Hydronic heat duty

Calculate sensible heat transfer from measured liquid flow and temperature difference.

How this calculation works

A measured liquid flow and temperature difference can be converted into sensible heat transfer when the fluid properties are known. Enter volume flow in liters per second, density, specific heat and the magnitude of the temperature change. The calculator first changes liters per second to cubic meters per second and multiplies by density to obtain mass flow. Multiplying mass flow by specific heat and temperature difference gives duty in kilowatts. Use properties suitable for the actual liquid, concentration and mean operating temperature; water and glycol mixtures should not automatically share one set of values. Because temperature difference is entered as a magnitude, the output does not indicate heat-flow direction. The balance assumes a single liquid phase and does not include pipe losses or choose equipment size.

Inputs and units

Liquid volume flow (L/s)
Liquid density (kg/m³)
Liquid specific heat (kJ/(kg·K))
Temperature difference magnitude (K)

Method and formula

m = density × volume flow/1000; duty = m cp ΔT, with volume flow in L/s and cp in kJ/(kg·K).

Worked example

Example inputs

Liquid volume flow: 1 L/s
Liquid density: 1000 kg/m³
Liquid specific heat: 4.18 kJ/(kg·K)
Temperature difference magnitude: 5 K

Calculation steps

Liquid mass flow = 1000 × 1 / 1000 = 1 kg/s.
Heat-duty magnitude = 1 × 4.18 × 5 = 20.9 kW.

Example results

Heat-duty magnitude: 20.9 kW
Liquid mass flow: 1 kg/s

Assumptions

Single-phase liquid; constant supplied density and heat capacity.

Limitations

Use properties for the actual water/glycol concentration and mean temperature.
No phase change, pipe loss or equipment sizing.
Preliminary educational check; verify inputs and equipment data before design or operation.

Sources

DOE Fundamentals: Thermodynamics: Specific heat, equation 1-17; first-law energy balance