Does Your Basement Need Two Sump Pumps?
Many basements with a working drainage system run fine on one sump pump. Two sump pumps in a basement make sense when a single pump is a single point of failure you cannot afford, when water comes in faster than one pump can move it, or when the drain tile cannot reach one pit. The second pump is usually a backup that sits higher in the same pit. Liberty Pumps, in the installation manual for its Model 441 battery backup system, says the backup float in its off position should sit at least 1/2 inch above the water level at which the primary pump turns on, so the backup only runs when the primary falls behind.
Pump ratings on the box can be misleading, because flow drops sharply as the pump has to lift water higher. The performance table in that same Liberty 441 manual lists 1,500 gallons per hour at 8 feet of vertical head, 1,300 at 10 feet, 1,050 at 12 feet, 450 at 16 feet, and zero at 18 feet. Below, we cover why homes add a second pump or a second pit, how the two pumps are staged, how to read capacity, and when the better fix is outside the house.
What is a dual sump pump system?
A dual sump pump system is two pumps set up to remove groundwater from the same basement, either in one shared pit or in two separate pits. The pumps can be two plug-in (AC) pumps, or one AC pump plus a battery or water-powered backup.
There are three common layouts:
A primary and a backup pump in the same pit, with the backup mounted or floated higher
Two pumps in the same pit that alternate or share heavy loads
A second sump pit on the far side of the basement, each pit with its own pump
Why would you put two sump pumps in a basement?
Redundancy is the usual reason. A sump pump can fail from a stuck float, a burned-out motor, a clogged intake, or a tripped circuit, and you usually find out when the floor is already wet. A second pump in the same pit gives the water somewhere to go when the first one quits.
Power outages are the other big one. A standard sump pump stops the moment the power goes out, which often happens during the same storms that raise groundwater. A battery backup pump covers that gap. We explain run times, battery care and what they can and cannot handle in our post on whether battery backup sump pumps work in a power outage, so we will not repeat it here.
High inflow is the third reason. If your pump runs almost nonstop in a wet spring, or the pit refills within a minute of the pump shutting off, one pump may simply be undersized for the water coming in. Pittsburgh International Airport averages 39.61 inches of precipitation and 44.1 inches of snowfall a year, according to NOAA's 1991 to 2020 climate normals, so snowmelt plus spring rain can keep a Western PA pit busy for weeks.
How are primary and backup pumps staged in one pit?
Staging means setting each pump to turn on at a different water level. The primary handles everyday water. The backup waits above it and only starts if water keeps rising.
Manufacturer instructions spell out the spacing:
Liberty Pumps' Model 441 manual: the backup float, in its off position, should be at least 1/2 inch above the level where the primary turns on. It also says to install the backup pump as close to the bottom of the pit as possible.
Zoeller's installation manual for its Model 508 12-volt DC backup pump: the intake of the DC pump must be set above the intake of the primary pump to reduce the risk of air lock, and an optional high water switch should turn on at least 1 inch above the operating float's on level.
For scale, Zoeller's spec sheet for its Model M53 primary pump lists automatic on and off points of 7 1/4 inches and 3 inches. So a backup paired with a pump like that would be set to start somewhere above that 7 1/4 inch line. Your exact numbers depend on the two pumps you have and the depth of your pit, so follow both manufacturers' manuals for your models.
One discharge pipe or two?
Liberty's 441 manual shows three ways to plumb a backup. Its recommended method gives the backup its own separate discharge line. The other two tee the backup into the primary's discharge pipe. Liberty also says a check valve is required in the primary pump's discharge so water does not run back into the pit.
How much water can each pump actually move?
Capacity is measured in gallons per hour (GPH) or gallons per minute (GPM) at a given head. Head is the height the pump has to push water. The more lift, the less flow.
The Liberty 441 table above shows the pattern clearly: the same pump that moves 1,500 GPH at 8 feet moves 450 GPH at 16 feet, less than a third as much, and nothing at 18 feet. That last number is called shut-off head. Zoeller lists the M53's maximum head as 19.25 feet, and its maximum flow as 43 GPM, and that maximum flow is only reached at low lift.
What this means for a second pump:
Measure the vertical lift from the bottom of your pit to where the discharge pipe leaves the house.
Look up each pump's flow at that height on the manufacturer's performance curve or table, not the headline number.
Compare the backup's flow at your head with how fast your pit fills during heavy rain.
If a backup moves far less water at your actual lift than the primary does, it may buy you time during an outage without keeping up in a true storm. For help matching a pump to your basement, see how to choose the best sump pump for your basement.
When do you need two sump pits in a basement?
Two sump pits in a basement usually solve a layout problem, which is different from adding a backup. Interior drain tile slopes toward a pit. In a long or oddly shaped basement, water from the far corner may have to travel a long way, and a single pit can leave that side wet even when the pump works perfectly.
A second pit can make sense when:
One side of the basement stays wet while the pit area stays dry
An addition or a lower section of the basement sits on its own footing and drain line
The existing pit already needs two pumps running at once to keep up
Before cutting a new pit, it helps to confirm the drain tile is actually clear and connected. Our post on basement drain tile installation near North Huntingdon explains how that system feeds the pit, and do I need a sump pit in my basement covers the signs that point to a pit in the first place.
Is a second pump the right fix, or is it a drainage problem?
Sometimes a busy pump is a symptom. If roof water is dumping next to the foundation, the pump may be recycling the same water over and over. The Department of Energy's Building America Solution Center recommends that downspouts discharge at least 5 feet from the foundation, or underground at least 10 feet away, and that the ground slope away at least 1/2 inch per foot for the first 10 feet.
Check these before buying a second pump:
Where your sump discharge pipe ends outside. If it drains back toward the house, the water returns.
Downspout extensions and gutters, especially after a storm.
Grading along the wall closest to the pit.
If the pump still runs constantly after the outside water is handled, a second pump or a second pit is a reasonable next step. If not, you may not need one.
FAQ
Is it beneficial to have two sump pumps in one house?
For many homes, yes, because a second pump covers mechanical failure or a power outage. It helps most in basements with finished space, stored belongings, or a pit that runs often.
How many sump pumps does a basement need?
Many basements get by with one working pump in a properly drained pit. A backup is added for protection, and a second pit is added only when the drainage layout or inflow calls for it.
Can two sump pumps share one discharge pipe?
They can. Liberty Pumps shows tee connections as acceptable options in its Model 441 manual, but its recommended method runs the backup on a separate discharge line. Zoeller's Model 508 manual, which uses a tee, builds a check valve into the backup's discharge.
Should the backup pump sit higher than the primary?
The backup's turn-on point should be higher. Liberty says the backup float's off position should be at least 1/2 inch above the primary's turn-on level, and Zoeller says the intake of its DC backup must be above the primary pump's intake.
Do I need a 1/3 or 1/2 horsepower sump pump?
Horsepower alone does not answer it. Compare each pump's flow at your actual vertical lift on the manufacturer's performance table, then match that to how fast your pit fills.
Why does my basement have two pits?
Often each pit collects water from drain tile on a different side of the basement. If one pit has a sealed, bolted lid and connects to a bathroom or laundry, it may serve plumbing rather than groundwater, so have it identified before you change anything.
What does it cost to add or replace a sump pump?
It depends on whether you are adding a pump to an existing pit, adding a battery backup, or cutting a new pit with drain tile. Our post on sump pump replacement cost in Pittsburgh breaks down what affects the price.
Get Help Deciding on a Second Sump Pump
A Better Choice, Inc. has served homeowners in the Greater Pittsburgh area since 1996, across Westmoreland, Allegheny, and Butler counties. Our basement waterproofing team can look at your pit, drain tile, and discharge line during a free inspection and explain whether a backup pump, a second pit, or a drainage fix fits your basement.
Visit A Better Choice, Inc. or call 1-800-343-2357 today. Contact us for more information.
Comments