How do I clean a pool cartridge filter?

Switch the pump off, cut power at the breaker, release the pressure, and then hose the pleats clean one cartridge at a time. The job needs a garden hose, a bucket and a wrench. The one part that deserves care is the pressure: a closed pool filter is a pressure vessel, and the order of the first steps is a safety matter, not a formality.

When to clean it

Go by the pressure gauge. Pentair says to clean the filter when the pressure gauge reads approximately 10 psi higher than the original starting pressure, or when a significant reduction in flow is noticed.^[1] Hayward gives a range, saying to clean or replace the cartridge when pressure rises 7 to 10 psi above the starting pressure.^[2] That means you need the "clean" reading. After each cleaning, write down the gauge pressure with the pump running and the cartridges fresh, and tape it next to the gauge.

If you have a variable-speed pump, record the pump speed with it. The same pump at a lower speed shows a lower pressure, so comparing readings only makes sense at the same setting.

Before you open anything

Pentair puts the warning first in its guide: pressurized air can cause the lid to separate from the tank with great force.^[1] Air gets into the system whenever it is serviced, so release it before the clamp ring comes off. To do that, turn the pump off, shut off any automatic controls such as timers, and disconnect power at the circuit breaker so the system cannot start during maintenance.^[1] Then open the manual air relief valve by turning it a quarter turn counterclockwise, stand clear, and wait until the pressure gauge reads zero.^[1]

Open the drain plug at the bottom and let the tank empty. Loosen the clamp ring, set it aside, and lift off the lid. Do not use a screwdriver or pry bar to lift the lid, because that can damage the O-ring.^[1] If it sticks, wiggle it gently from side to side.

Cleaning the cartridges

Lift off the top manifold and take out each cartridge one at a time. Then:

• Hose them down. Hold the nozzle at a 45-degree angle to the cartridge and wash the entire cartridge from top to bottom, paying special attention to the area between the pleats.^[1] Use a straight-flow nozzle rather than a jet.
• Do not pressure wash. Hayward says not to pressure wash a cartridge, because it can damage the filter element.^[2]
• Let it dry and brush it. For a stubborn cartridge, let it dry, then carefully brush the pleated surface to remove fine particles.^[2]
• Deal with oils. Sunscreen and body oils coat the pleats and hosing alone will not shift them. Pentair says to soak the cartridge for at least one hour in a commercial filter cleaner, a solution of 1 cup of trisodium phosphate to 5 gallons of water, or 1 cup of dishwasher detergent to 5 gallons of water, then rinse it thoroughly.^[1]

Wash out the inside of the tank and the bottom manifold while everything is apart, and look at the gasket and O-ring for grit or cracks. A dirty O-ring is the usual reason a lid leaks after reassembly.

A word on acid

Mineral scale and algae can build up over the years, and manufacturers describe an acid soak for it. Treat that as an advanced step. Pentair warns that acid soaking without first removing all oils and cleaner will permanently restrict water flow through the filter and cause the cartridge to fail early.^[1] Muriatic acid is corrosive and can burn skin, and Pentair says to wear rubber gloves, safety glasses and an N-95 dust mask when handling it.^[1] If a cartridge still will not recover after a proper hosing and a cleaner soak, replacing it is simpler and safer. You can expect an average cartridge life of three years under normal conditions.^[1]

Putting it back and restarting

Seat the cartridges and manifold, make sure the O-ring is clean and seated, refit the lid and clamp ring, and tighten the clamp as the manual shows. Before starting the pump after servicing, make sure the clamp band is correctly fastened.^[1]

Open the air relief valve, stand clear of the filter, and switch the breaker and pump back on. When a steady stream of water comes out of the valve, close it. Then read the gauge and write down the new clean starting pressure.

Stop and call a pro if

• Pressure does not drop to zero when you open the air relief valve, or the valve is blocked. Do not open the filter.
• The system misbehaves on restart. Pentair says to shut off the pump immediately if the gauge shows pressure before water flows out, if no solid stream of water appears within 30 seconds after the pump's inlet basket fills, or if water leaks where the two halves of the filter meet.^[1]
• The breaker trips, the pump hums or the motor is hot. That is an electrical or motor problem. Virginia allows no individual to work, or offer to work, as an electrical tradesman without a tradesman licence, unless working under the supervision of a tradesman licensed in that specialty.^[3]
• The clamp ring, lid or tank is cracked or distorted. Replace the part; do not run the filter with it.
• The cartridges are torn or flattened and the pressure still climbs within days of cleaning.

Getting help in Virginia

If the filter keeps clogging or the equipment is old, a pump and filter service can look at the whole system. See how much a new pool pump costs in Virginia and whether a variable-speed pump is worth it. For routine upkeep, read how to clean a swimming pool, browse pool and spa services, or see more pool and spa answers.

When you are ready, describe your pool equipment once and local pool services quote it.