HVAC DELTA T (ΔT) Explained for Air Conditioners!

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? For HVACR purposes Delta T can be referred to as the change in temperature that any medium (air, refrigerant or water etc.) experiences between two measurable points. It is important to note that for this measurement to be accurate it is critical for the medium to remain consistent and free of infiltrations from unwanted sources. 

Unwanted sources of infiltration can include, but are not limited to:

• Holes and tears in ductwork

• Radiant heat gains from attics 

• Any source of undesired heat gains or losses 

• 
  

The purpose of measuring Delta T is to allow the technician to develop some theories about deficiencies present in system operations.

Delta T matters and can be a valuable test if measured in conjunction with other appropriate diagnostic tests. It can provide the technician with a glimpse of the overall efficiency and performance of the system.

Understanding Delta T

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:

∆T  =  T1  - T2

Example

∆T  = Return air - Supply air

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. Dry bulb is the  measurement of sensible temperature and doesn’t include latent heat. Sensible heat is the heat that causes a measurable change in the substance’s temperature that can be felt while latent or hidden heat is energy absorbed or released during a phase change. Later on we'll discuss the effect humidity has on Delta T, but first let's talk about how to measure it.

Measuring Delta T

Measurements can be taken with a temp meter equipped with a k-type bead temp sensor or any other device capable of taking accurate and reliable dry bulb temperature measurements. The 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 then back it all the way out. 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. Temperature sensor placement is critical as it is important to only measure air that has crossed the indoor coil, which is free of gains caused by unwanted infiltration. It is not recommended to take measurements from return and supply registers as the air will likely be mixed with attic air due to compromised ducts.  After taking our dry bulb temperature measurements, we assign the return temp as T1 and the supply temp as T2.

For example, the temperature in the return duct may read 74°F (T1) while the temperature of the supply may be 54°F (T2).

In this case, 74°F (T1) - 54°F (T2) = 20°F therefore Delta T (ΔT) = 20°F

You can write this as ΔT = 20°F or Delta T = 20°F and either would 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 air conditioning mode may be between 18-21°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

· Indoor Humidity

· Indoor Heat Load

· Indoor and Outdoor Unit 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 15°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. Low Delta T could also be found if the indoor airflow is too high or the capacity of the evaporator coil, metering device, and outdoor unit is too low to keep up with the demand of a 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. For example, if a system has low airflow it will be unable to move the required CFM of heat laden air across the evaporator coil. 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 high 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.)

There are also cases where having a high Delta T is perfectly acceptable. In areas like Arizona that often experience conditions of high sensible with low latent loads, you might see a Delta T of 25°F. This is why understanding how conditions affect Delta T is crucial to diagnostic decision making.

Not only should the technician understand conditions, but he or she should familiarize themselves with equipment types and variations. For instance, a system equipped with an EEV (Electronic Expansion Valve) may produce a Delta T of 23°F and still be performing at acceptable levels. This is because the EEV may hold the superheat lower than that of a TXV (Thermostatic Expansion Valve), which would allow more refrigerant to stack in the coil and absorb more heat.

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. Delta T is only intended to help estimate your overall performance. While a reading between 18-21°F is an indication that heat is being transferred, it does not mean the system as a whole is performing efficiently.  In the case of a system with a piston metering device, you may measure a 19°F, but the system could be slightly overcharged and liquid refrigerant could be entering the compressor and damaging it. To determine if the compressor is receiving refrigerant that is fully vapor, measure the total superheat. If the refrigerant has several degrees of total superheat, the refrigerant is in its fully 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? 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