How to Tell If Your Pool Filter Needs Cleaning (And What Happens When You Wait Too Long)

Most pool problems that stump homeowners come back to one thing: the filter. Cloudy water that won't clear up even after you shock it. Algae that keeps coming back. A pump that sounds like it's working harder than it used to. Nine times out of ten, the filter is either dirty, overdue for a cleaning, or both.

Here's what you need to know about each filter type, how to read the signs, and what actually happens to your pool when cleaning gets put off too long.

The pressure gauge is telling you something

Every pool filter has a pressure gauge on it. When your filter is clean and running normally, the gauge settles at a baseline number, usually somewhere between 8 and 15 psi depending on your system. That baseline number is what you want to write down the first time you run your pool after an opening or a fresh cleaning. It's your reference point.

When the gauge climbs 8 to 10 psi above that baseline, your filter needs attention. Not tomorrow, not this weekend. Now. That pressure spike means water is having to push harder to get through the filter because it's clogged with debris, oils, and whatever else has built up in there. The pump is working against resistance it wasn't designed for.

If you don't have a baseline written down, check the gauge on a day when the water is clear and the pool has been running clean for a while. That number is your target to return to after every cleaning.

Sand filters

Sand filters are the most common filter type around South Jersey, especially on above-ground pools. They work by pushing water down through a bed of sand, which traps particles as small as 20 to 40 microns. As debris builds up between the sand grains, the filter actually gets slightly better at trapping small particles, right up until it gets too clogged and starts to restrict flow.

Cleaning a sand filter means backwashing: you reverse the water flow so it flushes the trapped debris out of the sand and out to waste. For most pools, you should be backwashing every 4 to 6 weeks during swim season, or whenever the pressure gauge tells you to, whichever comes first. After a big storm, which is common around here, backwash before you add any chemicals.

One thing a lot of people miss: sand needs to be replaced every 5 to 7 years. Old sand gets smooth and rounded from constant water flow and starts channeling, meaning water finds a path of least resistance straight through without actually being filtered. If you've had the same sand in your filter for years and can't get your water clear no matter what you do, the sand itself may be the problem.

Cartridge filters

Cartridge filters use a pleated polyester element to filter water and can trap particles as small as 10 to 15 microns, which is tighter filtration than sand. They're common on smaller pools and newer installs. There's no backwashing with cartridges. You pull the cartridge out, hose it off between the pleats, and let it dry before reinstalling if you have a spare, or reinstall it damp if you don't.

During peak season with heavy use, cartridges need rinsing every 2 to 4 weeks. At minimum, plan on cleaning them every 1 to 3 months. The pressure gauge still applies: when it climbs 8 to 10 psi above baseline, it's time.

Cartridge elements wear out. The pleated fabric degrades over time and the element stops holding its shape. Most cartridges last 2 to 3 years with proper care. If you're hosing it off and it still won't bring the pressure down, or you can see tears, flattened pleats, or the end caps are cracking, it's past time for a replacement.

DE filters

Diatomaceous earth filters trap particles down to around 3 to 5 microns, which is the finest filtration you can get in a residential pool. They use a powder made from fossilized algae that coats a set of internal grids. When the powder gets saturated with debris, you backwash to flush it out and then add fresh DE powder to recoat the grids.

Backwash a DE filter monthly during the season, or whenever pressure climbs. Every 3 to 4 months, the filter needs to be fully disassembled, the grids pulled and hosed down individually, and inspected for rips or tears. A torn grid lets DE powder blow back into the pool, which shows up as a fine white or gray cloud in the water. If you see that, the filter needs to come apart.

What happens when filter cleaning gets skipped

Short version: everything else stops working right.

When the filter is clogged, water circulation slows down. Chemicals you add to the pool aren't distributed evenly. Dead spots form where water barely moves, and algae takes hold there first. You add shock, algae comes back in a week, you add more shock, and the cycle keeps going because the root cause is a filter that can't do its job.

The pump is the other casualty. Running a pump against a clogged filter puts mechanical stress on the motor and the impeller. It runs hot, it works harder, and it wears out faster. Pump replacements run several hundred dollars or more depending on the system. A filter cleaning costs a fraction of that.

For above-ground vinyl liner pools especially, which are common around South Jersey, a neglected filter is also how you end up with staining on the liner. Oils, metals, and organic debris that the filter should be catching instead sit in the water and deposit on the liner surface.

South Jersey specifics worth knowing

Our area throws a few things at pool filters that other regions don't. The pine trees in the Pinelands and across much of Atlantic County shed tannins and fine debris that push through screens and tax cartridge filters faster than you'd expect. The heavy pollen season in May and June hits hard. And after a summer thunderstorm, filters take a real hit from runoff, leaves, and whatever else washes in.

After any significant storm, check your pressure gauge before you check anything else. If it's spiked, backwash or clean the filter before you run chemicals. Shocking a pool with a clogged filter just wastes product.

When to call someone

If your pressure gauge climbs back up within a day or two of cleaning, something else is going on: high bather load, an algae bloom, a chemical imbalance that's causing organic buildup, or a filter that's past its service life. That's a good time to have someone look at the full system rather than just keep chasing the gauge.

We handle filter cleaning for all three filter types, including full DE teardowns and cartridge replacements. If you want it handled properly and off your to-do list, give us a call at (609) 525-4124 or check out our services at aqualocksolutions.com.