Pump selection and system curves

Pumps move the water that carries heating and cooling through a building. They run for thousands of hours a year, so a pump selected far from its best efficiency point, or oversized by a generous safety factor, wastes energy for decades and often wears out early. Good pump selection starts with understanding two curves: the pump curve and the system curve.

This guide explains how those curves work, how to select a centrifugal pump for HVAC service, and how variable speed drives and parallel pumps change the selection.

The pump curve

A centrifugal pump curve, published by the manufacturer, shows the head a pump develops at each flow rate for a given impeller diameter and speed. Head is the energy the pump adds to the water, expressed in metres (or feet) of water column, or as pressure in kPa.

A typical pump curve chart includes:

• Head versus flow curves for several impeller diameters.
• Efficiency contours or curves, with a best efficiency point (BEP) for each impeller.
• Power curves showing shaft power at each flow.
• NPSH required (net positive suction head) curve.
• Minimum and maximum flow limits recommended by the manufacturer.

Head falls as flow increases. A pump selected near its BEP runs efficiently, quietly and with low radial loads on bearings and seals.

The system curve

The system curve shows the head required to move water through the piping system at each flow rate. For a closed hydronic loop, the static height of the system does not count, because water going up one side comes down the other. The head required is mostly friction, which varies approximately with the square of flow:

H2 = H1 x (Q2 / Q1)^2

In variable flow systems with differential pressure control, there is also a fixed component: the differential pressure setpoint the pump must maintain across the remote coil and valve. This flattens the system curve and shifts it upward at low flows.

For open systems, such as cooling tower condenser water, the static lift from the tower basin water level to the top of the tower distribution system must be added.

The operating point

The pump runs where its curve intersects the system curve. If the system curve was overestimated, which is common because designers add safety factors, the actual system curve is lower and the pump runs further right on its curve: higher flow, lower head, often lower efficiency and higher power than expected. Balancing valves are then throttled to bring flow back down, wasting the excess head.

Calculating pump head

1. Identify the critical path: the circuit with the highest total pressure loss, usually the most remote or most heavily loaded branch.
2. Add pipe friction losses for straight pipe at design flow, using friction charts or software.
3. Add fitting losses: elbows, tees, valves, strainers, using equivalent lengths or loss coefficients.
4. Add equipment losses: chiller or boiler, coil, control valve, balancing valve, heat exchanger, from manufacturer data.
5. Add control valve pressure drop sized for adequate authority.
6. Apply a modest, documented allowance only where real uncertainty exists, not a blanket percentage at every step.

Stacking a safety factor on each item can easily inflate pump head well beyond what is needed. The guide on hydronic piping sizing and velocity limits covers friction rates.

Selecting the pump

Aim near the best efficiency point

A common target is to select the design point slightly to the left of the BEP, often within about 85% to 105% of BEP flow, so that as the system operates with less head than calculated, the pump moves toward the BEP rather than away from it. Many manufacturers recommend a preferred operating region around the BEP.

Avoid the largest and smallest impellers

Selecting with an impeller somewhat below the maximum diameter for the casing leaves room to increase capacity later. Very small impellers in a large casing are inefficient.

Check NPSH

Net positive suction head available (NPSHa) must exceed NPSH required (NPSHr) with margin to avoid cavitation. In closed HVAC systems with proper pressurization this is rarely a problem, but it matters for open condenser water systems, pumps drawing from tanks, and high-temperature water. Cavitation sounds like gravel in the pump and erodes impellers.

Choose pump type

Motor sizing

Select the motor to avoid overload across the full operating range of the curve, called non-overloading selection, particularly if the pump might run out to the right of design. Premium efficiency motors are required by Canadian energy efficiency regulations for many motor sizes.

Variable speed pumping

Variable frequency drives (VFDs) or integrated variable speed pumps change the pump curve by changing speed. The affinity laws describe the effect:

• Flow varies directly with speed.
• Head varies with the square of speed.
• Power varies with the cube of speed.

In practice, savings are less than the cube law suggests when the system curve includes a fixed differential pressure setpoint, because head cannot fall as fast as flow. Resetting the differential pressure setpoint based on valve positions captures more of the potential savings. See variable speed pumping and differential pressure setpoints.

Variable speed also allows a slightly oversized pump to be trimmed to the actual design point by speed rather than by throttling a balancing valve, which saves energy.

Parallel and series pumping

Parallel

Pumps in parallel add flow at the same head. Parallel pumps provide redundancy and allow staging at part load. When two identical pumps run in parallel on a typical system curve, total flow is less than double the flow of one pump, because the system head rises with flow. Each pump must be selected to operate acceptably both alone and in parallel.

Running multiple variable speed pumps at the same speed, and staging pumps based on efficiency rather than simply on speed, often minimizes energy.

Series

Pumps in series add head at the same flow. Series pumping is less common in HVAC but occurs inadvertently in primary-secondary systems if the decoupler is restricted.

Common selection errors

• Excessive safety factors on head, leading to overflow and throttled balancing valves.
• Selecting far to the right of BEP, causing high power, noise and possible cavitation.
• Ignoring operation with one pump of a parallel pair off.
• Using a single large pump where two smaller pumps would give redundancy and better part-load efficiency.
• Poor suction piping: elbows close to the pump inlet, causing uneven flow. A straight run or suction diffuser helps.

Frequently asked questions

Can an oversized pump be fixed without replacement?

Often, yes. Trimming the impeller to the actual operating point, or adding a VFD and reducing speed, can bring the pump close to the required point. Measured flow and head during water balancing give the data needed.

Does pump efficiency matter in small systems?

Yes. Small circulators run continuously, and modern electronically commutated circulators use substantially less energy than older fixed-speed models.

Key takeaways

• The operating point is where the pump curve meets the system curve.
• Calculate head carefully along the critical path and avoid stacked safety factors.
• Select near the best efficiency point and check NPSH, motor sizing and impeller size.
• Variable speed and differential pressure reset reduce energy, but the cube law overstates savings.
• Parallel pumps must work properly alone and together.