Why Is There Air in My Pool Cartridge Filter?
If you have installed a brand-new filter cartridge and noticed an air gap inside the filter tank, this is normal behaviour. It does not mean the filter is faulty. It means the new filter cartridge element isn't restrictive at all! Some people call it "cavitating" but this is not accurate either.
Brand New Filter Cartridges Have Full Flow
A new cartridge lets water pass through very easily because it has no debris in it. There is very little resistance. This high-flow condition creates low internal back pressure, which often results in an air pocket forming at the top of the filter housing.
This air pocket can confuse pool owners and even some inexperienced pool technicians, but it is normally nothing to worry about.
How to Remove the Air Gap and Increase Back Pressure
When a new cartridge is installed, the filter tank may take longer to fill completely because the cartridge offers almost no resistance. This is expected.
Manually bleed the filter:
Open the air relief valve at the top of the filter while the pump is running. This allows trapped air to escape. Keep the valve open until a steady stream of water flows out.
Temporarily restrict the return flow:
If the air gap remains, you can increase internal pressure by slightly closing the valve on the return line after the filter. This added resistance helps the water force out the remaining air inside the tank. If your system does not have a return-line valve, we strongly recommend having one installed.
Over time, your filter will collect fine debris, which is how filtration works. As this debris builds up, resistance increases and the cartridge begins to create its own back pressure. This process can take several weeks or months depending on pool use and water clarity.
This gradual rise in pressure is completely normal. As back pressure increases, the water level inside the tank rises and any remaining air is displaced.
Most modern cartridge filters also have an internal automatic air bleed system. This is usually a small pipe or capillary tube that slowly purges air while the pump is running. It works continuously but at a slow rate, so the filter tank will not always fill instantly, and that is perfectly fine.
I Bled the Air, but there is More Air in the Tank!
If you bleed the air from the filter and it keeps coming back, that means new air is being pulled into the system from somewhere before the filter.
With an old and dirty cartridge, this issue is often hidden because the clogged filter creates higher back-pressure. That pressure helps keep small leaks sealed. Once you install a brand-new full-flow cartridge, the pressure drops and the system becomes much more sensitive to air leaks. The air you never noticed before now shows up as a constant air pocket in the filter housing.
Air can enter the system anywhere between the skimmer and the filter. Any fitting, mechanical seal, lint pot, etc.
Why Proper Plumbing Matters
Good pool plumbing includes valves on the return line so you can control flow and back pressure. Many systems lack these valves because shortcuts were taken during installation.
If your system does not have a valve on the return line, it is worth adding one. It makes the system easier to maintain and improves performance.
Full-Flow Cartridges Are a Good Thing
This behaviour is not a problem. In fact, it is beneficial. Full-flow cartridges reduce strain on the pump and improve circulation. They are especially helpful with variable-speed pumps because they reduce energy consumption while still providing effective filtration.
In Summary
If you see air at the top of the filter tank after installing a new cartridge, this is completely normal. Bleed the air, use your return valve if needed, and give the filter time to settle in. Modern cartridge filters are designed to manage this safely and automatically.
If you are unsure, contact the team at Heater and Spa Parts for advice on filters, flow control, and system upgrades.