What Should the Air Delta T be? (Air Temperature Split)

Bryan Orr asks the all-important question: "What should the air delta T be?" The discussion in this video is all about finding the correct delta T or air temperature split for a residential HVAC system.

No matter what you call it, the delta T or air temperature split is the difference between the air temperature as it enters the evaporator coil and the air temperature as it leaves the evaporator coil. You can't get an accurate delta T from return and supply grilles because heat gains and losses occur in the ducts. You also don't want to use an infrared thermometer to measure the supply and return temperature at vents (even though home inspectors tend to do that rather often).

When measuring your air temperature split or delta T, your temperature probes should be before and after the evaporator coil but far enough away from the coil to allow for proper air mixing. In other words, you don't want the coil surface temperature to interfere with your measurements.

An old rule of thumb requires there to be a 20-degree delta T, but that's too simple to be a hard and fast rule. Generally, the delta T will be between 14 and 23 degrees, but that number can vary wildly depending on blower type and load conditions.

HVAC equipment doesn't just remove sensible heat; when water condenses on the evaporator coil, some latent heat is removed from the air. Latent heat is still part of the load, but it doesn't affect the actual air temperature. When your relative humidity is higher, you can expect there to be more latent heat removal, and the delta T will decrease due to the equipment's focus on removing latent BTUs over sensible BTUs. A system running outside its usual running conditions (especially if the capacity is affected) will also affect the split.

It's also worth noting that many of our rules of thumb are only applicable for systems running 400 CFM per ton and 12,000 BTUs per ton. The operating conditions also affect the mass flow rate and compression ratio, which affects the capacity and can impact the temperature split in turn. Variable speed systems that also have a dehumidification mode will also have a delta T that differs from the norm.

Overall, we can view delta T as a ratio between compressor mass flow rate (higher capacity = higher delta T and vice versa) and airflow (higher airflow = lower delta T, lower airflow = higher delta T). Relative humidity also tends to have an inverse relationship with delta T (higher RH = lower delta T). Taking the delta T by itself can't tell you that much about the system, but using the delta T with other valuable measurements (like superheat and subcooling) can point you in the right direction when diagnosing a system.

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