HVAC Delta T Calculator (ΔT, Heat Transfer & Flow)

Delta T (ΔT) is the temperature difference between the supply and return of an HVAC fluid — the driver of every heat-transfer calculation. This calculator finds ΔT and the heat transferred using Q = ṁ · cp · ΔT for chilled water, hot water and air.

A larger design ΔT lets a system move the same heat with less flow, cutting pump and fan energy — the basis of low-flow / high-ΔT design. Typical values: chilled water 5–7 K, condenser water 5 K, LPHW heating 10–20 K, and air across a cooling coil 8–12 K.

Chilled water entering a coil at 12 °C and leaving at 7 °C gives a ΔT of 5 K. At 5 L/s that removes about 104.6 kW (≈29.7 TR).

How the HVAC Delta T Calculator Works

The calculator takes the difference between your supply and return temperatures to give ΔT, then multiplies the mass flow (flow rate × fluid density) by the fluid specific heat and ΔT to give the heat transfer in kW, also expressed in tons of refrigeration and BTU/h. Selecting Water or Air switches the specific heat and density used.

Why Delta T Matters

• Higher design ΔT means lower flow, so smaller pipes, pumps and pump energy.
• Low ΔT at part load is the classic symptom of oversized flow or fouled coils.
• Air-side ΔT and water-side ΔT are linked at the coil by the same heat-transfer rate.