Air Bubbles
If you’ve ever noticed air bubbles shooting out of your pool return jets, you’re not alone — and you’re right to be concerned. While a few tiny bubbles during startup can be normal, persistent air in your return lines is a sign that something isn’t right within your pool’s circulation system.
Air Bubbles indicate a potential equipment issue
Air bubbles are more than just an annoyance. Left unresolved, they can:
• Reduce circulation efficiency
• Cause noisy pump operation
• Lead to pump cavitation
• Accelerate wear on seals and internal components
• Increase energy consumption
• Eventually result in costly repairs
Today we are going to jump into:
• What air bubbles in pool return lines mean
• The most common causes (from simple to severe)
• How to identify where the air is coming from
• Practical troubleshooting steps
• When it’s time to call a professional
• How to prevent the issue from happening again
Whether you’re a homeowner trying to understand what’s happening or someone who wants to protect their pool investment, this guide will walk you through everything you need to know.
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What Are Pool Return Lines — and Why Do Air Bubbles Matter?
Your pool return lines are the final step in your circulation system. After water is pulled from the pool through the skimmer and main drain, it travels through the pump, filter, and heater (if equipped), then returns to the pool through the return jets.
Under normal conditions, this system should be completely sealed and air-free once the pump is primed.
When air enters the system:
• The pump can no longer move water efficiently
• Water flow becomes inconsistent
• The pump basket may not stay completely full
• You may hear a rattling, gurgling, or grinding noise
Air bubbles exiting the return jets are a visual symptom of a suction-side problem nearly 90% of the time.
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Normal vs. Problematic Air Bubbles
Before jumping to conclusions, it’s important to know what’s normal.
Normal Air Bubbles
• Small bubbles for 30–60 seconds after startup
• Tiny bubbles after vacuuming (temporary)
• Minor air release after opening the system for maintenance
These typically disappear quickly as the equipment pushes the trapped air out of the plumbing.
Problematic Air Bubbles
• Continuous bubbles during normal operation
• Large bursts of air
• Pump basket never fully fills with water
• Bubbles worsen when valves are adjusted
• Loss of prime after pump shuts off
If you’re seeing any of these, you’re dealing with an air intrusion issue.
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The Pool System Rule You Must Understand
Water leaks on the suction side suck air in — they don’t leak water out. The suction side of the pool is all the plumbing from the front of the pool pump and back. This includes things like the drains at the bottom of the pool body, the skimmers and wall ports for suction side pool sweeps.
This is why suction-side problems are harder to spot. You may not see water on the ground, yet air is still entering the system.
Before you go any further, this post will require your pool to be at the proper water level for the skimmers to flow unobstructed. If you don’t know where that is, we recommend reading this blog first and seeing if that resolves the issue. Now let’s break down the most common causes.
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1. Pump Lid O-Ring and Pump Lid Issues (Most Common Cause)
The first place we are going to start and rule out is the pump lid. The pump lid creates a seal that allows the pump to maintain vacuum. Over time:
• O-rings dry out
• Lids warp from heat and sun exposure
• Dirt or debris prevents a full seal
• Lids crack from overtightening
Even a tiny imperfection can allow air to enter.
Signs This Is the Problem
• Bubbles worsen when the pump is running
• Pump basket shows swirling air
• You see bubbles immediately after startup
• Air disappears when you spray water over the lid
How to Diagnose
1. Turn off the pump
2. Remove the lid
3. Inspect the O-ring for:
• Flat spots
• Cracks
• Brittleness
4. Clean the O-ring groove thoroughly on the lid
5. Lubricate with silicone pool lubricant only
How to Fix
• Replace worn O-rings (cheap and effective)
• Do not overtighten the lid
• Replace warped or cracked lids
This one fix alone solves air bubble issues in a large percentage of cases.
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2. Suction-Side Valve Seals and Diverter Valves
If the pump lid wasn’t the culprit, we will next want to look at the valves.
A pool valves’ internals will have:
• An internal diverter or check valve flapper
• Two O-rings on the diverter and a lid oring
• Seals that age and shrink on the diverter or check valve flapper
These components sit under constant vacuum pressure.
Common Valves Involved
• Skimmer / main drain selector valves
• Multiple suction line diverter valves
• Vacuum line valves
Symptoms
• Air bubbles change when valves are adjusted
• Bubbles worsen when a specific line is selected
• Slight hissing sound near the valve, usually audible while the pump is off
• No visible water leaks
How to Diagnose
• Run the pump
• Slowly move each suction valve, turning off the pump between valve changes
• Note which valve increases/decreases bubbles and proceed to the next step
How to Fix
• Unscrew the lid of the valve and replace the internal O-rings
• While we have it open, replace diverter valve and/or check valve flapper if worn
• Replace cracked valve bodies
Note: installation requires care to avoid future leaks. Do not overtighten the valve lid, cracked housings require replumbing with a new valve.
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3. Loose or Improperly Sealed Unions and Fittings
Another common area of air leaks is the pump unions or fittings before the pump due to:
• Cracked/Pinched O-rings
• Old thread sealant
• Cracked fittings
Signs
• Bubbles appear intermittently
• Bubbles worsen when the pump runs at higher speed
Diagnosis Method
• Visually Inspect any Union on the Suction side of the pool pump for cracked threads
• Unthread the Unions and inspect the internal O-ring
Resolution
• Tighten unions carefully
• Replace cracked fittings and/or O-rings
• Re-seal threaded connections using proper sealant and lubricate O-rings with silicone lubricant
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4. Clogs and Obstructions on the Suction Side
A clog or restriction in the suction line increases vacuum pressure and limits the amount of water flowing into the pump.
Common Obstructions
• Skimmer/Leaf Cannister baskets filled with debris
• Pump baskets clogged with leaves
• Debris lodged in suction lines, especially suction ports used for suction-side pool cleaners
Warning Signs
• Reduced water flow
• Loud pump noise
• Increased air bubbles
• Pump struggling to prime when a specific suction valve is open
Steps to Resolve
1. Clean all baskets thoroughly
2. Inspect pump basket for debris under the basket
3. Check skimmer weir doors for free movement
4. Not the suction line, but backwashing or cleaning filters can help with water flow
Never ignore clogs — they often lead to larger mechanical failures. Call a professional if you think you have a clogged suction line. Most pool services like us carry CO2 and plumbing snakes for situations like this.
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5. Underground Suction Line Leaks (The Most Serious Issue)
Underground suction leaks:
• Are invisible
• Worsen over time
• Can destroy pumps if ignored
• Often require pressure testing
Common Causes
• Soil movement
• Improper installation
• Aging PVC
• Tree root intrusion
Signs of Underground Suction Leaks
• Persistent air no matter what you fix
• Pump never stays fully primed
• Bubbles increase with higher pump speed
• Air remains even after replacing all above-ground seals
How Professionals Diagnose It
• Line isolation testing
• Pressure testing suction lines
• Dye testing
• Specialized listening equipment
Solutions
• Spot repairs if leak is localized
• Rerouting suction lines
• Installing new dedicated lines
This is not a DIY repair and requires professional equipment.
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7. Pump Shaft Seals (Less Common, Still Important)
A failing shaft seal typically leaks water — but under certain conditions, it can also allow air to enter.
Signs
• Water dripping under the pump
• Rust stains
• Grinding noises
• Intermittent air bubbles
Fix
• Replace shaft seal
• Inspect motor bearings
• Ensure proper alignment
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Step-By-Step Troubleshooting Checklist
If you want a logical approach:
1. Check water level
2. Clean skimmer, leaf-cannister and pump baskets
3. Inspect and lubricate pump lid O-ring, Valve O-rings and Union O-rings
4. Inspect Suction Fittings and Valve housings for Cracks
5. Adjust suction valves one at a time, if isolating a single line resolves the issue, a clog is likely.
If the issue persists after these steps or you think there is a clog in the plumbing, professional diagnosis is recommended.
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Preventing Air Bubble Issues Long-Term
Best Practices
• Lubricate O-rings annually
• Replace seals proactively
• Avoid overtightening lids and unions
• Keep water levels stable
• Address small changes early
A properly sealed suction system should run quietly, efficiently, and bubble-free.
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When to Call a Professional Pool Service
You should call a professional if:
• Air persists after basic troubleshooting
• You suspect underground leaks
• Pump loses prime regularly
• Equipment noise increases
• You’re unsure which component is failing
Experienced technicians can diagnose issues in minutes that might take homeowners weeks.
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Final Thoughts: Air Bubbles Are a Symptom, Not the Problem
Air bubbles in pool return lines are never random. They’re a clear signal that air is entering where it shouldn’t. Sometimes the fix is simple. Other times, it requires expertise and specialized tools.
Either way, addressing the issue early protects your equipment, saves money, and keeps your pool running the way it should.
