all of your questions!

How Do Central Air Conditioner Systems Work?

Before we get into what temperature your air conditioner should be putting out, let’s touch on how air conditioning works first. 

Central air conditioners use the vapor-compression refrigeration cycle to cool the air in your home. The vapor-compression refrigeration cycle is the process that chills the food in your refrigerator and freezes the ice in your freezer. 

A central air conditioner is a closed-loop system consisting of a compressor, condenser, thermal expansion valve, and evaporator tied together with refrigerant lines. These refrigerant lines contain refrigerant that travels around the closed-loop like a racecar on a circular track. 

On its journey, the refrigerant experiences various pressures, temperatures, and states of matter (liquid and gas). These changes facilitate the removal of heat from your home. 

When the refrigerant reaches the evaporator coils inside your home, it is very cold. Warm air from inside your home blows across the coil. When this happens, the heat from the air transfers to the refrigerant. It then moves to the condenser outdoors and expels the heat. At the same time, the cold air blows out of the supply vents. The cycle repeats until your thermostat’s setpoint is reached. 

Central air conditioners can only remove so much heat from your home at once. For example, the air conditioner will take a while to lower the home’s temperature by 10ºF. This is because the cold air they provide your home can only get incrementally colder than the current indoor air temperature. 

What Are Supply & Return Air Temperatures: Why Do They Matter? 

The supply and return air temperatures are important because they are critical in the cooling of your home. The supply air temperature is the temperature of the air coming out of your supply air ducts. In other words, it is the cold air that flows into the rooms of your home. 

On the flip side, the return air temperature is the temperature of the air that is being sucked back to the air conditioner through the return vents. 

Assuming your cooling system is working correctly, the supply temperature will always be lower than the return temperature. Their temperature difference is known as Delta T.

The measurement of supply, return, and Delta T temperatures is a common way to check if a central air conditioner is cooling adequately. If the supply temperature is too hot, which will make the Delta T smaller, then the air conditioner likely has an issue. 

How Cold Should The Air Coming From My Air Conditioner Be?

Manufacturers design central air conditioners to supply air into your home at a fixed Delta T of 15ºF to 20ºF colder than the return air temperature. 

When your supply vents have air that is 15ºF to 20ºF cooler than the return air temperature, that means the AC unit is working correctly. 

Here is how to calculate the Delta T temperature differential: 

Return Temperature – Supply Temperature = Delta T

For example, if you measure an air temperature of 80ºF in your return vent and 60ºF in your supply vent (when the AC is on), your Delta T is 20ºF (80º – 60ºF = 20ºF). In this case, your air conditioner is producing the ideal temperature difference and is working correctly.

On the flip side, if your return temperature is 80ºF and the supply temperature is 75ºF, your Delta T is only 5ºF. This indicates an issue with the air conditioner and requires an inspection by an HVAC professional. 

What Causes Supply Vents To Blow Out Warm Air?

Central air conditioners are not flawless machines. In their lifetime, they may develop an issue that takes it offline until an HVAC technician repairs it. One of these issues that could pop up is the blowing of warm air through the supply vents when your AC is running. 

You can verify it is blowing warm air by performing the Delta T calculation (return minus supply temperature).

If your air conditioner is blowing warm air, give our team of HVAC experts a call today for help. 

If it is blowing warm air, here are some potential causes: 

Low Refrigerant 

The refrigerant is like the lifeblood of the air conditioner. It is critical for moving the heat from inside your home to the outdoors. When refrigerant levels are normal, the refrigerant entering your evaporator coils is a cool, low-pressure fluid.

Warm, indoor air blows across the coils, and the refrigerant absorbs the heat. The now-warm refrigerant flows off to the condenser, where its heat leaves the refrigerant and floats away outdoors. A refrigerant leak complicates the cooling process.

When there is low refrigerant, this process fails, and the heat cannot transfer successfully. In this case, the air conditioner would blow out warm air. 

Failing Compressor 

Another common reason for an air conditioner blowing warm air is a failing compressor. If the refrigerant is the lifeblood of the AC, then the compressor is the heart. It pumps the refrigerant around the closed-loop system. 

If the compressor starts failing and inhibits the refrigerant flow, it will slow or stop the heat transfer and cause your supply air to be warm. 

Bad Thermal Expansion Valve

The thermal expansion valve controls how much refrigerant flows to the evaporator coils in your air conditioner. It also controls the pressure and temperature of the refrigerant, which is vital in the heat transfer process. 

If the thermal expansion valve has an issue, it can get stuck and slow the flow of refrigerant into your evaporator coils. When this occurs, it could lead to your AC unit pumping warm air into your home. 

How Can You Keep Your Home Cool In Warmer Months?

Homeowners could implement various practices and improvements to keep their homes cooler in the summer months. Here are some steps you can take immediately to keep your home cool. 


Hot air leaking into your home from the outdoors counteracts all the hard work your air conditioner does to cool your home. To reduce your cold air loss to the outdoors, make sure you properly seal common escape paths. These include windows, exterior doors, garage doors, fireplaces, attic accesses, ductwork, and more. 


Insulation prevents hot air from the outdoors from getting into your home. To improve your insulation, start with the attic. You can add fiberglass or expanding spray foam insulation to the ceiling and blown-in insulation to the floor. 

If your exterior walls do not have adequate insulation, you cannot do much about it unless you’re willing to remove your drywall. Lastly, consider replacing your old windows with double-pane versions to increase your insulation. 


Improving the shade of your home with foliage is a long-term strategy that could really pay off. Trees strategically planted to block direct sunlight will reduce the amount of infrared radiation your home receives. Therefore, it won’t warm up as much in the summer months. 

Another shading tactic is to keep your blinds and curtains closed. It’s the same idea as planting trees; it blocks out the sun’s heat. 

Get an Attic Fan

When the sun beats down on your roof on a bright day, it warms your attic. The heat from the attic then warms the rest of your home. With an attic fan, you can have this hot air routinely exhumed to help keep your home cool. 

Maintenance Practices To Keep Your AC System Working Properly

An air conditioner with proper maintenance will continue to cool your home for years with minimal issues. Failing to perform routine maintenance and tune-ups can lead to costly repairs and complete equipment failure. 

Here are the best maintenance practices.

Replace the Filter

The HVAC air filter collects dust, dirt, and other particulates from the air in your home. In fact, all the air blowing out of your supply vents passes through the air filter first. When it gets too dirty, it will clog and inhibit airflow. This also puts undue stress on the HVAC system, which leads to greater wear and tear. Dirty filters also increase energy bills.

Check the filter monthly and replace it before it is entirely saturated with dust. Depending on the filter and the indoor air quality of your home, you should replace it every 3-12 months. 

Clean the Condensing Unit

The condensing unit is also known as the outdoor unit. It is the big cube with a fan on the top that sits near your home outside. The inside of the condenser houses the compressors and other components. 

Over time, the condenser coils will trap dirt, leaves, twigs, and other outdoor debris. Follow our condenser cleaning guide for details on how to clean it yourself, or call our HVAC pros for assistance. 

Schedule Routine Maintenance

To ensure your air conditioner gets the best care, schedule routine maintenance to ensure it runs efficiently and to prevent any unnecessary failures. Our HVAC technicians are highly trained and skilled in HVAC tune-ups. They can oil blower motors, inspect fan belts, service the compressor, inspect electrical components (relays, capacitors, etc.), and keep your air conditioning system running like new.

Meet Your HVAC Expert

Lester Mclaughlin

HVAC systems are highly technical and often is the most misunderstood part of the house. From ductwork to heat pumps, I've been exposed to all sorts of issues facing homeowners. It really irks me when a homeowner is given bad advice like refilling freon vs fixing a leak in the system. I'm here to help our website readers with their heating and a/c problems.
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