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Why Delta T is the Magic Number for Proactive Diagnostics and Problem-Solving

Delta T, or the difference between return air temperature and supply air temperature, is one of the most commonly used measurements in the HVAC industry – and for good reason. Monitoring Delta T over time can clue you in to things like sub-optimal system performance, unnecessary energy usage or impending equipment failure, helping you address these issues proactively instead of reactively.

Calculating Delta T is simple: just subtract the return air temperature from the supply air temperature. The difference, or delta, between the two is Delta T. Thermostats like the Honeywell Home Prestige® IAQ that come with duct sensors can automatically calculate Delta T for you and send the homeowner alerts for abnormal measurements, provided you set appropriate fault limits.

Ideal Delta T limits for heating and cooling

On the cooling side, the ideal Delta T range varies depending on who you ask, but a good rule of thumb is between 16F and 22F. On the heating side, the ideal Delta T range varies by system, so check the data plate on the furnace to see the temperature rise minimum and maximum (it’s usually a 30-degree spread). Generally speaking, if Delta T is high, it can indicate poor airflow. If Delta T is low, it can indicate poor system performance or capacity.

Using Delta T to diagnose heating issues

Low Delta T: If Delta T is on the low end of the recommended range for the heating system in question, the air coming off the furnace will likely feel cool to the homeowner, and they might complain about drafts. If Delta T drops below the low end of the recommended range, condensation could start to form in the primary heat exchanger, eventually causing rust and limiting its life span. Try slowing the blower down.

High Delta T: When Delta T is near the high end of the recommended range, the furnace can start cycling on the limit control, which can lead to overheating and component damage. The system might not be moving enough air, but that can be remedied with basic adjustments.

Using Delta T to diagnose cooling issues

 Low Delta T: Delta T below approximately 16F can indicate a number of potential issues, including:

  • Undercharge or low refrigerant levels
  • A malfunctioning metering device
  • Too much airflow through the evaporator
  • Abnormally high humidity
  • Liquid line restriction
  • A malfunctioning compressor
  • Leaking reverse valves
  • Leaking return air ducts

If you’re seeing a low Delta T and you’re not sure where the issue is, try checking the air filter, humidity levels and system charge for starters.

High Delta T: Delta T above approximately 22F can indicate a couple issues, including the following:

  • Low air flow resulting from a dirty filter, evaporator or blower wheel, not enough supply ducts or a too-small return
  • Abnormally low humidity
  • A blower that’s not running the correct speed (or running backward)
  • If you’re seeing a high Delta T and you’re not sure where the issue is, try checking the air filter, humidity levels or blower motor to ensure proper airflow.

Why you should monitor Delta T over time

Seeing an abnormal Delta T value can help you address an immediate problem, but monitoring Delta T over time is how you can get in front of problems before they happen. Select thermostats, like the Honeywell Home Prestige IAQ mentioned above, have a performance log feature where Delta T (and other important measurements) are tracked over time, making it easy to see the progression.

You can also program the Honeywell Home Prestige IAQ to alert the homeowner to call and schedule a service appointment when Delta T falls outside the limits you set during install. It’s one more way you can provide proactive service and help customers keep their equipment up and running.