A sump pump runs constantly when water keeps entering the pit, or when the pump can't shut off. Common causes are a stuck or obstructed float, a check valve that lets water drain back, an airlocked pump, a blocked discharge line, or a pump too small for the inflow. A pit that refills with no rain may mean a leak.
Is water still coming into the pit?
Look into the pit first, with the pump unplugged if you need to reach in. Is the water level dropping when the pump runs, then rising again quickly? Or is the pit close to empty while the pump keeps humming?
If the pit refills fast, the pump is responding to real inflow. Wayne's troubleshooting chart lists "excessive inflow or pump not properly sized for application" as one cause of a pump that will not shut off, and its fix is to recheck the sizing calculations to determine the proper pump size. In other words, the pump may be running because more water is arriving than it can clear, or because it is too small for the job. If the pit rises faster than the pump can keep up, a high-water alarm will sound; see why a sump pump alarm goes off.
Rain explains many cases. But a pit that keeps filling when it has not rained is worth chasing. Some possible sources are a leaking supply pipe or fixture, a sprinkler or irrigation leak near the house, or a drain that is feeding the pit. In an Arizona home, where groundwater is usually deep and a sump pit is uncommon, a pit that fills on a dry day may point to a leak instead of the water table. Our guide on how to do a water meter leak test shows how to check for a hidden leak: shut off every fixture and see whether the meter still moves. For a leak that does not show itself, how plumbers find hidden leaks explains the methods.
Liberty's chart has a related entry. Under "pump runs periodically when fixtures are not in use," it lists a check valve that is not installed, stuck open or leaking, and "fixtures leaking," with the fix of repairing the fixtures to eliminate the leakage. Leaking fixtures are not the only possible source, but they are a cheap thing to rule out.
If the pit is not the source of the trouble, the next question is whether the water is going where it should. Clear water pumped out of the pit has to go somewhere legal. Our guide on where a sump pump can legally discharge covers that. It also matters here, because a discharge pipe that ends too close to the house can send water right back toward the foundation and into the pit again.
Can a bad check valve make a sump pump run constantly?
Yes, and it is one of the most common causes. The check valve sits on the discharge pipe just above the pump. Its job is to stop the water in the pipe from falling back into the pit after each cycle. Wayne's manual says a check valve is required "to prevent the back-flow of water after each pump cycle." Liberty's manual says the same, and adds that a gate valve should follow the check valve "to allow periodic cleaning of the check valve or removal of the pump."
When the check valve leaks or sticks open, the water in the discharge pipe drains back into the pit as soon as the pump stops. That raises the float, and the pump starts again. The cycle repeats. Both manufacturers list this cause:
- Liberty: under "pump cycles too frequently" and "pump runs periodically when fixtures are not in use," it lists a check valve not installed, stuck open or leaking, and the fix is to "remove check valve and examine for freedom of operation and proper installation."
- Wayne: under "pump starts and stops too often," it lists back-flow of water from the discharge pipe and a check valve not functioning properly or leaking. Its fix is to install a check valve, or to remove it and examine it for proper installation and free operation, and replace it if necessary.
Wayne says a check valve is required with its pumps. If yours has none, that is the first thing to add.
A quick clue: if the pump stops, and a few seconds later you hear water falling back down the pipe and then the pump starts again, the check valve is not holding.
A small air-relief hole on the discharge pipe can look like a leak but is a different thing. Zoeller says it recommends drilling a weep hole "just above the pump's body and below the check valve," and Liberty says a small spray of water from its built-in anti-airlock hole is normal while the pump runs. That spray is not a problem.
Could the float be stuck?
If the pit is empty and the pump still runs, suspect the float. Liberty's chart lists two causes for a pump that "runs, but does not turn off": the float switch unable to move to the "off" position because of interference with the side of the basin or another obstruction, and a defective float switch. The fixes are to position the pump or float so it has clearance to move freely, or to replace the switch.
Wayne's chart for a pump that will not shut off gives the same list. It names the switch, switch obstructions, a restricted discharge, and excessive inflow or an undersized pump. The fixes are to replace the tether switch or the pump, remove the obstruction so the switch moves freely, clear the discharge piping, and recheck the sizing.
Wayne also says to make sure the pump stands upright. It explains that vibration can make a pump tilt onto its side, and a tilted pump can jam the float. Wayne's manual also says to replace the tether switch every two years.
What about an airlocked or thermally tripped pump?
Two less common causes deserve a mention.
Airlock. Liberty lists "pump airlocked" as a cause of a pump that runs but does not turn off. Its fix is to turn the pump off, let it sit for several minutes, and restart it. An airlocked pump can hum without moving water, and it may look like it is running forever because the pit never clears.
A blocked or restricted discharge. If water cannot leave the pipe, the pump runs and the pit stays full. Liberty says to check the discharge line for foreign material, "including ice if the discharge line passes through or into cold areas." Wayne lists "restricted discharge (obstruction in piping)" for a pump that will not shut off. Check that any valve on the line is open and that the outlet at the far end is clear.
Wayne also describes a pump that "shuts off and turns on independently of switch" when it trips its thermal overload protection. Its listed causes include excessive water temperature, a switch problem, an obstruction in the discharge pipe, and low supply voltage. Wayne says the pump should not be used for water above 77°F. A pump that shuts itself off and restarts is protecting its motor, and it needs its cause found.
### What should you do now?
If the pump has been running a long time, unplug it before you look into the pit. Then work through the checks above: the float, the check valve, the discharge line and the pit itself.
If you cannot find the cause, or the pit is filling from a source you cannot see, have it checked. Our guide on why a sump pump won't turn on covers the opposite problem. To catch these problems early, see how to maintain and test a sump pump.
HQ Plumbing and Air can track down where the water is coming from and replace a failed pump, check valve or float. Free estimates, upfront pricing, and someone to call at any hour. If the pump in question moves sewage instead, see how to replace a sewage ejector pump float switch and why a sewage ejector pump runs but does not pump.