Why Is There Air in My Pool Cartridge Filter?
If you’ve just installed a brand-new filter cartridge and noticed an air gap in your filter tank, don’t worry — this is normal behavior and doesn’t mean your filter is faulty or leaking.
Brand New Filter Cartridges Have Full Flow
When a new cartridge is installed, it allows water to pass through very easily. There’s little to no resistance because it’s clean and unrestricted. This high-flow situation causes low internal back-pressure, which can result 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’s nothing to be concerned about. In fact, it’s a sign your system is running efficiently.
How to Remove the Air Gap and Increase Back-Pressure
When a new cartridge is installed, it may take time for the filter tank to fill completely due to the lack of internal resistance. This is expected and not a fault.
- Manually bleed the filter: Open the air relief valve (usually at the top of the filter) while the pump is running. This will allow trapped air to escape. Continue until a steady stream of water flows from the valve.
- Temporarily restrict the return flow: If the air gap remains, you can help increase internal pressure by slightly closing the valve on the return line (after the filter). This added resistance creates back-pressure, helping the water displace remaining air inside the filter tank. If your system doesn't have a valve on the return line, we highly recommend having one installed.
With time, your filter will naturally accumulate small particles and debris during normal operation — this is how it “filters.” As this happens, resistance builds up inside the cartridge, creating its own back-pressure. This process may take several weeks or even months, depending on how often the pool is used and how clean the water is.
This gradual increase in pressure is normal and expected. As back-pressure increases, the water will begin to fill the tank more fully, and any remaining air will be displaced slowly.
Most modern, good-quality cartridge filters include an internal automatic air bleed system — often a small pipe or capillary tube that helps purge air from the top of the filter over time. This works continuously while the pump is running but does so gradually. So, it’s important to understand that the filter tank may not fill instantly, and that’s completely okay.
Why Proper Plumbing Matters
Well-designed pool systems include valves on the return line for controlling flow and back-pressure. Unfortunately, many poorly plumbed systems lack these features, often because shortcuts were taken during installation or service.
If your system doesn’t have a valve to control flow after the filter, it’s worth considering an upgrade. It’ll make maintenance much easier and improve performance overall.
Full-Flow Cartridges Are a Good Thing!
It’s important to understand that this behavior isn’t a problem — it’s actually an advantage. Full-flow filters put less strain on your pump and improve water circulation. They’re especially beneficial when using variable-speed pumps, helping reduce energy use while maintaining good filtration.
In Summary
Seeing air at the top of your filter tank after changing to a new cartridge? That’s normal. Just bleed the air, use your return valve if needed, and allow time for the filter to do its job. Modern filters are designed to manage this process safely and automatically — no need to worry.
Still not sure? Contact our team at Heater and Spa Parts for expert advice on filters, flow control, and system upgrades.