I was recently asked for a formula to determine the temperature drop between the return air and supply air of an air conditioning system. While it is logical to check the temperature drop, determining exactly what it should be is not as simple as plugging in readily available numbers into a formula. Two operating conditions can have a pronounced effect on the results: the relative humidity of the return air and the amount of airflow. Most air conditioning systems condition the air two ways. They cool the air, referred to as sensible cooling; and they take water out of the air, referred to latent cooling. Only sensible cooling creates a temperature drop. Removing water from the air takes system capacity. The more water the system removes from the air, the less capacity is left for reducing the air temperature. Standard airflow is 400 CFM per ton for most systems, but that does not mean your system is actually operating at 400 CFM per ton. If you move less air across the coil, the air will be cooled a little more. To determine the temperature drop you must know the outdoor ambient temperature, the return air dry bulb, the return air wet bulb, the CFM of airflow, and the system’s sensible cooling capacity at that condition.
Take for example, a system that is removing no water out of the air operating at 100% sensible cooling with a standard 400 CFM per ton of airflow and producing 12,000 Btuh per hour. The temperature difference is calculated as TD = 12000/(400 x 1.08) = 28°F TD. If the airflow is reduced, the TD becomes 12000/(350 x 1.08) = 32°F. Increasing the airflow would make the TD 12000/(450 x 1.08) = 25°F. In humid climates, the latent capacity can easily be as much as one third of the total capacity, reducing the sensible cooling capacity to 8,000 Btuh. These same airflows would then give TDs of: 8000/(400 x 1.08)=19°F, 8000/(350 x 1.08)=21°F,8000/(450 x 1.08)=16°. In reality, these TDs would be a little off because the overall system capacity would be a bit less with the decreased airflow and a bit more with increased airflow. The system capacity will also change depending upon the outdoor ambient. The 12.000 Btuh per ton is a nominal figure based on the AHRI rating condition of 95°F outdoor ambient, 80°F indoor dry bulb and 67°F wet bulb.
You can try to account for duct gain by reducing the expected TD by some amount: say 3°F - 5°F. However, it is really difficult to use TD at the registers because the duct gain from one system to another can vary a lot. Ducts in the attic will pick up more heat than ducts in a crawl space. Duct leakage also has a big effect. If 10% of the air entering the coil comes from a 150°F attic, that obviously will affect the delivered air temperature. The rule of thumb people have used for many years is a TD of 15°F to 20°F across the coil, not at the registers. Looking at the above calculations, you can see where this comes from. However, it is also easy to see how little you actually know if you don’t really know all the operating conditions, the system airflow, and the system capacity at those conditions. If all you do is measure the return and supply air temperatures at the registers, you don’t really know much.