Sump Pump Troubleshooting

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Short-Cycling — The Motor That Never Gets a Minute

Short-cycling is on, off, on, off, every few seconds to half a minute. Heat builds. The start capacitor (where there is one) works too hard. The check valve slams. Impellers and floats die early. It looks like “the pump is too small and cannot keep up,” which is the opposite of what is happening: the pump is emptying a tiny volume too fast, then the float falls, then a cup of inflow brings it back on.

Check, in order:

• Drawdown. A tethered float tied in a knot, or a vertical float with the stops an inch apart, turns an 18 in pit into a 2 in pit. Let the float travel. The calculator’s cycle time assumes 8 in unless you tell it otherwise.
• Basin diameter. A 12–15 in “utility pump in a bucket” cannot hold a civilized cycle at 10+ GPM inflow no matter what HP you drop in. Larger liner, not larger motor.
• Oversized pump in a small pit. Putting 3/4 HP in a 15 in crock to “be safe” shortens on-time. Match GPM at head, then give the float room. See the sizing guide.

Pump Running Constantly

If the motor never shuts off, either water never falls below the off level, or the float thinks it is still up. Those are different jobs.

Water never drops

Inflow equals or exceeds pump GPM at this head. Time a rise with the pump unplugged (guide). If inflow is 20 GPM and a tired 1/3 HP is making 12 GPM at 18 ft of real head, the pit will not empty. That is a real undersize or a real head problem (clog, ice at the outlet, crushed flex line). Run the calculator with measured numbers, not the carton.

Water drops, float stays up

Tether hung on the discharge, float stuffed against the liner, or a vertical rod bent. The pump is pumping. The switch never sees “off.” Free the float. Do not oil it; replace a waterlogged float.

The riser is refilling the pit

Failed check valve. Next section. The pump empties the basin, shuts off, the 8 ft of standing water in the discharge falls back through a stuck-open check, the float rises, the pump starts. From the basement door it looks like the pump “never stops.” From the pit it is a 30-second cycle of the same gallons.

Pump Not Keeping Up During Heavy Rain

A pump that idles through a drizzle and loses the pit in a two-hour downpour is either seeing a real inflow spike, or the discharge is choking exactly when you need it.

• Measure storm inflow. Timed rise during the event, or as close as you can get. Quiet-day 4 GPM × 1.5 is not a 25 GPM footing drain.
• Outlet iced or buried. Daylight pipes freeze. A buried pop-up emitter silts shut. The pump is running against a closed pipe. Head goes to shutoff. GPM goes to zero. Clear the outlet before you buy 3/4 HP.
• Impeller worn or jammed. Gravel and silt live in sump pits. A pump that used to keep up and now does not may still be the right HP with a dead impeller. Pull it (power off) and look.
• Downspouts into the crock. That is a storm sewer in a 18 in bucket. Keep roof water on splash blocks or a separate storm line. This calculator’s 1/3–3/4 HP band is groundwater, not a roof leader.

Interior fixtures gurgling during the same storm is probably DWV, not the sump. Slope vs vent: drain slope vs vent sizing. Do not enlarge the sump pump to fix a wet vent.

Check Valve Failure — The Silent Recirculator

A check valve on the discharge keeps the riser full when the pump stops. Without it, every gallon you just lifted falls back into the pit. Symptoms:

• Short, regular cycles even in a large basin (the drawdown is the riser volume, not the float travel).
• A slam or bark in the pipe when the pump starts (valve was closed and is slamming open) or when it stops (valve slamming shut). Some slam is normal. Constant machine-gun cycling is not.
• Pump runs, pit drops, then the waterline climbs with the pump off faster than groundwater can explain.

Replace a failed swing check. Install it in the direction of flow, with a union so the next pump change does not cut the stack. Vertical installation per the valve’s instructions — some checks are horizontal-only. A missing check is not a sizing problem and not a reason to jump from 1/3 to 3/4 HP.

FAQ