Updated: Oct 31, 2022
Delta T is a term that gets thrown around in the HVAC field quite a bit, but what does it really mean, what is its purpose, and does it really matter?
So, what is Delta T? When it comes to removing heat in air conditioning mode, it is a measurement used to help estimate the efficiency of an air conditioning system and is measured while checking the refrigerant charge.
The first step to understanding Delta T is to look at it in terms of a mathematics or physics equation where Delta or the symbol “Δ” represents “change in” and “T” represents “Temperature”. Simply put, Delta T is a change in temperature typically displayed in the following equation:
T1 – T2 = ΔT
In terms of HVAC/R and specifically when dealing with air conditioning, we calculate Delta T as the change in the indoor dry bulb air temperature as it crosses the indoor evaporator coil. Measurements can be taken with a temp meter equipped with a k-type bead temp sensor. This tool will measure a dry bulb temperature. This bead temp sensor can be inserted into a hole in the duct created by a zip screw. To make a hole in the air duct, simply screw a zip screw into the duct and remove it from the duct. Now with a hole present, insert the bead temp sensor to take a temperature measurement. After taking the measurement, remove the bead temp sensor and screw the zip screw back in to seal the hole.
The first measurement is taken in the return air duct a few feet prior to the evaporator coil and the second measurement is taken in the supply air duct a few feet after the evaporator coil. After those dry bulb temperature measurements are read, we assign the return temp as T1 and the supply temp as T2.
For example, the temperature in the return duct may read 72°F (T1) while the temperature at the supply may be 53°F (T2).
In this case, 72°F (T1) - 53°F (T2) = 19°F therefore Delta T (ΔT) = 19°F
You can write this as ΔT = 19°F or Delta T = 19°F and either would both be correct.
At this point, we have covered how to find the Delta T, but it is still critical to understand what the Delta T should be and how to apply it in terms of servicing an A/C system.
At the beginning of this article, I explained that Delta T is used to estimate the overall efficiency of the system and is measured while checking the refrigerant charge. In an ideal situation, the target for Delta T for a single or two speed compressor system in cooling mode is between 18 to 21°F, however there are many factors to consider when looking at the final Delta T reading that may have nothing to do with the refrigerant charge of a given system. Those factors include:
· SEER Rating
· Fin Condition and Possible Deterioration
· Metering Device
· Head Load
· Air Flow
Each of these factors can impact the overall Delta T reading despite an accurate refrigerant charge.
For example, consider what it means if a system has a low Delta T?
1. A low Delta T could be an indication of a low refrigerant charge, however, that may not always be the case. If a system has a fixed orifice metering device, such as a piston or capillary tubing, the system may only reach a Delta T of 16°F but still be properly charged. This is because a fixed orifice metering device cannot allow more refrigerant into the system to compensate for a high heat load or high humidity like a TXV can. This results in a low Delta T during hot and humid days or when the system is first turned on, after being off for a while.
2. Low Delta T can be caused by outdoor unit fin deterioration or low outdoor airflow which will not allow the system to reject heat absorbed from the inside of the building.
3. If the indoor airflow is too high, the capacity of the evap coil, metering device, and outdoor unit are too low to keep up with the large heat load from the indoor airflow.
So, what does it mean if the Delta T is high?
A Delta T above 21°F could be a case of low indoor airflow. If a system has low indoor airflow, the system is unable to move enough heat from the air inside the building across the evaporator for the refrigerant to absorb. This will result in a lower supply air temperature especially on a system with a piston metering device. This will cause a higher overall Delta T. (Note: For a system with a low Delta T and a piston metering device, the refrigerant compressor could be in danger of saturated refrigerant entering it which will cause the compressor to be damaged. Check out our book or quick reference cards for troubleshooting if a problem exists.)
In the case of a mini-split unit with a VRF (variable refrigerant flow) compressor, the Delta T may be 20-24°F. This is normal for these types of systems. Mini-splits are usually designed to have less superheat in the evaporator coil which allows for more heat to be removed from within the building. (Note: These systems have an accumulator tank which safeguards the compressor while the system runs at a low superheat. To learn more, check out our video on the accumulator tank!)
So, does a Delta T between 18 and 21°F mean that a system has a correct refrigerant charge?
The simple answer is maybe. Remember, Delta T is only intended to help estimate your overall performance. While a reading between 18 and 21°F is an indication that heat is being transferred well, it does not take into account if the system is operating completely correct. In the case a system with a piston metering device, you may measure a 19°F but the system could be slightly over-charged and liquid refrigerant could be entering the compressor and damaging it. To determine if the compressor is getting fully vapor refrigerant to enter, measure the total superheat. If the refrigerant has several degrees of total superheat, the refrigerant is presently fully in the vapor state. The only way to effectively measure the refrigerant charge of a system is with total superheat and subcooling measurements. Delta T alone should not be solely relied on to determine the refrigerant charge level or for troubleshooting. Delta T should be one of multiple measurements taken when servicing a system in order to get an overall idea of how the system is operating.
Finally, does the Delta T really matter? Yes! While it should not be a sole source for checking a system’s performance, it can play a major role in getting the most accurate picture of all aspects of a system’s health, beyond the refrigerant charge. At the end of the day, Delta T is another tool in a technician’s pocket and can make the difference when trying to troubleshoot a system.
Can’t get enough on Delta T? Stay tuned for our article on Delta T for heating coming out soon! Also check out our Video "HVAC Delta T Explained! What Temperature Should it be?"
Published: 3/25/2020 Author: Craig Migliaccio
About the Author: Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Be sure to check out Our Book and Other Products Here! Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 17 years and holds an NJ HVACR Master License. AC Service Tech LLC creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel