Mass Flow Rate & Pump Sizing Calculator

Mass flow rate (kg/s) is calculated from volumetric flow rate using the formula: mass flow = (flow rate in L/min × 0.001 × fluid density) / 60. Once you have mass flow rate, thermal power is Q = mass flow × Cp × ΔT, where Cp is the specific heat capacity and ΔT is the temperature difference across the heat exchanger.

The required flow rate depends on the heat pump's thermal output and the temperature difference (ΔT) across the system. For a 15 kW heat pump with water at a typical 5°C ΔT, you need approximately 43 L/min. At a 10°C ΔT, the flow rate halves to about 21.5 L/min. Use the "Find Flow Rate" tab above with your specific parameters.

The optimal pipe velocity for hydronic heating and cooling systems is between 1.0 and 2.0 m/s. Below 0.5 m/s, air entrainment becomes a problem. Above 3.0 m/s, you risk pipe erosion, noise, and excessive pressure drop. Use the "Velocity & ΔP" tab to check your pipe sizing.

Pressure drop is calculated using the Darcy-Weisbach equation: ΔP = f × (L/D) × (ρ/2) × v², where f is the Darcy friction factor (typically 0.02–0.04), L is pipe length, D is internal diameter, and v is fluid velocity. The calculator automates this and shows results in kPa.

Use the "Time to Heat/Cool" tab. The formula is Energy (kJ) = volume (m³) × density × Cp × ΔT, then Time = Energy / Power. For example, heating 500 litres of water by 40°C with a 15 kW heat pump takes approximately 56 minutes, not accounting for standing losses.