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Water Powered Backup

How a water-powered backup works

A water-powered backup, sometimes called a venturi or ejector backup, connects to your home's pressurised water supply. When the pit water rises to a float set above the primary pump's range, a valve opens and municipal water rushes through a narrow nozzle inside the ejector. The fast-moving stream creates low pressure, which draws pit water up a suction pipe and pushes the mixture out through the discharge. Because there is no motor, there is nothing to burn out and no battery to go flat. Operation continues as long as the supply pressure holds.

Where they make sense

These units are attractive in neighbourhoods with frequent multi-day outages, for homeowners who travel, and for basements where a dead battery would be expensive. They are generally a poor fit for homes on private wells, because the well pump itself needs electricity, and for areas where water pressure is low or where local rules restrict this kind of connection.

Pressure and capacity

Capacity rises with supply pressure. A typical product literature pattern is something like: at 40 psi, a unit removes a modest amount of pit water per minute; at 60 psi it removes more. Many units also have a ratio of roughly one to two gallons of city water used for every gallon or two removed, depending on design and head. Check the manufacturer's chart against your measured inflow and your total dynamic head. Measure your house pressure with an inexpensive hose-bib gauge during a period of normal neighbourhood use.

Backflow prevention

Because the device connects drinking water to a pit of groundwater, plumbing codes typically require an approved backflow prevention assembly — often a reduced pressure zone (RPZ) or another device your local authority specifies. Some jurisdictions require permits, testing by a certified tester, and periodic re-inspection. This is not a DIY connection; a licensed plumber should handle the tap, the backflow device and any permit.

Water use and cost

Every minute a water-powered backup runs, it consumes metered water and sends it to your discharge. During a long storm that can add up to hundreds or thousands of gallons. In some cities, sewer charges are billed on water consumption, which can raise the cost further. Ask your utility whether discharged sump water is billed and whether there are any restrictions during drought or emergencies.

Worked example

A homeowner has 55 psi of static water pressure. A hypothetical ejector's chart shows it removing about 1,000 gallons per hour at 10 feet of lift at that pressure while consuming about 1 gallon of city water for every 2 gallons removed. Their measured storm inflow is 420 GPH, so the backup can keep up with room to spare. Over a 10-hour power cut with the backup running 45% of the time, it removes about 4,500 gallons from the pit and uses about 2,250 gallons of city water. At an illustrative combined water-and-sewer rate of $0.01 per gallon, that outage costs about $22.50 in water — far less than a flooded finished basement, but not free.

Illustrative note (2026-10-29): This example was built on 2026-10-29 using made-up ejector figures and a hypothetical utility rate. Actual consumption ratios, capacities and local prices vary widely; confirm with the product chart and your utility bill.

Comparison with battery backup

For a fuller picture see the battery backup guide.

Installation notes for planning

• The ejector mounts above the primary pump with its own float and suction line.
• The discharge should not tie in below the primary's check valve.
• A shut-off valve upstream allows service.
• The backflow device must be accessible for testing.
• Freezing risk: supply lines in cold basements or crawlspaces need protection.

Testing

Test by lifting the float or adding water to the pit after closing off the primary. Watch for a steady discharge and listen for unusual noise. Check that the supply valve is open; a closed valve is a common reason a water-powered backup fails when needed. Have the backflow assembly tested on the schedule your local authority requires.

Drawbacks to weigh

Water-powered systems can be noisy, waste treated water and lose capacity if neighbourhood pressure drops during a widespread emergency. Pressure can fall when many homes draw water at once or when the utility has its own power problems. They also move less water than many electric primaries, so they are a backup, not a replacement. Some homeowners combine a battery backup and a water-powered unit to cover both short and long outages.

Questions to ask before buying

Ask your plumber what your static and working water pressure are, which backflow assembly your municipality accepts, whether a permit and annual test are required, and where the extra discharge will go. Ask your utility whether outdoor water restrictions apply during emergencies. If any answer is unclear, a battery backup may be the simpler first step.

Frequently asked questions

Can I install a water-powered backup on a well?

Generally no, because the well pump needs electricity. Ask a licensed plumber about alternatives.

Do I need a permit?

Often yes, because of the potable water connection and backflow requirements. Rules vary by municipality.

Will it keep my basement dry in any storm?

No device can promise that. It extends protection during outages within its rated capacity.

Related pages

• Battery backup sump pumps
• Sump pump alarms
• Check valve guide
• Generator size