DEV Community

Cover image for Quick Tip: HVAC Delta T Ranges You Should Know
Evgenii Konkin
Evgenii Konkin

Posted on

Quick Tip: HVAC Delta T Ranges You Should Know

#hvac #engineering #quicktip #diagnostics

Delta T is the temperature difference between return air and supply air:

ΔT = T_return − T_supply
Enter fullscreen mode Exit fullscreen mode

Normal ranges by system type:

System                    ΔT (°F)
──────────────────────────────────
Residential cooling (DX)  14–22
Gas furnace               40–70
Heat pump (heating)       25–35
Chilled water             10–12
Hot water boiler          20–40
Condenser water           10–15
Enter fullscreen mode Exit fullscreen mode

What out-of-range delta T tells you

In cooling mode:

ΔT < 14°F → low refrigerant charge
             excessive airflow
             dirty evaporator coil

ΔT > 22°F → restricted airflow
             dirty filter
             frozen coil
Enter fullscreen mode Exit fullscreen mode

In heating mode (gas furnace):

ΔT > 70°F → low airflow through heat exchanger
             potential cracking risk
Enter fullscreen mode Exit fullscreen mode

From delta T to capacity

Combine delta T with airflow to get sensible capacity:

Q (BTU/hr) = 1.08 × CFM × ΔT
Enter fullscreen mode Exit fullscreen mode

Example:

CFM = 1,000
ΔT  = 20°F

Q = 1.08 × 1,000 × 20
Q = 21,600 BTU/hr
Q = 1.8 tons
Enter fullscreen mode Exit fullscreen mode

If the system is rated at 2 tons, it is delivering 90% of capacity. Normal.

If ΔT drops to 12°F:

Q = 1.08 × 1,000 × 12
Q = 12,960 BTU/hr
Q = 1.08 tons → 54% of rated capacity
Enter fullscreen mode Exit fullscreen mode

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)
Enter fullscreen mode Exit fullscreen mode

Breaking either rule gives a false delta T.

Top comments (0)