Sump Pump Weep Hole: Why the Pump Runs But Moves No Water

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Quick answer from a licensed plumber: A weep hole is a small hole drilled in the discharge pipe above the pump and below the check valve. Its job is to let trapped air escape so the pump can prime. Without one, a pump can run at full speed and move no water at all — a fault that looks like a dead pump and is not. But check your manual first: some pumps have air relief built in and their makers advise against drilling one.

What an air lock actually looks like

This is the symptom worth recognizing, because it is genuinely alarming and completely fixable:

• The pump runs — you can hear it, it is not dead, the float is working.
• The water level in the pit does not drop, or drops far slower than it should.
• It often happens after the pit has been dry a while, or right after a new install or a pump swap.

What is happening: air is trapped in the pump housing above the impeller. The impeller spins in air, which it cannot push, so it never develops enough pressure to open the check valve and start moving water. The pump is fine. It is just spinning against a bubble it cannot get rid of.

If your pump is not running at all, that is a different problem — start with a sump pump that will not work instead.

What the weep hole does, and where it goes

Boshart describe the purpose plainly: “Its purpose is to provide a passageway to expel air that can get trapped in the sump pump housing, preventing the pump from priming and removing water from the pit.”

And the location, which is the part people get wrong: “This hole is drilled in the discharge pipe approximately 5″ above the pump discharge connection and below the check valve by the installer.”

Both halves of that matter:

• Above the pump discharge — so it sits in the section of pipe where the air collects.
• Below the check valve — this is the half people miss. Drill it above the check valve and it does nothing for the air trapped underneath, while still letting water out on every cycle. You get the downside and none of the benefit.

It should also be below the water line of the pit, so the small jet of water it releases goes back into the pit rather than spraying across your basement floor.

⚠️ Check your manual before you drill — this is not universal

This is the caveat that gets left out of most advice, and it is stated directly by the source above: “Not all sump pumps require an air release hole in the valve or discharge pipe to prevent air-locking. Some brands of sump pumps have a specially designed impeller or pump casting with a hole drilled in it that prevent the pumps from becoming air locked.”

And more pointedly: “Always follow the pump manufacturers installation instructions, some manufacturers will not recommend a check valve with an air release hole in it.”

So the order of operations is: read the manual, then drill — not the reverse. Drilling a hole in a discharge pipe is not reversible without replacing that section, and on a pump that already has relief built in you have simply added a permanent leak.

📌 On hole size: a figure around 3/16″ is widely repeated in the trade, and some pump makers specify it in their own instructions. We are not going to attribute a specific diameter to a source that does not print one — use the size your pump’s manual gives. The principle is that it must be big enough to pass air and small enough that it is not meaningfully stealing pumping capacity on every cycle.

Why an oversized hole is a real problem

Every gallon that goes out of the weep hole is a gallon the pump moved and did not get rid of. A hole that is too large means:

• Reduced effective capacity — the pump works harder for less useful output, which matters most in the storm where you need every gallon.
• Longer run times, and on a pump that is already marginal for the pit, more wear and more heat.
• An audible squirt on every cycle that people often mistake for a leak.

A small stream of water visibly returning to the pit while the pump runs is normal and is the hole doing its job. A jet that noticeably slows the pit from emptying is not.

The related failure: a check valve installed backwards

While you are looking at that section of pipe, check the check valve’s arrow. It must point away from the pump, in the direction of flow.

Installed backwards, the symptoms overlap almost exactly with an air lock — pump runs, water does not leave — so the two get confused constantly. The arrow is moulded into the body and takes two seconds to read. Check it before drilling anything.

A failed or missing check valve produces a different signature: water in the vertical discharge pipe falls back into the pit when the pump shuts off, the float rises again, and the pump short-cycles. That is hard on a pump and is worth fixing promptly — see choosing a sump pump if yours is cycling itself to death.

The winter version of this problem

A weep hole solves air. It does nothing about the other classic discharge fault, which is a line that freezes where it exits the house. A frozen discharge blocks the outlet completely, the pump runs against a closed pipe, and the pit backs up regardless of how well the pump primes.

If your pump only fails in hard cold, look at the outdoor run and its termination rather than at the weep hole — they are unrelated faults with a similar-looking result.

FAQ

Where do you drill a weep hole in a sump pump discharge pipe?

In the discharge pipe “approximately 5 inches above the pump discharge connection and below the check valve,” per Boshart’s guidance — and below the water line of the pit so the released water returns to the pit. Drilling above the check valve does not relieve the trapped air and still bleeds water on every cycle.

Do all sump pumps need a weep hole?

No. Boshart state that “not all sump pumps require an air release hole,” because “some brands of sump pumps have a specially designed impeller or pump casting with a hole drilled in it that prevent the pumps from becoming air locked,” and that some manufacturers “will not recommend a check valve with an air release hole in it.” Read your manual before drilling — it is not reversible.

My sump pump runs but no water moves. Is that an air lock?

It might be — an air lock means the impeller is spinning in trapped air and never builds enough pressure to open the check valve. But check the check valve’s direction arrow first: installed backwards it produces almost identical symptoms. Those two causes are confused constantly.

How big should the weep hole be?

Use the size your pump’s manual specifies. A figure around 3/16″ is widely repeated in the trade, but it should come from your manufacturer rather than a general article. Too large and you lose real pumping capacity on every cycle; too small and it will not pass air.

Is water squirting out of the hole normal?

A small stream returning into the pit while the pump runs is normal and means the hole is working. A jet strong enough to noticeably slow the pit emptying means the hole is oversized — and if the water is landing outside the pit, the hole was drilled above the pit’s water line.

Sources

• Boshart — What is a sump pump weep hole?: “Its purpose is to provide a passageway to expel air that can get trapped in the sump pump housing, preventing the pump from priming and removing water from the pit.” · “This hole is drilled in the discharge pipe approximately 5″ above the pump discharge connection and below the check valve by the installer.” · “Not all sump pumps require an air release hole in the valve or discharge pipe to prevent air-locking.” · “Always follow the pump manufacturers installation instructions, some manufacturers will not recommend a check valve with an air release hole in it.” No hole diameter is stated on this page — use your pump manual’s figure.