Select the correct circulation pump rather than oversizing it

Two dimensions determine the Pump’s specifications

A circulation pump is selected on the basis of exactly two parameters: the flow rate required by the system under design conditions, and the head required to achieve this. Together, these determine the operating point. All other factors – series, connection type, casing material – are derived from this.

Step 1: Determine the flow rate

The flow rate is determined by the heating load of the circuit being supplied and the intended temperature difference between the flow and return. The greater the difference, the lower the required flow rate – and the smaller the Pump. Anyone who simply carries over values from old documentation rather than calculating them for the current system often ends up oversizing the system at this stage. This effect is particularly noticeable in refurbished buildings with a reduced heating load.

Step 2: Determine the delivery head

The delivery head covers the pressure loss in the most unfavourable circuit: pipework, Fittings, valves, heat generators, heat exchangers and the valves with the required valve authority. What it does not need to cover is the height of the building. In a closed circuit, the water column in the return counteracts that in the flow; the geodetic height cancels itself out. The static system pressure is maintained by the expansion vessel, not by the Pump.

This misunderstanding is one of the most common reasons for a pump to be significantly oversized: a six-storey building is quickly interpreted as ‘a head of around six metres’, even though the actual pressure loss in the circuit is well below that.

Why the rule of thumb is almost always too high

Oversizing is rarely the result of one major error, but rather of several minor ones that add up.

• Surcharge on the heating load: To be on the safe side, the heat output is calculated to be higher – the flow rate increases accordingly.

• Addition to the pressure loss: A reserve factor is added to the calculated grant amount, often without the underlying calculation being checked again.

• Round up to the next size: As the operating point lies just above the Characteristic curve, the larger Pump is selected.

• Transferred from the inventory: The old pump serves as a reference, even though it was already too large in its own right and the system has since been refurbished.

Each step on its own seems sensible. Taken together, however, they result in a Pump that consistently operates outside its optimal operating range, forces excess pressure into the network and causes noise at the valves. Excessive margins waste energy – and continue to do so throughout the system’s entire service life.

Step 3: Select the series according to the medium and application

Once the operating point has been determined, the type of pumped fluid determines the design. At Biral, the colour in the product name indicates the series for which it is intended: RED for heating systems, GREEN for air-conditioning and refrigeration applications, and BLUE for domestic hot water circulation. In the case of air-conditioning and chilled water circuits, this is not simply a matter of preference: if the fluid temperature is lower than the ambient temperature, condensation forms – and this is precisely what the GREEN designs are designed for.

Step 4: Check using the design tools

Biral offers several options for the actual selection process.

• Quick overview: Enter the operating point and calculate the hydraulically suitable Pump directly on the website – the quickest way for individual operating points.

• PumpSelector 6: The planning software offers four ways to get started – hydraulic selection, direct pump selection, search by part number or description, and QuickSearch. It provides precise Characteristic curves and reliable data for CAD and BIM systems.

• Replacement mirror: For replacing an existing pump. This covers wet-rotor pumps used in heating, air conditioning and drinking water systems, including those from other manufacturers.

• Advice: Biral’s HVAC and plumbing specialists have been familiar with the requirements of the Swiss market for over 40 years and will work with you to determine the operating point.

Step 5: Check after commissioning

The design process does not end with the order. The PrimAX RED T2 displays the flow rate and power consumption directly on its screen; with the ModulA T2 and VivarA, you can view the same values via Bluetooth in the Live Cockpit of the Biral ONE app. Anyone checking the system after the first heating season will quickly realise if a pump has been oversized – and can at least partially compensate for this by adjusting the control mode and the characteristic curve level.

Conclusion

A correctly sized circulation pump is almost always smaller than the one calculated using a rule of thumb. Calculate the flow rate and head for the current system, consistently separate the head from the building height, and allow for reserves only once rather than at every stage. The quick design tool and PumpSelector 6 make the check a matter of minutes – and if the operating point is in the borderline range, the experts at Biral will review it with you.