Duplex sump pump installation Buying, Installation & Maintenance Guide for Sump Pumps

  • SUMP PUMPS - home - CONTENTS: What is a sump pump? How do sump pumps remove building water or prevent water entry? De-watering pumps or submersible sump pumps are explained here. What types of sump pump can I buy? Pedestal pumps and ground level or submersible sump pumps; simplex & duplex sump pump installations, battery backup sump pumps, water operated sump pumps; Does using a sump pump cause foundation undermining & increase of water movement towards the building?
  • POST a QUESTION or READ FAQs about how to choose, buy, install, & use sump pumps to keep water out of buildings

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Sump pumps:

This article explains how sump pumps are used in buildings, describes the types of sump pumps, and describes how sump pumps should be installed, inspected, and maintained. We explain the difference between a sump pump, simplex and duplex sump pumps, a septic effluent pump, a sewage grinder pump, and an effluent pump.

This article explains the various types of pumps and their purchase, installation, inspection, and maintenance.

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What is a Sump Pump, how are they installed, used, piped, wired & repaired?

Sump pump with float (C) D FriedmanSump pumps, which we discuss on this page, are designed to remove unwanted water, such as surface or ground water that leak into a building.

Sump pumps only have to pump water, never solids. Sump pumps are normally used to pump clear liquid, such as ground water from a wet basement sump pit or graywater from a basement laundry sink.

Our sump pump photo (left) is not a wonderful installation, but you can see the pump motor (red arrow), pump float switch (orange arrow), and flexible pump discharge pipe (white arrow) clearly. The water inlet in tihs case is at the bottom of the pump assembly (blue arrow).

Sump pumps are light-duty and unlike septic or sewage pumps, sump pumps have no ability to pass solid debris other than perhaps fi

What's the Difference Between a Sump Pump, Septic Pump, Sewage Pump or Effluent Pump?

If you are confused between SUMP PUMPS used to remove ground water and septic pumps used to move sewage or septic effluent then


Typical Sump Pump Installation & Use

Sump in flooding basement (C) D Friedman

A sump pump is typically installed in a pit at the low end of a building's 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. On occasion we find sump pumps installed 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.

In a bad building water entry situation water runs across the basement/crawl space floor into the sump pit where it is pumped away (after already wetting the building and inviting a mold contamination problem).

This condition pertains when water is entering a building through foundation walls, often because the roof drainage or surface runoff are directed right against the building foundation itself.

Keeping gutters and leaders working and correcting outside drainage errors are critical in keeping water out of a building. Doesn't it make more sense to prevent water from coming into a building than to let it in and then pump it out?

In our flooding basement photo at above left, notice that there is a flood-line about half way up that oil storage tank? The little sump pump shown in the white bucket in the center of our photo is never going to handle such a huge volume of water.

And even for modest water entry, the projection of that sump bucket lip above the floor level means water has to rise a few inches in this basement before it can even flow into the sump pit!

Sump pump in pit (C) D Friedman

In a good situation, openings in the sides and bottom of the sump pit (photo at left) , or an under-floor drainage system direct subsurface water into the sump pit before the ground water level rises enough to send water into the building. Over several years of operation, and partly by pumping a little soil silt as it operates, a sump pump may actually improve the flow of under-floor water into the sump pit, thus reducing building water entry.

Septic pumps, sewage pumps, grinder pumps, and effluent pumps are not sump pumps, and they are discussed beginning
at Sewage Ejector Pump Grinder Pump.

The distinction among these pump types is important. Choosing the wrong pump can mean a short operating life for the pump, an unreliable system, and unnecessary expense.

There may be some confusion, depending on with whom you speak, because people don't always use just the right terms for construction or septic system parts - and the right sewage pump term, or right septic handling product versus the wrong one can be an important distinction.

Sump Pump Drain Venting

Depending on the lift height and other site conditions there are two sorts of vents one may find on any lift, grinder, or ejector pump or sump pump:

  1. a small drain hole specified by the manufacturer drains back wastewater from the vertical drain line, below the check valve, into the sump or pumping pit or chamber
  2. a separate vent may vent the sewage ejector piping to the open air outdoors

Model plumbing codes define a sump vent:

A vent from pneumatic sewage ejectors, or similar equipment, that terminates separately to the open air. - UPC 2006 

Two Types of Sump Pump Installations - Simplex and Duplex Pumps

Photo of a common sump pump used in a modern basement

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, many installations require that only one single pump be installed.

We discuss the details of submersible and pedestal sump pump types below
at Four Common Types of Sump Pumps.

Duplex sump pump installation

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.

Duplex sump pumps are illustrated and discussed further
at Septic Pump Duplex System Designs.

Two Types of Duplex Sump Pump Installations: Alternating and Reserve

Duplex septic pump installation

Please see SEPTIC PUMP DUPLEX DESIGNS for details.

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 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.

Four Common Types of Sump Pumps

Submersible sump pump installation
  1. Submersible sump pumps, such as shown in the photo at left, use a motor housed in a water proof enclosure and a separate float that turns the pump on and off.

    The sump pump float contains a position-activated switch and is connected to the submersible pump by a flexible wire. Some submersible sump pumps, such as the one shown here at left, use other types of float switches.

    The submersible sump pump motor is capable of working when entirely under water.

    A submersible sump pump uses a float switch intended to turn the pump on when ground water rises in the sump pit (or flows stupidly across your basement and into the pit) where it is discharged to a storm drain or the property surface.

    Watch out for debris or wiring in the sump pit that block movement of the float switch - your sump may fail to turn on.

Photo of a common sump pump used in a modern basement
  1. Pedestal sump pumps, such as shown in the photo at left, use a motor atop a pipe inside which the pump turns a shaft which operates the pump impeller.

    The pump impeller is located in a bottom pedestal which is under-water. You'll notice that the electric motor that powers the pump impeller is mounted on top a tall shaft that extends well out of the water itself.The float that turns the pump on and off usually looks suspiciously a lot like a toilet tank float, connected to a vertical rod.

    Pedestal sump pumps are an older non-submersible type of pump used for removing water from buildings.

    As rising water in the sump pit lifts the float, the float lifts the rod and the rod includes an adjustable screw-clamp fitting which pushes on the electrical contact of a mechanical switch to turn the pump on. As the water level drops the float falls and another screw-clamp fitting above the switch turns the pump motor back off.

    Watch out: if the pedestal pump is not adequately secured it may tip over and jam its float; if debris or other obstructions interfere with movement of the float and its vertical rod that operates the pump switch, the pump may fail to operate when needed.

Water powered sump pump

  1. Battery-backup sump pumps, use a rechargeable battery which is normally connected to live electrical power in order to remain fully charged. If electrical power fails, the batter can still operate the sump pump. We recommend this type of sump pump at homes where electrical power is frequently lost. You're most likely to lose electrical power during a storm, which may be exactly when you most-need the sump pump.

    The duplexed battery-backup sump pump shown above and immediately below was installed in a converted church in Staatsburgh, New York in a neighborhood subject to recurrent flooding. Note that the owner took advantage of the new sump pit to dispose of condensate from the full-time basement dehumidifier as well.

Water powered sump pump

Above: battery charger for a battery-operated sump pump.

Below: a water-powered sump pump design in a pre-1900 home

Closeup of water powered sump pump
  1. Water powered sump pumps, (as shown in the photos above) use municipal water pressure and a venturi fitting to pick up and eject water from a building during flooding. Usually they are turned on manually by opening a water valve near the pump. Water driven sump pumps work only where municipal water is provided.

    Water powered sump pumps offer the advantage that the pump can operate when there is no electrical power. At least the older versions of these devices are illegal in many municipalities because their installation constitutes a cross-connection which can back-contaminate public water mains with unsanitary floodwaters.
    There may be newer versions that are code-approved: we invite more data and comment on this product.

    See CROSS CONNECTIONS, PLUMBING for more details about plumbing cross connections and sump pumps.

Water powered sump pump

  1. Water & flooding alarm products are available in a variety of forms including battery-powered devices (we show one at Sewage Ejector Pump Grinder Pump) and even devices which can turn a light in a home or make a telephone call or inform an alarm company if a building is being subjected to flooding. Considering the very high cost of flood damage cleanup and mold remediation, we consider flood alarms a great idea for buildings which are often left unattended.

Where does the sump pump send its Discharge Water?

sump pump into storm drain

See SUMP PUMP DISCHARGE for details.

Sump pumps that have been added to an older structure often pump their discharge to the ground surface where it runs to a storm drain or area drainage setting.

If you have such a system be sure that the sump pump discharge empties where it meets these criteria:

sump to a storm drain

How to Calculate the Water Inflow Rate = Calculate the Necessary Sump Pump Capacity

Typically most of us just buy a 1/4 hp or 1/2 hp sump pump and throw it in the pit and see what happens. But you can guesstimate the rate at which water is flowing into the sump pit by knowing the sump pit dimensions and observing how long it takes the pit to fill to a measured depth.

At Static Head of Water in the Well we give details of how to calculate the volume of liquid in a cylinder (if your sump pit is round) using pi (3.14) and the radius (1/2 the diameter of the cylinder) squared.

Volume cylinder = 3.14 x (water height in inches ) x (cylinder diameter in inches /2)2.

Example using an 18" diameter joint compound bucket as a sump pit, and assuming that it takes water 1 minutes to fill the pit to 1 inch..

Volume = 3.14 x (1 inch of water) x ( 18 / 2)2

Volume = 3.14 x 1 x 81 = 254 cubic inches of water rose in the sump pit in one minute

Water in-flow rate to the pit = 254 cubic inches per minute

Convert cubic inches to gallons as follows.

1 cubic foot = (12 x 12 x 12) cubic inches or 1728 cubic inches

254 / 1728 = .15 cubic feet.of water flowing into the sump pit per minute

1 cubic foot = 7.5 gallons (U.S. Liquid)

.15 cu.ft. x 7.5 = 1.1 gallons of water flows into the pit per minute

This example let's us state a simple "rule of thumb" for joint-compund bucket-sized sump pits.

One inch of water in an 18-inch diameter joint compound bucket or sump pit = about 1 gallon of water

This means all you have to do is calculate the number of inches that you see water rise in your joint compound bucket sized sump pit in one minute - that's roughly the number of gallons per minute that water is flowing into the sump pit.

Choosing a De-Watering Sump Pump: Typical Pump Capacities in HP, Lift, & GPM Pumping Rate

Sump pumps for residential use range in horsepower from 1/4 HP to xx. Typical sizes are 1/3 HP and 1/2 HP. Prices (2012) run from about $100. to $200. for submersible pump models. Pedestal pumps and light duty sump pumps may sell for less than $100.

Sump pumps are rated for several important factors including horsepower and pumping capacity in gallons or liters per hour - a figure that varies by pump lift height. An individual sump pump's capacity will vary depending on the height to whch it has to lift its discharge water (higher lift means fewer gallons per hour) and other factors such as diameter and number of elbows in the discharge piping.

Watch out: be sure to consider both the anticipated de-watering flow rate you'll need and the pump's lift requirements. Typically if you are pumping out of a basement sump pit to ground level that's more than 5 ft. but less than 10 ft. of lift. But installations that have to lift higher distances and/or pump over longer distances and through multiple piping elbows need a more powerful pump.


Table of Sump Pump Performance Capacities and Typical Ratings for Residential Use

Pump Horsepower GPM Capacity/Model Capacity/Model Capacity/Model Capacity/Model Capacity/Model
Sump Pump Lift Capcity
0 ft. 
5 ft. 
10 ft. 
15 ft. 
Submersible or flat-on-surface sump pumps
1/4 HP [3] GPM   32 25 10  
1/3 HP [4] GPM   43 34 19  
1/2 HP [5] GPM   93 70 64 36
Pedestal style sump pumps
1/3 HP Pedestal [1] GPM   50 36 10  
1/2 HP Pedestal [2] GPM   60 51 38 17

Notes to the sump pump size table: (Note that pump manufacturers offer a wider range of types and capacities of pumps than just the examples listed here. Also pumps of the same HP or capacity may be produced using different materials suitable for different applications and with different durability.)

[1] Zoeller® Old Faithful Pedestal thermoplastic Model 81 1/3HP, maximum "shut off" head: 16 ft.
[2] Zoeller® Old Faithful Pedestal Model 84 1/2 HP , maximum "shut-off" head: 24 ft.
[3] Zoeller Model 49, Watrer RiddrIII
[4] Zoeller Model 50 Series 1/3 HP
[5] Zoeller FlowMate 137, 1/2 HP

* Some manufacturers give sump pumping rates as "total dynamic head/flow per minute dewatering" in gallons per hour instead of gallons per minute. When comparing pumps be sure you are comparing the same units. Also, among some manufacturers such as Zoeller, some pumps can handle both de-watering jobs and sewage effluent, but the reverse is not the case, that is, models that are designed only for de-watering, such as pedestal and some sump pumps, should not be used for septic system applications.

Inspecting & Troubleshooting De-Watering Sump Pumps

See SUMP PUMP INSPECTION for a detailed sump pump inspection & troubleshooting guide.

Sump pumps on newly constructed buildings are often connected to the building foundation drain. We consider this a bad practice. It is a rare home more than 20 years old whose footing drains are intact.

If a footing drain discharge itself becomes clogged or damaged, sending the sump pump discharge into that system will not work: you'll simply flood another section of the building foundation, basement, or crawl space, or you may overload the existing foundation drain causing building water entry.

Connecting a sump pump to a municipal sewer drain is bad practice and illegal in some communities. You're adding to the municipal sewer plant's water overload during wet weather and you may thus be contributing to the discharge of raw sewage from the overloaded municipal treatment facility right into the environment.

Where permitted, we prefer to route a sump pump to a storm drain, or where soil conditions permit it might be discharged to a drywell.

These simple sump pump dewatering trouble diagnostics may resolve pump capacity questions

If the sump pump motor is running or too frequently, constantly check the following:


Continue reading at SUMP PUMP DISCHARGE or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES.



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