Sizing a Heating Circulation Pump from the Heat Load

Step 1: flow from the heat load

Water carries about 1.163 kWh per cubic metre for each degree of temperature change. So the flow in m³/h is the heat load in kW divided by 1.163 times the temperature drop across the system.

Example: a 500 kW heating system designed for a 10 °C drop between supply and return. Flow = 500 ÷ (1.163 × 10) ≈ 43 m³/h.

Step 2: head from the loop resistance

Add up the pressure losses through the boiler or heat exchanger, the pipes, valves and the worst-placed radiator or coil. In a closed loop the height of the building does not count, because the water coming down balances the water going up. Say the total comes to 18 m.

Step 3: choose the pump

43 m³/h at 18 m points to an 80 mm frame. The 80-125 is rated 50 m³/h at 20 m with 5.5 kW, and covers the duty with a small margin. For hot water, it is built as an IRG.

Then check

• The highest water temperature, which decides the seal
• Static plus pump pressure, within 1.6 MPa
• NPSH at the hottest water, especially if the pump sits near the boiler outlet

All frames are in the selection table.