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Understanding Temperature Differential in HVAC Systems

What is a Temperature Differential?

Temperature differential (also called temperature split, Delta T, ∆T, or DT) refers to the difference between the return air temperature and the supply air temperature in an HVAC system. Monitoring the temperature differential over time can provide valuable insights into system performance, energy efficiency, and potential equipment failures.

What How to Calculate Temperature Differential

To determine the temperature differential, subtract the return air temperature from the supply air temperature:

Temperature Differential = Supply Air Temperature - Return Air Temperature

Definitions

  • Return Air: Air drawn into the HVAC system from the home to be conditioned, typically through return vents located throughout the home.

  • Supply Air: Cooled or heated air distributed into living spaces through supply vents or registers.

Acceptable Temperature Differential Ranges

The correct temperature split range depends on the type of HVAC system (heating vs. cooling):

  • Cooling Systems: An acceptable temperature differential typically falls within 14ºF - 22ºF.

  • Heating Systems: Temperature rise requirements vary by system; check the data plate on the furnace for the manufacturer’s minimum and maximum temperature rise (often around a 30º spread).

Why is Temperature Differential Important?

Monitoring temperature differential is a critical diagnostic tool for home inspectors and HVAC professionals. Deviations from the expected range may indicate:

  • Sub-optimal system performance

  • Unnecessary energy consumption

  • Improper refrigerant charge (for cooling systems)

  • Dirty air filters or clogged coils

  • Ductwork obstructions or leaks

  • Impending system failure

What to Do If the Temperature Differential is Outside the Normal Range

If the measured temperature differential falls outside the expected range, it may indicate an issue requiring further inspection. In such cases, inspectors should add a defect to the report to inform the client of any potential issues with the system.