Return air temperature minus supply air temperature = delta-T. This single number points to one of three problem categories before you touch gauges or open panels.
The three zones (cooling)
Delta-T Status What to check
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Below 14F LOW Refrigerant, coil, compressor
16 to 22F NORMAL Coil is working properly
Above 24F HIGH Airflow is restricted
Low delta-T (below 14F)
The supply air is barely cooler than return air. The coil is not absorbing enough heat.
Possible causes:
→ Low refrigerant charge (suction pressure low)
→ Dirty evaporator coil (insulated by dirt layer)
→ Failed compressor valve (low mass flow)
→ Excessive airflow (air moves too fast through coil)
High delta-T (above 24F)
The supply air is very cold. Sounds good — it is not. The coil is overcooling a reduced volume of air.
Possible causes:
→ Dirty filter (most common)
→ Collapsed or undersized duct
→ Closed damper or blocked return grille
→ Low fan speed (wrong tap, failing capacitor)
Danger: coil surface drops below 32F → ice forms
→ further restricts airflow → more ice
→ complete coil freeze in 30-60 minutes
The capacity check
Delta-T alone is not capacity. You need airflow too:
Q = 1.08 × CFM × Delta-T
Normal: 1.08 × 1,200 × 20 = 25,920 BTU/hr (2.16 tons)
Restricted: 1.08 × 800 × 28 = 24,192 BTU/hr (2.02 tons)
High delta-T partially compensates for low CFM in the formula — but the system is stressed, the coil risks freezing, and humidity removal fails.
Heating reference
Gas furnace:
Below 40F → low output (check gas pressure, ignition)
40-70F → normal
Above 70F → DANGER: heat exchanger overheat, CO risk
Heat pump:
Below 20F → check charge, defrost cycle
25-35F → normal
Two thermometers. One subtraction. The answer points you to the right system before you open anything.
Calculate delta-T and system capacity with the delta T calculator on CalcEngineer.
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