Sump pumps · buyer guide
Choosing a primary sump pump
The primary sump pump does the everyday work of keeping groundwater out of the basement. Picking one is mostly about four things: how much water comes in, how high the pump has to lift it, what the pump is built from, and what kind of float switch turns it on. Here is how to size it from your own pit, and three proven cast-iron pumps that cover most homes.
Step 1: Measure how much water comes in
Bigger is not automatically better — an oversized pump in a small pit short-cycles and wears out. Measure your inflow during a wet stretch:
- Measure the inside diameter of the pit.
- Unplug the pump (or wait right after it shuts off) and time how many inches the water rises in one minute. Plug the pump back in right away.
- Multiply inches of rise by the gallons per inch for your pit: about 1.1 gallons per inch for an 18″ pit and about 2 gallons per inch for a 24″ pit.
- Multiply by 60 to get gallons per hour at that moment.
Example: water rises 3″ per minute in an 18″ pit → about 3.3 gallons per minute → about 200 gallons per hour. Choose a pump that moves comfortably more than your worst-case inflow at your actual lift.
Step 2: Account for lift (head)
Pump flow drops as the vertical lift increases. Measure from the pump to the highest point of the discharge pipe (usually where it exits the rim joist), then add a little for elbows, the check valve, and long horizontal runs. Read each pump’s performance chart at that height, not the zero-foot number on the box. Wayne’s manual for the CDU980E, for example, lists about 5,490 gallons per hour at 0 feet and about 4,600 at 10 feet.
Step 3: Material and build
- Cast iron sheds motor heat into the water and tends to outlast thermoplastic bodies in pumps that run often.
- Oil-filled motors (common in Zoeller’s cast-iron line) help cool and lubricate the motor.
- Vortex or solids-handling impellers tolerate the grit and small debris that collect in most pits.
- Stainless hardware and a factory-tested pump are small details that matter on a part you rarely see.
Step 4: Float switch type
| Switch | How it works | Good fit | Watch out for |
|---|---|---|---|
| Tethered float | Ball on a cord swings up to start the pump | Larger pits; longer run cycles | Can snag on the pit wall or cords in narrow basins |
| Vertical float | Float rides up and down a rod | Narrow pits; predictable on/off levels | Needs a clean rod; debris can bind it |
| Built-in snap-action / mechanical float | Enclosed float trips a switch at a set level | Compact installs; the Zoeller M53 and M98 use this style | Fixed on/off levels; keep the float free of debris |
| Electronic sensor | No moving float; senses water level | Pits where floats keep jamming | Electronics can fail; still needs an alarm |
Three primary pumps worth considering
| Pump | HP | Build | Notes from the listing / manual |
|---|---|---|---|
| Zoeller M53 Mighty-Mate | 1/3 | Cast iron, oil-filled, vortex impeller | Long-time residential standard and an Amazon Best Seller in sump pumps; 9 ft cord with LED-lit plug |
| Zoeller M98 Flow-Mate | 1/2 | Cast iron, oil-filled, stainless hardware | Listed up to 72 GPM and 23 ft maximum lift; 1-1/2″ NPT discharge; snap-action float; factory tested |
| Wayne CDU980E | 3/4 | Stainless steel and cast iron | Vertical float; about 4,600 GPH at 10 ft per Wayne; needs a basin at least 11″ across; for high-inflow pits |
Check the warranty terms on the specific listing before you buy — they differ by model and are often measured from the install or manufacture date.
Don’t skip these
- Check valve on the discharge so pumped water does not fall back into the pit and restart the pump.
- Dedicated GFCI-protected outlet where required by code, with the cord not sharing a circuit with a freezer or other big loads.
- High-water alarm such as the Reliance water alarm so you hear about a failure before the floor gets wet.
- Backup pump — a battery backup or a combo system like the Basement Watchdog CITS-50. See battery backup sump pumps.
- Softener and appliance discharge: some pump makers, including Wayne for the CDU980E, say not to use the pump for water softener discharge. If your softener drains into the pit, check the pump manual or route the softener drain elsewhere — see softener drain air gaps.
When to call a pro
- The pit is undersized, cracked, or has no lid, or you need a new basin cut into the floor.
- The pump runs constantly in wet weather even after sizing up — that points to drainage or grading problems.
- You need a new circuit, or the discharge line needs rerouting or freeze protection.
For pit work, discharge plumbing, and electrical, use a licensed plumber or licensed electrician. R.E. Merrill & Associates has handled industrial and municipal pumps since 1987 — for questions about pump selection or performance curves, you can contact R.E. Merrill & Associates.
Related across the site
- Pressure tank precharge — another short-cycling cause, on well systems
- Softener drain air gaps — where softener discharge should go
Sources
- Zoeller M53 Mighty-Mate Amazon listing — 1/3 HP, cast iron, oil-filled, vortex impeller, 9 ft cord with LED plug.
- Zoeller M98 Flow-Mate Amazon listing — 1/2 HP, 72 GPM max, 23 ft max lift, 1-1/2″ NPT, snap-action float, stainless hardware.
- Wayne CDU980E listing and owner’s manual — 3/4 HP, vertical float, 5,490 GPH at 0 ft and 4,600 GPH at 10 ft, minimum 11″ basin, not for softener discharge or sewage.
- Pit volume: gallons per inch = π × (radius in inches)² ÷ 231.