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?
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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).

Article Series Contents

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 also see SEPTIC SYSTEM PUMPS.

Typical Sump Pump Installation & Use

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.

Sump in flooding basement (C) D FriedmanIn 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 FriedmanIn 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.

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 installationReserve 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

  1. Submersible sump pump installationSubmersible 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.

  2. Photo of a common sump pump used in a modern basementPedestal 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.
  3. 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.

    Water powered sump pump Water powered sump pump
    The duplexed battery-backup sump pump shown above 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.
  4. 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.

    Closeup of water powered sump pump Water powered sump pump

    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 & 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 drainIn this photo at left a temporary sump pump discharge line has been left on the basement floor - no good destination has been assigned to sump outlet hose. If this sump pump is called-on to operate in this condition the building will simply be flooded.

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:

  • The water leaving the sump pump should be discharged to a location where it does not flow back towards the building. Otherwise it may simply cycle the same water endlessly, possibly even undermining the building foundation

  • sump to a storm drainWater discharge from a sump pump must go to a legal destination. In the photo at left we're emptying the sump pump via a small diameter (Limited flow capacity) to a local storm drain.

    This might be legal and fine in the summer for an unusual event, but this is not a reliable, permanent sump pump installation.

    As this system was found in Maine, we can expect it to freeze or simply not work in winter or early spring when it may be most needed.

    Discharging onto a neighbor, and in some communities, discharging into local storm drains, may be prohibited and are certainly a bad practice.

  • In freezing climates, the sump discharge needs to be protected from freezing or the system may not work when most needed.

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 cu.in. / 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 Example Sump Pump Performance Capacities and Typical Ratings for Residential Use

  Parameter Capacity/Model Capacity/Model Capacity/Model Capacity/Model Capacity/Model
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.


Guide to Inspecting & Troubleshooting De-Watering Sump Pumps

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.

De-Watering Sump Pump Inspection Checklist

Bad sump pump instlallation (C) D FriedmamnOur photo, left, shows a horrible sump system, flooding crawl/basement, and water-logged heating boiler. Here is a checklist of sump pump installation details:

  • Alarms for basement water: Water or Sump Pump Alarm protection: where a sump pump is relied-upon to keep water out of a building, good practice includes an alarm to inform someone when the pump is not operating.

    Various systems use a detector which senses water on a building floor near the sump pump. Water alarms can sound an alarm to building occupants, turn on a light in a window for a neighbor to see, notify a security service, or even place a telephone call to report this condition. We recommend this protection for any building which is left unattended for long periods.
  • Battery backup sump pump systems: in areas prone to power failures the sump pump may be a battery-backup installation. The batteries are connected to a charging system and are available to operate the pump when area electrical power has been lost.

    Turn off the electrical power to the charger and confirm that the pump is running on battery power. Review the specifications for the system to determine whether or not the pump may be able to continue to operate on battery power for the typical length of time that electrical power is lost.
  • Check valve: is a check valve installed on the sump pump discharge line? If not water will run back into the sump pit each time the pump shuts off.

    This defect causes extra cycles of pump operation and may reduce pump or switch life; in odd circumstances such as a sump pump discharge line into a stream, it can even back-siphon outdoor water into the sump pit and into the building.
  • Discharge line security: sump pumps cause a sudden surge in water through the discharge pipe when the pump comes on. Many sump pump installations use a flexible discharge pipe which is not adequately secured. As a result, each time the pump cycles on and off the flexible discharge pipe jerks and moves across various contact surfaces.

    Watch out: We've seen this result in holes worn into the discharge pipe so that each time the sump pump cycled on it sprayed water across the basement.
Sump pump installation by extension cord, water over floor (C) D Friedman
  • Electrical safety & GFCI protection: Is the sump pump connected to a GFCI-protected electrical circuit or outlet? According to some writers a sump pump should be connected to a GFCI circuit. Is the sump pump powered by a safe, secured, properly wired electrical circuit and receptacle, not a makeshift job nor an extension cord?
    Our sump pit photo (left) shows extension cord wiring to the sump pump and a wet buiding whose water runs across the floor (wetting the building) before it can enter the sump pit.

    Wet locations sometimes keep tripping off the GFCI - a safe condition, but it means the building is likely to become flooded because the circuit powering the sump pump has shut down. Some writers and electrical inspectors make an exception to the more general GFCI-requirement rules for sump pumps:

    In some jurisdictions, to assure that the sump pump will have power when needed, a dedicated, non-GFCI electrical receptacle can be installed provided that it is a single-receptacle unit (just for the pump) - nothing else can be plugged into that electrical outlet, and it should be labelled as Not-GFCI-Protected.
  • Mechanical security: If a pedestal sump pump is installed, is it secured against tipping over? A tipped pump will jam its float and stop working.
  • Motor switch & Electric Motor: Does the pump's electric motor turn on in response to the float?
  • Pedestal style sump pumps: is the pump secured against movement or tipping over? For units that are designed for indoor use, are the motor and electrical connections protected from water, water spray and high humidity?
  • Sump Pump impeller assembly: does the pump actually move water when the motor runs? Impellers can and should be cleaned of dirt, pebbles, and mineral deposits to keep the pump operating efficiently.
  • Sump pump capacity: the sump pump (or collection of them) must be capable of handling the anticipated incoming flow rate of water that needs to be removed.

Sump pit debris (C) D Friedman
  • Sump Pump pit openings: Is the sump well or bucket properly opened to permit ground water to enter the pit directly?

    Our first home had a basement sump pump installed in a water tight steel bucket the builder had pushed through the basement floor. Water had to rise under the basement floor, leak into the basement, run across the floor, and then be pumped away. Making holes in that bucket allowed the pump to draw water from below the basement slab. It lowered the water table and stopped water from entering the basement through the foundation walls.

    Is there debris in the sump pit that could interfere with the sump pump control switch operation. (See our photo, above left).
  • Sump pump water destination: is the pump delivering water to a legal destination and one which will not send water flowing back towards the building? Water should be discharged no less than 20 ft. from the building and to a spot which drains away from the building.
Sump pit trip hazard (C) D Friedman
  • Trip hazards: Is the sump pit protected against someone tripping or falling into it?

    Watch out: We have received reports of injuries to adults and children who stepped into a sump pit, and one reported death of a pet, a small dog who fell into the sump pump pit.[1]

    Our photo (left) illustrates an open sump pit that is unsafe for that reason.
  • If radon is a problem in the area, is a radon-cover installed over the sump pit?

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:

  • Is water coming out of the pump discharge pipe? If not, is the pipe clogged, disconnected, or broken? If not, is the pump impeller damaged or clogged?
  • Does water level drop to the bottom of the pump but the motor keeps running? , either the switch is defective (or float stuck)
  • Does water level in the sump pit fail to drop low enough to shut off the pump? The pump is unable to keep up with incoming water, or an outlet pipe is damaged, or the pump may be damaged. (Low voltage can make some pumps run with loss of capacity too.) If the water flow rate does not exceed the pumps nominal capacity the problem is in the pump or drain piping. If the water inflow rate or the pump lift height plus water inflow rate exceed the pump's ability, don't blame the pump.
  • If the sump pump runs frequently but otherwise normally, every minute or so, you may be able to improve its operation by deepening the sump pit so that at each pump cycle the pump runs longer, removing more water, but running a bit less often - which is easier on the pump and its life. Deepening the sump pit also may reduce the tendency of groundwater to come up through the floor at other locations.


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