Delta T is the temperature difference between return air and supply air:
ĪT = T_return ā T_supply
Normal ranges by system type:
System ĪT (Ā°F)
āāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāāā
Residential cooling (DX) 14ā22
Gas furnace 40ā70
Heat pump (heating) 25ā35
Chilled water 10ā12
Hot water boiler 20ā40
Condenser water 10ā15
What out-of-range delta T tells you
In cooling mode:
ĪT < 14Ā°F ā low refrigerant charge
excessive airflow
dirty evaporator coil
ĪT > 22Ā°F ā restricted airflow
dirty filter
frozen coil
In heating mode (gas furnace):
ĪT > 70Ā°F ā low airflow through heat exchanger
potential cracking risk
From delta T to capacity
Combine delta T with airflow to get sensible capacity:
Q (BTU/hr) = 1.08 Ć CFM Ć ĪT
Example:
CFM = 1,000
ĪT = 20Ā°F
Q = 1.08 Ć 1,000 Ć 20
Q = 21,600 BTU/hr
Q = 1.8 tons
If the system is rated at 2 tons, it is delivering 90% of capacity. Normal.
If ĪT drops to 12Ā°F:
Q = 1.08 Ć 1,000 Ć 12
Q = 12,960 BTU/hr
Q = 1.08 tons ā 54% of rated capacity
Something is wrong. The homeowner feels it.
Two measurement rules
1. Supply temp: measure 6+ ft downstream from coil (not at coil exit)
2. Wait 15+ min after startup before reading (steady-state required)
Breaking either rule gives a false delta T.
