Duplex sump pump
When does a duplex sump pump system help?
A duplex sump pump system uses paired pumps and controls to share duty or provide a standby pumping path. As of Nov 2026, the control sequence is decisive: Liberty’s 5050 alternates pumps but runs only one at a time, so duplex does not automatically mean combined pumping capacity.
Sources: Liberty Pumps. Checked Nov 4, 2026.
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What does the controller do when water rises?
The lead pump is the pump selected to run first; the standby or lag pump’s response depends on the controller. Swaby describes a system that alternates duty and can bring both pumps into operation for peak demand.
Request the normal-cycle sequence, high-water response and alarm action in writing. “Two pumps” leaves the operating behavior unresolved.
Can a duplex system run both pumps together?
Liberty’s 5050 transfers operation to the other pump on a high-water alarm. That differs from a controller designed for simultaneous pumping.
For peak inflow, verify the required pumping duty. For power loss, compare the separate backup-power scope; alternation does not establish outage protection.
What must fit inside the existing basin?
Liberty’s installation drawing shows paired pumps, a pump-control float and a higher alarm float. Its instructions require the float to move freely and caution against placing the vertical switch directly in incoming flow.
Have the installer check basin space, pump positions and float travel. Confirm whether the installation scope includes basin changes, discharge changes and controls.
Which commissioning and service checks matter?
Liberty requires qualified installation/service and power disconnection before that work. Use a qualified pump installer and licensed electrician for electrical alterations; this guide supplies no live-panel procedure.
The manual calls for testing pump and alarm operation and periodic checks for damage and free float movement. Request a demonstration of alternation, high-water response and alarm reset, then keep the instructions with your maintenance plan.
Frequently asked questions
Does duplex always mean both pumps operate together?
No. Liberty’s 5050 controller alternates pumps and runs only one at a time. Other system designs can provide simultaneous operation; check the selected control sequence.
Is an alternating system the same as battery backup?
No. The 5050 literature describes an AC pump controller, not battery-backed operation. Verify a separate outage-power plan instead of treating alternation as power-loss protection.
Should the high-water alarm be demonstrated at handover?
Yes. Liberty’s instructions call for testing both the pump float and alarm float. Ask the installer to explain the alarm response and how normal operation is restored.
Sources and dates
Key facts checked . Recorded calculator components retain their original Oct 24, 2026 retrieval date; new technical references were accessed separately for each article.
- 5050-Series: Alternating Control & Alarm, Sales Literature — Liberty Pumps. Source publication: 2020-01 (LLIT006680-R01/20). Accessed Nov 4, 2026.
- Duplex System — Swaby Pumps. Source publication: not stated. Accessed Nov 4, 2026.
- 5050 Control and Alarm System Installation Instructions — Liberty Pumps / SJE-Rhombus. Source publication: 2010-11 (PN 1030682A scanned edition). Accessed Nov 4, 2026.
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Plan the next step
Ask for the proposed control sequence and basin layout before buying a second pump. The calculator covers a separate standard installation reference, not a duplex-system quote.