What Temperature Should My Central AC Be Blowing Out?

Curious about what temperature your AC should be at and if you’re setting it too low or too high? Wondering if there’s an energy-efficient temperature you should set?

Your personal comfort should always come first, so there isn’t a universal fixed temperature recommendation. However, it’s best to have a 16 to 22°F difference between your supply air and return air temperature when you set your thermostat.

HVAC professionals call this the “evaporator Delta T.” When a temperature difference is within the Delta T, your AC is working properly. When the difference falls outside this range, your AC might struggle to perform effectively.

To further explain, in this blog, we’ll review:

What exactly is the difference between supply and return air temperature? And why does it matter?

Simply put, all homes have both supply and return air vents that take in warm air and push out cooled air that the air conditioner has treated.

Return air vents draw in warm air from your home and into your HVAC system. Once the air is cooled, it gets pushed back into the ducwork and re-enters your home via the supply ducts, which is typically the only time you’ll “feel” your vents working.

When calculating your evaporator Delta T, an HVAC technician is looking to see if there’s a drastic difference between the temperature of the air entering and exiting your HVAC system through the return and supply ducts. The evaporator Delta T essentially measures your air conditioner’s efficiency at handling hot indoor air temperatures. In specific, we’re measuring the efficiency of an air conditioner’s evaporator coil.

Each air conditioner has an evaporator coil that houses a cold chemical refrigerant that absorbs the warmth in your indoor air so that your AC can reach your set thermostat temperature. An efficient evaporator coil can better close the gap between unconditioned and conditioned air temperature so that an AC won’t need to strain to cool your home.

Learn more in our blog, "What Is a Central Air Conditioner Evaporator Coil?"

So, now that you know the difference between supply and return air temperatures, let’s calculate your home’s evaporator Delta T.

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

In most cases, this indicates that something is not working correctly inside your air conditioner. Properly performing air conditioners would easily have a Delta T that meets this range.

Let’s look at some common problems.

If you have a high Delta T with more than a 22°F difference, the gap between the incoming (return) air temperature and outgoing (supply) air temperature is excessively large. Your AC is most likely blowing hot air at this point. It's primarily due to low airflow entering your AC system and passing over your evaporator coil.

When there isn’t enough warm air entering the air conditioner, the evaporator coil won’t be able cool enough air to meet your desired temperature. The main reasons for restricted airflow are:

• Dirty air filters
• Blower motor set to the wrong speed
• Dirty evaporator coils
• Improperly-sized ductwork

You can try replacing your air filter (if it’s dirty) to see if that reduces the Delta T. Otherwise, contact an HVAC professional to clean your evaporator coil, increase the blower motor speed, and inspect your system for problems.

Learn more about how you can troubleshoot in our blog, “Why Is My Central Air Conditioner Blowing Hot Air? A Phoenix Pro Explains."

If you have a low Delta T with less than a 16°F difference, there is an excessively small difference between the incoming (return) air temperature and outgoing (supply) air temperature. A small Delta T can indicate that your AC has:

• Low refrigerant levels
• Weak compressor valves
• Leaking reverse valves
• Leaking return ductwork

You’ll want to hire an HVAC professional to check your AC system for refrigerant leaks as the substance is toxic and inspect your valves and ductwork for any leaks to repair.

Learn more about how you can tell if you have low refrigerant levels in our blog, "4 Signs Your AC May Need a Refrigerant (Freon) Charge."