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This article explains when, where, and how multiple or duplex sump pumps are used in buildings, describes the types of sump pumps, and describes how sump pumps should be installed, inspected, and maintained.
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A sump pump is normally installed in a pit at the low end of a basement or crawl space floor or in another location where water needs to be removed such as in a boiler pit or an outdoor well pit. Also see SUMP PUMPS GUIDE and SEPTIC SYSTEM PUMPS.
On occasion we find that we need to install two or more sump pumps inside or outside of a building foundation to remove water from around the foundation of a poorly-sited (too low) building which has no natural drainage path to dispose of ground water by gravity.
Single submersible or pedestal sump pump: The photo on the left is what you're likely to see if your basement has a modern sump pump.
A pedestal type pump must keep its motor out of water and dry.
Regardless of which type of pump we select, pedestal or submersible sump pump, many installations require that only one single pump be installed.
We discuss the details of submersible and pedestal sump pump types at Four Common Types of Sump Pumps.
Duplex sump pumps: The photo at left shows a duplexed sump pump system using pedestal type sump pumps. This was a really wet basement - a single sump pump simply could not keep up. In this installation .
When a building footprint or foundation layout is complex, or where the building is constructed over both basement space and one or more crawl spaces, it may be necessary to install multiple sump pumps to protect these various areas.
In a single large basement whose floor did not slope uniformly to a single low corner, it may be more economical to install two or even more sump pumps in problem areas than to tear up the entire basement floor to install a sub-slab drainage system.
Two Types of Duplex Sump Pump Installations: Alternating and Reserve
Reserve septic backup design: the backup pump never runs unless the primary pump has failed or is overloaded. A simple installation provides a pump control float switch that turns on the backup pump only water in the pumping chamber reaches a level above that normally handled by the primary sump pump.
This approach provides both pump backup and the ability to handle surges in building water entry loads on the sump pump system.
Alternating septic pump design: the two sump pumps are installed at the same location but are wired so that the pumps take turns, first one, and next cycle the other pump is turned on by the float switch.
This dual pump hookup is more common among septic pumping stations than among home sump pump de-watering systems, but it may be appropriate where a large volume of ground water has to be kept constantly out of a building.
An example we've seen was in the basement of a home on Long Island, NY in which the level of the basement slab was so low that flooding from Long Island sound would be nearly constant if the pumps failed.
The alternating sump pump approach has the advantage that both pumps are being exercised regularly, which reduces the chance of the ugly discovery that in the event you have to rely on a backup sump pump which has been sitting idle, waiting its chance to run, has in the interim, died.
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