How to Winterize a Pool Filter - Sand, DE, and Cartridge

Why Winterizing Your Filter Matters

A filter tank that freezes cracks. A multiport valve with standing water splits. DE grids with ice inside them tear.

The repair cost for freeze damage to a pool filter ranges from $150 (valve) to $800 or more (tank replacement). The time cost of winterizing correctly is about an hour. Every year, pool owners skip winterization and pay for it in spring.

How Do You Winterize a Pool Filter?

Winterizing a pool filter means getting every drop of water out of the tank, the valve, and the plumbing before the first hard freeze. The filter media itself stays put. Water is the only thing that has to leave.

1. Backwash and rinse the filter one final time, or pull and clean the cartridge
2. Drain the tank completely through the drain plug at the base, with the air relief valve open
3. Remove the drain plug and the pressure gauge and store both indoors
4. Set a multiport valve to WINTERIZE, or park the handle between two positions
5. Blow out the skimmer and return lines and plug them
6. Store cartridges or DE grids clean and dry indoors, and leave the housing open

That sequence takes about an hour on a typical residential setup and is the same in principle for sand, DE, and cartridge filters. Only the media handling changes.

What Happens When Water Freezes in Your Filter

Water expands approximately 9% in volume when it freezes. A filter tank holds several gallons of water in the sand, around the laterals, and in the underdrain.

When that water freezes:

• The expanding ice cracks the filter tank (fiberglass or ABS plastic both fail)
• Lateral arms inside the sand filter crack at their connections
• The multiport valve body splits if standing water is trapped in the valve ports
• DE grid fabric tears under the pressure of ice forming inside the grid channels

None of these failures are immediately obvious - they look fine until spring startup, when the cracked filter either leaks immediately or fails within the first run cycle.

Do You Have to Drain the Filter Tank?

In any climate that freezes, yes, and it is not a step you can half do. Any water left behind in the tank, in the valve body, or in the little Bourdon tube inside the pressure gauge can freeze and crack the housing it sits in. That single failure is the most expensive winterizing mistake there is, because a cracked tank or split valve body is a replacement part, not a repair.

Two things make people leave water behind without realising it:

• They skip the air relief valve. A sealed tank draining through one small plug pulls a partial vacuum on itself. Drainage slows to a trickle and stops with an inch or two still sitting in the bottom. Open the relief valve first and the tank empties in 10 to 15 minutes.
• They leave the tank standing level on soft ground. If the equipment pad has settled, water pools on the low side of the tank floor away from the drain port. Rock the tank gently once it stops draining and you will often get another cup or two out of it.

The gauge deserves its own mention. It is the cheapest component on the pad and the first one to fail, because the sensing tube inside holds a thread of water that has nowhere to expand. Unscrew it, wrap the threads for spring, and put it on a shelf indoors.

Owners in climates that never drop below freezing can skip nearly all of this. Florida, the Gulf Coast, and Southern California pools stay open year round. Cut the pump to a reduced winter schedule of 4 to 6 hours a day, keep chlorine in range, and leave the filter assembled and full.

How to Winterize a Sand Filter

Sand filters are where most winterizing damage happens, mainly because the tank looks empty long before it actually is. This is the sequence that matters.

1. Run one final backwash and rinse. Backwash until the sight glass runs clear, then rinse for 30 seconds to reseat the sand bed. If you are unsure of the cycle, the full procedure is in our guide on how to backwash a pool filter. Do not add anything to the filter afterwards.

2. Set the valve to WINTERIZE or between ports. Hayward and Pentair multiports have a WINTERIZE or CLOSED detent for exactly this. If yours does not, lift the handle and leave it halfway between two labelled settings. The point is to take the spring load off the diverter so the rubber gasket is not compressed against the port seats for five months, and so any water still in the valve has somewhere to go.

3. Pull the drain cap at the base of the tank. This is the large threaded cap or plug at the bottom of the tank body. Open the air relief valve on the lid at the same time so air can replace the water. Let it run until it stops completely, which usually takes 10 to 15 minutes on a 24 inch tank.

4. Remove the pressure gauge and take it inside. Trapped water splits gauges before it splits anything else on the pad. It threads out with a wrench and goes back in spring with fresh thread tape. If you need to identify it or the other components on the tank, see our breakdown of pool filter parts.

5. Leave the sand in place. Sand does not need removing, replacing, or covering for winter. It is inert, it lasts 5 to 7 years, and disturbing the bed every autumn only accelerates channeling. Drain the water and the sand is fine.

6. Leave the drain plug out all winter. Do not reinstall it in November because it looks tidy. Drop the plug and its O-ring into a small mesh bag and tie the bag to the tank collar or the valve handle. It cannot get lost, it cannot get thrown out, and it is impossible to forget at reopening because you have to untie it before you can do anything else.

Winterizing a Sand Filter - Step by Step

Tools needed: Flathead screwdriver or socket wrench for drain plugs, O-ring lubricant

1. Do a final backwash and rinse cycle
2. Turn off the pump and circuit breaker
3. Remove the drain plug(s) from the filter tank bottom
4. Open the air relief valve at the top of the filter
5. Allow the tank to drain completely (10 to 15 minutes)
6. Set the multiport valve to WINTERIZE (or position between settings)
7. Remove the pressure gauge and store indoors (cold can freeze the Bourdon tube)
8. Lubricate all O-rings with silicone lubricant
9. Store drain plugs somewhere you will find them in spring (the skimmer basket is a classic spot)

Do not: Leave the valve in any locked position. Do not add fresh sand or any chemicals to the filter before closing.

Winterizing a DE Filter - Step by Step

Tools needed: Filter cleaner, O-ring lubricant, storage space for grids (in hard freeze climates)

1. Do a final backwash cycle - do not recharge with DE
2. Turn off the pump and circuit breaker
3. Release pressure through the air relief valve
4. Remove the drain plug from the filter tank
5. In mild climates: allow the tank to drain, leave assembled
6. In hard freeze climates: disassemble the filter completely, remove the manifold and grids, rinse each grid with a hose, store indoors dry
7. Leave the drain plug removed and the air relief valve open
8. In spring: reassemble, check grids for damage, add fresh DE charge

Winterizing a Cartridge Filter - Step by Step

1. Remove the cartridge from the filter housing
2. Clean the cartridge thoroughly with a garden hose (spray between pleats)
3. Soak in cartridge filter cleaner overnight if it has been more than one season
4. Store the clean cartridge indoors in a bucket of clean water (prevents the media from drying and cracking)
5. Drain the filter housing completely
6. Leave the drain plug removed
7. Store the housing lid loosely in place (not sealed)

Recommended cartridge cleaning products: Pool cartridge filter cleaner on Amazon - use a dedicated cartridge cleaner, not bleach or household cleaners.

Cartridge and DE Filter Winterizing

Sand filters are forgiving because the media is inert. Cartridges and DE grids are not. Both are fabric, and fabric that goes into storage dirty or damp comes out of storage ruined.

Cartridge filters. Take the element out of the housing before anything else. Deep clean it properly, not a quick hose down: spray between every pleat until the water runs clear, then soak it overnight in a dedicated cartridge cleaner to strip the oils and scale a hose cannot touch. Then dry it fully. A cartridge that goes onto a shelf still wet grows mildew in the pleats and comes out in spring smelling like a pond and flowing at half its rated rate. Store it indoors, flat or standing upright, never squashed on its side and never damp. Leave the housing itself drained with the drain plug out and the lid resting loosely on top so the inside can breathe.

DE filters. Backwash first, then commit to the full teardown. Open the clamp band, lift the tank, pull the manifold, and separate the grids. Rinse each one, then soak the set in filter cleaner to break down the oils bound into the fabric. Dry the grids completely before they go anywhere. From there you can either reassemble the empty filter and leave it drained with the plug out, or store the grid assembly indoors, which is the safer option in hard freeze country.

What you cannot do is leave DE caked on the grids over winter. Dried DE bonds to the fabric, sets hard over five months, and blinds the grid permanently. Come spring the pressure runs high from the first minute and no amount of backwashing brings it down. New grid sets run $150 to $300, which is a lot of money for skipping a 40 minute cleaning.

O-Ring Maintenance at Closing

Winterization is the best time to inspect and lubricate all O-rings.

O-rings degrade over time - they dry out, crack, and lose their seal. Cold winters accelerate this degradation. Inspecting them at closing means you know the condition before spring startup, not after the pump is already running and leaking.

Check:

• Multiport valve O-ring (large O-ring on the valve base)
• Drain plug O-rings
• Union O-rings in the plumbing
• Filter lid O-ring (cartridge and DE filters)

Apply a thin coat of pool-safe silicone lubricant. Do not use WD-40, vegetable oil, or Vaseline on rubber O-rings - these degrade rubber over time. Pool silicone lubricant is inexpensive and specifically formulated for this use.

Pool O-ring silicone lubricant on Amazon

Spring Startup Checklist

Before reopening in spring:

• Replace drain plugs (with fresh O-rings if the old ones are cracked)
• Reinstall the pressure gauge
• Reinstall the cartridge (cartridge filters) or add fresh DE (DE filters)
• Set multiport valve to FILTER position
• Prime the pump
• Check all connections for leaks before leaving the equipment running unattended

Spring Reopening Checklist

Winterizing well makes reopening short. Work in this order:

1. Reinstall the drain plug and pressure gauge with fresh tape. New PTFE tape on the gauge threads and a lubricated O-ring on the plug. Hand tight plus a quarter turn, no more.
2. Refill and prime the pump. The filter cannot do anything until the pump has water in the housing and a solid prime. If it will not catch, work through how to prime a pool pump.
3. Backwash before you filter. The first thing through a winter dormant sand bed is stale water and debris. Backwash for 2 minutes, rinse, then switch to FILTER.
4. Record the new clean baseline pressure. Write the reading on the tank with a marker. Every backwash decision for the rest of the season is measured against that number, and last year’s baseline is no longer valid.

The full opening sequence, including chemistry and startup order, is in our guide on how to open a pool with a sand filter.

Related Guides

• How to Backwash a Pool Filter - final backwash procedure
• How to Change Pool Filter Sand - spring sand replacement if needed
• How Much DE for Pool Filter - spring recharge quantities
• Pool Filter Maintenance Complete Guide - full maintenance schedule