Pool pump losing prime (overnight or when off)
Updated : · Piscines VP
If the strainer drains when the pump is off, air is getting into the suction side and the water runs back to the pool. Common causes: lid O-ring or suction fitting letting air in, pump sitting above the water level, missing or faulty check valve. Find the air leak before the pump runs dry at start-up.
In the morning, the pump basket is half empty and the pump takes a while to get going? It’s losing prime when it’s off.
What’s happening
When the pump stops, the water in the suction line wants to flow back to the pool. If air can get in somewhere, it replaces the water, and the strainer drains.
Causes
- Lid O-ring dry or damaged: air gets in when off even if it seems sealed while running.
- Suction fitting letting air in. See the shaving cream trick in Air bubbles in the basket.
- Pump above the water level, far from the pool.
- Check valve missing or faulty.
What to do
- Lubricate or replace the lid O-ring.
- Find and fix the air leak.
- If the pump sits above the water, a check valve on the suction side can help (installed by a technician).
- See also Pool pump won’t prime.
Frequently asked questions
Is it serious if the pump restarts empty?
If it re-primes within a minute or two, it isn't urgent, but each dry start wears the shaft seal. If it struggles to re-prime, fix it soon.
What's a check valve?
A valve that lets water pass in one direction only. On the suction side, it keeps water from draining back to the pool when the pump is off.
Not sure of the cause? Take the 2-question diagnosis →
Be the first to know
The first batch will be limited. People on the list hear before anyone else. One email, no ads, no sharing.
Read next
Worn pool pump bearings: the signs and what to do
High-pitched squeal, whine that gets worse, hot motor: how to recognize worn pool pump bearings and why you shouldn't wait.
Cavitation: why your pool pump sounds like gravel
A gravel or marbles sound in your pool pump is often cavitation: the pump is starved of water. The causes and the checks you can do yourself.
Pool pump won't prime: what to check
Pool pump running but not moving water? The common causes of a pump that won't prime and how to re-prime it, step by step.