How to Select a Pump Group by Boiler Output and ΔT
The most common mistake is selecting a pump group by guesswork. We show you a simple formula that saves you from noise, excess electricity costs, and cold remote circuits.
A pump group is selected not by pipe diameter but by the calculated flow rate of the heat-transfer fluid. The formula is straightforward:
G = Q / (1.16 × ΔT), where G is the flow rate (m³/h), Q is the circuit output (kW), and ΔT is the temperature difference between supply and return (°C).
Example
Radiator circuit of 24 kW, ΔT = 20 °C: G = 24 / (1.16 × 20) ≈ 1.03 m³/h. For underfloor heating, ΔT = 5–7 °C is used, so the flow rate for the same output will be approximately three times higher.
Common Mistakes
- Using ΔT = 20 for underfloor heating — the pump delivers insufficient flow and remote loops stay cold.
- Installing an oversized pump »with margin« — noise, cavitation, unnecessary power consumption.
- Ignoring the hydraulic resistance of the circuit (head) — the pump never reaches its operating point.
How Termojet Solves This
Termojet pump groups feature a built-in balancing valve that allows precise adjustment to the calculated flow rate after installation. And the energy-saving APM/APE pumps with electronic control automatically maintain optimal speed, saving electricity.
Select your group by flow rate and head, not by habit — and your system will run quietly, evenly, and efficiently.
