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.
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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.
Simply put, supply air is the air entering your home through the registers/vents pictured above.
The air then goes back into your ductwork via the return vent, is cooled, and reenters your home through the supply registers.
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
- Get a temperature probe
- Go to your return vent and record the temperature with the probe
- Go to 3 supply vents and record the temperature
- Find the average temperature of the 3 supply vents (add the temperatures and divide by 3)
- Subtract the return vent temperature from the average supply vent temperature to get Delta T
“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.