How the calculation works
A water coil carries a heat load in proportion to the flow rate, the fluid's ability to hold heat and the temperature difference across it:
P [kW] = Q [m³/h] × SG × cp [kJ/kg·K] × ΔT / 3.6
For plain water this reduces to the rule of thumb most HVAC engineers carry in their head — 1.163 kW per m³/h per K, or in imperial units about 500 BTU/h per GPM per °F. Rearrange it and you can pull out the flow rate from a known load, or check what ΔT a given flow will actually produce.
Practical notes
- ΔT is a design decision, not a measurement. A wider ΔT means less flow, smaller pipes and less pump energy — but a larger coil and a slower control response.
- The heat load must be the coil's water-side load. If you are starting from a room load, add the coil's fan and duct gains first.
- Flow, not ΔT, is what the control valve actually modulates. Sizing the valve from this flow rate is the next step — see the Cv / Kv calculator.
- Glycol mixtures shift both numbers. A 30 % ethylene glycol solution is roughly 4 % denser and 10 % lower in specific heat, so flow rises about 15 % for the same duty.
- Below a ΔT of about 2 K the numbers get very sensitive to sensor error, so treat wide-flow results cautiously.