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A Split in Diagnosis

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A critical element when evaluating a cooling system’s performance is the temperature difference between the return and supply air. A good temperature difference can indicate that the cooling system is performing well, and the problem with comfort is related to something else like the ducts, or an issue with the dwelling. However, most technicians use a rule of thumb that has no relation to the equipment’s performance or the indoor/outdoor conditions. To determine if the temperature difference is acceptable, the technician must know how the manufacturer expects the equipment to perform.

Taking the time during your service or maintenance visits to check airflow and the manufacturer’s data will not add much extra time to your calls. This information establishes a baseline and provides something to compare your measurements to the manufacturers’ specifications. Measure and document the system operating temperatures during every service call, you owe it to your customer to do a complete system evaluation.

In our industry, there are too many “rules of thumb.” These have been adopted and passed down from one technician to another. For anyone who has been in the business for a long time will tell you, “rules of thumb” have given birth to many bad habits. One notoriously lousy practice in the HVAC business is the typical 15-20 degree temperature split we’ve all heard about.

If you had asked me 20 years ago, “Why is it a 15-20 degree temperature split?” I would have said, “I don’t know; it is just the way I was taught.” After a while, that question bothered me. Eventually, I decided to break my bad habit and learn about temperature differences.

Cooling systems frequently have a specific temperature drop across the evaporator coil for specific operating environments. Cooling systems will react to shifts in both indoor and outdoor temperatures.The specified capacity (and associated temperature drop) are captured in the manufacturer’s performance data.

In this example, we are going to look at a system’s expanded performance data on a matched system. You will notice a different capacity for different temperatures (indoor and outdoor) and airflow. A formula that you must know as a technician or installer is the ∆T formula.

∆T= Sensible Capacity÷1.08÷CFM


Sensible Capacity: The sensible load in BTU’s.

1.08: Constant for standard air (at sea level)

CFM: Airflow specified by the OEM

Scenario 1 


During a spring maintenance check, with an 85°outdoor ambient temperature, an indoor entering wet-bulb temperature of 67°an entering dry-bulb temperature of 75°and a CFM of 1,000 the system should have a ∆T of 16.5°F. 


For this scenario, the sensible capacity is 17,900, and the total capacity is 32,100Now, let’s do the math to calculate the ∆T. 

∆T= 17,900 ¸ 1.08 ¸ 1,000 


∆T= 16.5°F 


With the information provided by the manufacturer, the required temperature difference is 16.5°F. 


If we used the “rule of thumb” 15-20° for every cooling unit, we could make a misdiagnosis or make an unnecessary adjustment to the system. The 15-20° degree rule of thumb for temperature split is not a one size fits all. There are many factors in play to arrive at the correct temperature split. If this system’s sensible ratio capacity was different, and the temperatures and airflow were also different, the delta T would be different, too. This example’s answer happened to fall into the 15-20° temperature split.  


Most manufacturers publish installation, service, and troubleshooting literature and specifications online. The first thing to check with any refrigeration issue is the flow across the condenser and evaporator. Technicians should always think about airflow during their service and maintenance visits. There is no point in taking temperature and pressure measurements on a system that doesn’t have the appropriate airflow or water flow across one or both of its coils.  


The study of HVAC is a scientific, academic discipline. Everything you tell your customers about their HVAC system must be based on logic, facts, and or credible scholarly opinion.  


The critical component in determining a systems diagnosis is airflow. If you do not have the correct airflow, the system will not operate as designed. Calculating a temperature difference requires airflow verification. When you use the manufacturer’s specifications as the basis for your diagnosis, you serve your customers well and improve your business’s reputation. 


Author’s Note: Matt Akins is ACCA’s Manger of HVACR Education. He can be reached at or 703-824-8867. 


Matt Akins
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