Comparison

Pumps in Series vs Parallel: Which Arrangement Fits?

Parallel pumps add flow at the same head. Series pumps add head at the same flow. Includes a worked example on two systems.

Mechanical, HVAC & Fluids · Last reviewed October 17, 2026

Short answer: use parallel pumps when the system needs more flow and has low static head, and when you want redundancy. Use series pumps when the system needs more head, such as a high static lift. The system curve decides which gives more flow.

	Parallel	Series
Combined curve	Flow adds at each head	Head adds at each flow
Best for	Variable flow, redundancy, low static head	High head, long or high-lift systems
Flow from two identical pumps	Less than double, because the system head rises with flow	Less than double, with a larger gain when the static head is high
Operating range	Wider flow range as pumps start and stop	Narrower, and the pumps must match
Redundancy	Good, since one pump can carry part of the load	Poor, because one failed pump blocks the flow unless bypassed
Control	Staging and speed matching, with check valves	Pumps must run together, with matched flow
Risks	A small pump alongside a larger one can be dead-headed. Runout of a single pump	Pressure ratings of the casings and seals of the second pump

A worked example

Each pump has the curve H = 60 − 0.002 Q² (head in feet, flow in gpm). Two systems are compared: one with 20 ft of static head and one with 50 ft, both with friction head of 0.001 Q².

	System with 20 ft static	System with 50 ft static
One pump	115 gpm at 33 ft	58 gpm at 53 ft
Two in parallel	163 gpm at 47 ft (1.41 times the flow)	82 gpm at 57 ft (1.41 times the flow)
Two in series	141 gpm at 40 ft (1.22 times the flow)	118 gpm at 64 ft (2.05 times the flow)

In the low-static system, parallel gives more flow. In the high-static system, series gives far more, because the single pump works near its shut-off head and the second pump adds head where it is needed. In neither case is the flow doubled.

How to choose

Draw the system curve, including static head and friction.
Combine the pump curves for each option and find where each crosses the system curve.
Check each pump against its allowable operating range, its NPSH and its power.
Decide how much redundancy and turndown you need.
Plan the controls: staging, speed matching and protection against dead-heading.

Common mistakes

Expecting twice the flow from two parallel pumps.
Running mismatched pumps in parallel, so that one is pushed to the end of its curve.
Putting pumps in series without checking the pressure rating of the downstream pump.
Forgetting the check valves in a parallel system.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

Can a second pump make the first one cavitate?

Yes. More total flow raises the flow through the suction pipe and the required NPSH. Check the NPSH at the new operating point.

Is a variable-speed pump better than two pumps?

A single variable-speed pump may suit a varying load, and two pumps add redundancy and range. Many plants use both.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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