When pumps are arranged in parallel, sometimes called the “volume” position, the total flow rate increases while the pressure head stays the same. This is the defining characteristic: you add flow, not pressure. Two identical pumps running in parallel at a constant head will double the flow rate compared to a single pump operating alone.
This stands in direct contrast to pumps in series (the “pressure” position), where flow stays roughly the same but head pressure adds together. Understanding how parallel pumping works, and what can go wrong, matters whether you’re designing a system, troubleshooting one, or studying for an exam.
How Parallel Pumping Increases Flow
The combined performance curve for pumps in parallel is built by adding the flow rates of each pump at the same head. If Pump A delivers 100 gallons per minute at 50 feet of head, and identical Pump B also delivers 100 GPM at 50 feet of head, the combined output is 200 GPM at 50 feet of head. The head doesn’t change. The volume does.
In practice, though, the actual operating point isn’t simply “double the flow.” The system has its own resistance curve, which represents how pressure losses increase as flow increases. Friction in pipes, fittings, and valves all create more resistance at higher flow rates. The real operating point is where the combined pump curve intersects the system resistance curve. Because system resistance rises with flow, the actual combined output will be less than a perfect doubling. Each pump ends up delivering slightly less than it would running solo, but the total system flow is still significantly higher than one pump alone.
When one of the parallel pumps shuts off, the operating point slides back along the system resistance curve to the single-pump intersection. Both the head and flow rate decrease, though the remaining pump picks up some additional flow compared to what it was delivering as part of the pair.
Why Parallel Is Chosen Over Series
Parallel arrangements solve a specific problem: the system needs more volume than a single pump can deliver. This is common in large cooling loops, municipal water distribution, and industrial processes where demand fluctuates. Running two smaller pumps in parallel instead of one oversized pump also provides redundancy. If one pump fails, the other keeps the system running at reduced capacity rather than shutting it down entirely.
Series arrangements, by contrast, are used when you need to push fluid against high resistance or to a significant elevation. The choice between parallel and series comes down to whether your bottleneck is volume or pressure.
Check Valves Are Not Optional
Every pump in a parallel system needs a check valve on its discharge line. Without one, a running pump can force flow backward through a stopped pump, wasting energy and potentially damaging the idle unit. Even when both pumps are running, slight differences in output can cause the stronger pump to develop enough pressure to close the weaker pump’s discharge path, forcing it to operate at shutoff head (zero flow). A pump running at zero flow overheats rapidly, damages seals, and will eventually fail.
The Problem With Mismatched Pumps
Parallel operation works best with identical pumps. When two dissimilar pumps run in parallel, the combined performance curve becomes uneven and harder to predict. The weaker pump may get pushed far outside its intended operating range, running near its shutoff head while the stronger pump dominates the system. This leads to overheating, seal damage, and premature failure of the weaker pump.
Dissimilar pumps can be installed in parallel, but only if they have similar shutoff head characteristics. If the shutoff heads are significantly different, the stronger pump will effectively deadhead the weaker one whenever system demand drops. Provisions to prevent deadheading, such as minimum flow bypass lines or automated controls, are necessary if mismatched pumps must share a common header.
Even with matched pumps, certain curve shapes create risk. Pumps with drooping head-capacity curves (where the head drops and then rises again at low flow) should not operate in parallel under variable flow conditions where demand can approach zero. The unstable region of the curve can cause the pumps to “fight” each other, with flow surging back and forth unpredictably.
Variable Speed Drives Improve Efficiency
In systems where demand varies throughout the day, variable frequency drives (VFDs) allow parallel pumps to adjust their speed rather than cycling on and off. Instead of running two pumps at full speed and throttling excess flow with a valve (which wastes energy), VFDs slow the pumps to match actual demand. Optimized control strategies that account for each pump’s efficiency curve at different speeds can reduce energy consumption by roughly 10% compared to conventional on/off staging.
Good parallel pump control also includes logic to prevent frequent pump switching, which wears out mechanical components. Modern control strategies set minimum run times and speed boundaries for each pump, staging additional pumps on only when the running units can’t maintain the required pressure at their maximum efficient speed.
Key Operating Principles to Remember
- Flow adds, head does not. At any given head, the total flow equals the sum of each pump’s individual flow at that head.
- The system curve determines the real output. Higher combined flow means higher system resistance, so actual output is always less than the theoretical sum.
- Each pump shares the load. With two identical pumps in parallel, each pump delivers half the total system flow at the common operating head.
- Shutoff heads must be similar. If one pump’s shutoff head is much higher than the other’s, it will deadhead the weaker pump.
- Check valves prevent backflow. Without them, a stopped pump becomes a path of least resistance for reverse flow.