How to Install a Water-Powered Sump Pump

This Old House plumbing and heating expert Richard Trethewey shows how to install an emergency backup sump pump

Why It’s Worth It: The real value of a water-powered backup becomes clear during storms. As This Old House experts point out, flooding typically happens during heavy rain — exactly when power outages are most likely, rendering your electric sump pump useless. A water-powered unit runs entirely on city water pressure, so it keeps working even when the electricity goes out. If you’ve invested in finishing your basement, the cost of a backup system is modest compared to the potential damage.

Pro Tip: Richard Trethewey, This Old House plumbing and heating expert, explains why placement matters: Mount the water-powered backup so that its float sits above the height of your existing electric pump. “We only want this to come on if this one fails, it gets clogged,” he says. This ensures the backup activates only when the primary pump can’t keep up.

Steps:

1. Attach two 3-foot-long 2×3 cleats to the wall above the primary sump pump; fasten the cleats with 3-inch screws.

2. Screw the backup sump pump to the 2×3 cleats using the mounting hardware provided. Be sure to set the float on the backup pump slightly above the height of the primary pump.

3. Next, cut into a nearby cold-water line and run ¾-inch-diameter copper pipe down to the control valve on the backup sump pump. Solder each connection with a propane torch and lead-free solder.

4. Unplug the primary sump pump. Then, drain water from the existing discharge pipe by using a slotted screwdriver to loosen the band clamp on the check valve.

5. Cut into the into existing discharge pipe leading from the primary sump pump, and run a new 1½-inch-diameter PVC discharge pipe from the backup pump.

6. Install a new check valve onto the backup pump, then use PVC pipe primer and cement to clean and solvent-weld together the plastic pipe and fittings.

7. Disassemble the backflow preventer, then open the water valve to flush any solder or flux into a bucket.

8. Fill the sump pit with water, plug in the primary pump and wait for it to turn on.

9. Unplug the primary pump and raise the float on the backup pump by hand to simulate water rising in the sump pit. When the float reaches the appropriate level, the backup pump will kick on and drain the water from pit.

Safety Note: Before cutting into the existing discharge pipe, Richard Trethewey warns that two critical steps must come first: “One is to disconnect the electrical because if you cut that pipe and the pump comes on, you’re going to get wet, and the other is to get the water out of this pipe.” He explains that the check valve at the bottom keeps the pipe full of water above it, so loosening the clamp on the check valve lets that water drain safely before you make your cut.

How It Works: Richard Trethewey explains the ingenious physics behind a water-powered sump pump: Full city water pressure enters the control valve, turns 180 degrees, and exits toward the outside. As it does, it creates suction that pulls water out of the sump pit along with it — no electricity required. This venturi effect means the pump works as long as you have municipal water pressure, making it an ideal backup during power outages caused by storms.