Delta T Explained Simply
Delta T (ΔT) is the temperature difference between the air entering your HVAC system (return air) and the air coming out (supply air). It's the most basic measurement of "is this system actually cooling or heating the air?"
Delta T = Return Air Temperature − Supply Air Temperature (Cooling)
Delta T = Supply Air Temperature − Return Air Temperature (Heating)If your AC is running and the delta T is only 8°F, something is wrong. If it's 25°F, the system might be low on airflow. The target range varies, but it's one of the fastest diagnostic checks you can make.
Interactive Delta T Calculator
Normal Delta T Ranges
| Mode | Normal Range | Ideal | Alarm Level |
|---|---|---|---|
| Cooling | 14–22°F | 18°F | <12°F or >24°F |
| Gas Heating | 30–70°F * | Varies by unit | Outside rating plate range |
| Heat Pump Heating | 15–30°F | 20–25°F | <12°F |
* Gas furnace temperature rise is specific to each unit. The acceptable range is printed on the rating plate (e.g., "Temperature Rise: 35–65°F"). Always check the plate.
What Causes Low Delta T (Cooling)?
- Low refrigerant charge — The #1 cause. Not enough refrigerant in the evaporator = not enough heat absorption.
- Dirty evaporator coil — Insulation layer of dirt reduces heat transfer.
- Fan speed too high — Air moves through the coil too fast to absorb enough heat. Common on multi-speed systems set to the wrong tap.
- System just started — Give it 15 minutes to reach steady state before measuring.
What Causes High Delta T (Cooling)?
- Low airflow — Dirty air filter (the #1 cause), collapsed ductwork, closed registers, failing blower motor or capacitor.
- Undersized return ductwork — The return can't move enough air to the system.
- Duct leaks — Conditioned air escaping into the attic/crawlspace.
HVAC IQ Pro uses two temperature probes — one on the supply duct and one on the return — to calculate delta T automatically every 60 seconds. If delta T drifts outside the normal range, you get an alert on your phone before the customer calls.