How Cold Should Air From AC Be? (Temperature Differential)

One thing you may ask is how cold should the air be coming out of a mini-split or other type of AC be? Maybe you are feeling that it’s still too hot inside your home, even though the AC is on. But you don’t want to lower the AC’s temperature even further and end up spending too much money on electricity.

Finding the balance between a comfortable home and a reasonable energy bill with an efficiently running AC unit can be a tricky task. But we can help! Keep reading to learn how to calculate your home AC temperature differential. This will help you set your thermostat to the right temperature and keep your AC functioning properly.

How Cold Should The Air Coming From The AC Be?

When turning on your AC for the summer, you may ask what air temperature should be coming out of the vents when cooling. Unfortunately, there is not a universal answer to this question.

A good rule of thumb to live by is having your thermostat set at 20℉ below the outside temperature. For example, if it is a hot, 90℉ day, you do not want your thermostat set lower than 70℉.

An air conditioner begins its work by taking up air at a return vent. The air then runs through the ductwork of your home. Here is where it reaches the evaporator coil. The evaporator coil is what cools the air in your home. It passes through a fan, onto the evaporator coil, then reenters your home through the supply air vent.

The AC unit air temperature output all depends on the efficiency of your evaporator coil. A dirty coil, freon leak, or leaking reverse valves may cause an AC temperature differential too low.

A dirty air filter, dirty evaporator coil, or a fan set at an incorrect speed may cause an AC temperature differential that is too high.

daikin air conditioner

Temperature Differential Explained: What Is It?

In short, temperature differential, or Delta T, is the difference between the air temperature at the supply vent and the return vent. Your Delta T should be between 16℉-22℉. If your temperature differential is out of this range, something is not working.

You can calculate the temperature differential yourself or can hire a professional to do it for you. The steps to calculate your Delta T are simple, needing only the right tools. To calculate temperature differential, you will need a temperature probe, a piece of paper and pen, and a calculator.

To calculate, take your temperature probe and go to the return vent. Record the temperature with the probe at the return vent.

Find three supply vents and record the temperature at all three. Find the average of the three supply vents by adding the three temperatures and dividing by 3.

Subtract the return vent temperature from the average temperature of the supply vents. The difference between these two is your Delta T or temperature differential.

It is good to know your temperature differential. It is used to determine whether your air conditioner unit is properly working. Checking your Delta T will help you catch a problem in your air conditioner early. It can result in saving your air conditioner from breaking down.

Temperature Difference Between Supply and Return

A supply air vent is where the air that has already passed through your evaporator coil reenters your home. The return air vent is where the air is pulled from your home into the ductwork. It then goes through the fan and evaporator coil.

You may ask yourself, what should the temperature difference between supply and return be? As mentioned earlier, the ideal range is between 16℉-22℉. You do not want your Delta T to differentiate from this range. If you set your thermostat too low on a hot day, you require your air conditioner to work too hard.

For example, if the air at your return vent is 98℉ and your thermostat is set to 68℉, you ask your air conditioner to have a temperature differential of 30℉. This high-temperature change will make your air conditioner work overtime and eventually burn out much faster than if set at a higher temperature.

The Delta T Issues

Your Delta T number indicates the efficiency and workings of your air conditioner. If your AC temperature differential is too high, this can indicate multiple issues. If your AC temperature differential is too low, this means other problems.

A high-temperature differential (more than 22℉ difference) indicates one of the following issues:

  • A dirty air filter
  • A dirty evaporator coil
  • Too small ductwork
  • Incorrectly set fan speed

If this is the case, the first thing to try is replacing your air filter. It is a simple and inexpensive way to fix a Delta T that is too high. If this does not reduce the temperature difference, hire a professional to increase the blower speed of your fan, clean the coil, and inspect your system for other problems.

A low-temperature differential (less than 16℉ difference) indicates one of the following issues:

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

The only way to fix a low-temperature differential is by hiring a professional. This professional will check your system for refrigerant leaks and inspect the ductwork and valves.

Ameristar HVAC Technician

Can I Make My Air Conditioner Blow Cooler Air? (Here’s How)

If you just cannot get your air conditioner to cool your home, you may be wanting to try to get your air conditioner to blow cooler air. Is this a good idea?

Remember that your AC unit should only support a maximum temperature drop of 20℉ between your supply vents and your return air. So, it is not advised to simply lower your thermostat setting to get your air conditioner to blow cooler air.

There are steps you can take to get your air conditioner working most efficiently. These steps may allow your AC to blow cooler air.

Another way to cool your home in the hot summer months without overworking your air conditioner is the use of fans. Whether ceiling fans or box fans, fans are a great way to cool your home without increasing your energy bill.

In the evenings, when the air is cooler, open the windows in your home. This allows the natural, cool breeze into your home. Putting a box fan into the window provides great circulation of the cool air into your home.

If you have an outdated air conditioner, also consider upgrading to a newer model to get cooler temperatures. Even with more of an outright cost, getting a new air conditioner may save money in the long run due to better efficiency.

People Also Ask (FAQ)

Here are frequently asked questions on AC temperature differential.

What is a differential temperature controller?

A differential temperature controller is a device that is installed in the infrastructure that controls temperature responses automatically.

How cool should my house be if it’s 100℉ outside?

Remember, the Delta T should not be over a 22℉ difference. However, this does not mean your house has to be 78℉ exactly. Your air conditioner has already done some work and has kept the temperature inside your home a little cooler than outside. With this being said, the air at your return vent is probably cooler than 100℉. Set the thermostat from 75-78℉.


Setting your thermostat to the right temperature can be confusing. Remember to not overwork your air conditioner unit, and be sure to keep your Delta T between 16℉-22℉. This range means your AC unit is working correctly, most efficiently, and not overworking itself.

Josh Mitchell

Josh Mitchell

My name is Josh and I am obsessed with DIY and improving my family home. HVAC topics can be tricky for homeowners so I decided to share my knowledge on the subject. When I am not working on DIY projects, you can find me at the beach or my local coffee shop.