How to swap a pool filter

Your filter does most of the work of keeping your water clear, and swapping one is simpler than most people expect. The harder part is choosing the right type and size. This guide compares cartridge, sand and D.E. filters, shows where the filter belongs in your plumbing, and covers installation and a safe first start-up.

Cartridge vs sand vs D.E.

Texas tip: During droughts, many cities limit how and where you can drain pool water. A cartridge filter skips backwashing entirely, which saves water and keeps you out of trouble with those rules.

Sizing your filter

• Flow comes first. The filter's rated maximum flow must be higher than your pump's flow at the highest speed you'll run.
• Cartridge: bigger is better. More square footage means longer runs between cleanings and less pressure on the system. Sizing up costs little and pays off every season.
• Sand: respect the rating. Pushing more flow than a sand tank is rated for causes channeling, which lets dirty water slip through.
• D.E.: match grid area to flow. D.E. filters are sized by grid square footage, and the manufacturer lists the flow each size can handle.
• Check the rest of the pad. Your heater has its own flow limits too.

Not sure what your pool holds? Our pool volume calculator gives you gallons in under a minute.

Plumbing order: where the filter goes

Water should move through a standard equipment pad in this order:

1. Skimmers and main drain
2. Pump
3. Filter
4. Solar (if equipped)
5. Heater or heat pump
6. Check valve
7. Chlorinator or salt cell
8. Returns

The filter sits right after the pump so the heater and everything after it only see clean water. The chlorinator or salt cell goes last so concentrated chlorine never flows back through your heater, and the check valve protects the heater from back-siphoning when the pump shuts off.

Switching filter types? Sand and D.E. filters need a waste (backwash) line. Cartridge filters don't, so cap the old waste line if you're converting. Sand filters usually use a multiport valve with three ports: PUMP (inlet), RETURN (outlet) and WASTE. Follow the flow arrows on the valve body.

Tools and parts

• New filter with pressure gauge and air relief valve
• Media: filter sand or an approved alternative, cartridge elements, or D.E. powder
• Two unions, Schedule 40 PVC pipe and fittings
• PVC primer and pressure-rated cement
• PTFE tape and silicone-based O-ring lubricant
• Shop vac (to empty an old sand tank), bucket, PVC saw
• Dust mask. Sand and D.E. both release fine silica dust.

Installation step by step

1. Shut down. Turn off the pump at the timer or automation, then switch off the breaker. Close the valves.
2. Relieve pressure. Open the air relief valve and confirm the gauge reads zero.
3. Drain the tank. Remove the drain plug.
4. Disconnect. Unscrew the unions, or cut the pipe, leaving as much straight pipe as possible.
5. Remove the old filter. Sand tanks are heavy, so vacuum out the sand first.
6. Set the new filter. Place it on a level pad where you can read the gauge, reach the clamp and line up the ports without stressing the pipe.
7. Load the media:
   • Sand: Assemble the laterals and standpipe as the manual shows. Fill the tank about halfway with water to cushion the laterals, and cover the top of the standpipe so sand can't get in. Pour sand to the fill line on the tank label, wipe the tank rim clean, then install the valve.
   • Cartridge: Hose off the new cartridge, seat it on its locator, lubricate the tank O-ring and set the lid.
   • D.E.: Assemble the grids and manifold. You'll add the D.E. powder after start-up.
8. Plumb it. Use unions on the inlet and outlet. Dry-fit, then prime and cement. Let the joints cure for the time on the can.
9. Secure the clamp. Tighten the clamp band exactly as the manual specifies. Some require tapping around the band to seat it evenly.

First start-up

Every filter type

1. Open the air relief valve.
2. Prime and start the pump (see our pump replacement guide).
3. When a steady stream of water comes out of the air relief, close it.
4. Check every joint and the clamp for leaks.
5. Write down the gauge reading. That's your clean pressure. Mark it on the gauge or the tank with a paint pen.

Sand filters

1. With the pump off, set the valve to BACKWASH. Run for 2–3 minutes, until the sight glass runs clear.
2. Pump off. Set to RINSE and run for 30–60 seconds.
3. Pump off. Set to FILTER and restart.

D.E. filters

1. With the pump running on FILTER, mix the D.E. into a slurry in a bucket of water.
2. Pour it slowly into the skimmer closest to the equipment.
3. Add the amount on the tank label (often about 1 lb per 10 sq ft of grid area). Pressure will climb a few psi, and that new reading is your clean pressure.

Maintenance schedule

Clean or backwash when pressure rises about 8–10 psi above your clean reading. Don't clean more often than that. Sand and D.E. filters actually trap fine particles better once they're slightly loaded.

• Sand: Backwash at 8–10 psi over clean. Replace the sand every 5–7 years, or sooner if water stays cloudy.
• Cartridge: Hose off at 8–10 psi over clean. Soak in a cartridge cleaner a couple of times a season. Texas hard water leaves calcium scale, so degrease first and use an acid soak only if scale remains. Acid applied before degreasing locks oils into the pleats. Replace cartridges when the pleats are crushed or frayed or the bands break.
• D.E.: Backwash, then recharge with fresh D.E. Tear down and clean the grids at least once a year. D.E. showing up in the pool means a torn grid or a bad manifold.

Filter troubleshooting

Pool filter FAQs

Which filter is best for a salt pool?

All three work. Many salt-pool owners pick cartridge for the low maintenance and water savings.

Can I switch from sand to cartridge?

Yes. Remove the multiport valve and cap the waste line, then re-plumb the inlet and outlet to the cartridge tank. Unions make it a clean job.

How often should I change filter sand?

About every 5–7 years. If pressure stays low and water stays hazy after backwashing, the sand has probably worn smooth or clumped.

Is D.E. safe to handle?

Wear a dust mask when you add it, because the powder contains fine silica. Check your city's rules before backwashing, since some areas restrict discharging D.E.