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What Temperature Should My Central Air Conditioner Be Putting Out? A PHX Tech Explains.

Wondering what temperature your AC should be blowing into your home?

Well, there’s not a universal, fixed temperature your AC should always be blowing. The temperature your AC puts out is relative to the temperature you set on your thermostat.

So even though there’s no single ideal temperature, you do want a 16°–22° F difference from the supply air and return air. Professionals call this temperature difference the evaporator Delta T.

When evaporator Delta T is between 16°–22° F, that means your system is working properly. But if temperatures fall outside of that range, it means your AC has some issues.

In this article, we’ll go into how you can calculate evaporator Delta T and what issues you could have when air temperatures fall outside the ideal range.

Need a repair? Contact us today!

Let’s start by going into more detail about evaporator Delta T...

The difference between supply & return air temperature

Just so we’re on the same page, let’s discuss what we mean by supply and return air.

Supply vent register in a home

A supply vent register

Simply put, supply air is the air entering your home through the registers/vents pictured above.

Return grill of an air conditioner in a home

A return air grill

The air then goes back into your ductwork via the return vent, is cooled, and reenters your home through the supply registers.

Model of supply and return air process in an attic of a home

Air enters your home through the supply vent, then goes back into the system through the return grill.

The indoor AC part that actually cools your home’s warm air is called the evaporator coil (pictured as a snowflake in the image above).

When we calculate evaporator Delta T, we’re trying to see how efficiently the coil is working.

So now that you know supply vs. return air, you can calculate evaporator Delta T:

How to calculate evaporator Delta T

  1. Get a temperature probe
  2. Go to your return vent and record the temperature with the probe
  3. Go to 3 supply vents and record the temperature
  4. Find the average temperature of the 3 supply vents (add the temperatures and divide by 3)
  5. Subtract the return vent temperature from the average supply vent temperature to get Delta T

For more information, watch this helpful YouTube video about calculating Delta T. Of course, you can always have a professional calculate Delta T for you.

“My Delta T doesn’t fit into the 16F°- 22F° range. What does that mean?”

The short answer is, it means something’s not working correctly in your AC. Let’s look at some common problems:

High Delta T issues (more than 22F° difference)

High evaporator Delta T means that the incoming temperature and outgoing temperature is excessively large. It’s usually caused by low air flow across the coil, which includes problems like:

  • A dirty air filter
  • Fan set to an incorrect speed
  • A dirty evaporator coil
  • Ductwork is too small

Learn more about why your AC is blowing hot air.


Low Delta T issues (less than 16F° difference)

Low evaporator Delta T means that there’s an excessively small difference between the incoming and outgoing temperature. Low evaporator Delta T problems include:

  • Low refrigerant (Freon) levels
  • Weak compressor valves
  • Leaking reverse valves
  • Leaking return air ductwork


  • Hire a professional to check your system for refrigerant leaks and inspect your valves and ductwork.

Is your AC not blowing at the correct temperature?

Contact George Brazil to schedule an air conditioning repair. We will send one of our trusted professionals to your home to get your AC back in tip-top shape