Why Is My Pool Filter Blowing Sand? - Causes and Fixes

The Four Causes - Diagnosed by When It Started

If sand started appearing after a sand change

Cause: Wrong sand grade or overfilled tank

If the pool started blowing sand immediately or within a few days of a sand replacement, one of two installation errors occurred:

1. Wrong sand grade: Play sand, builder’s sand, or any sand finer than #20 grade (0.45-0.55mm) has grains that pass through the lateral slots. Even some bags labeled “pool filter sand” from hardware stores can be the wrong grade if they are not clearly marked as #20 silica. Solution: drain the filter and replace with correct #20 grade pool filter sand.

2. Overfilled: If the sand level is above the top of the standpipe opening, sand spills directly into the standpipe and returns to the pool. Solution: drain the filter and remove enough sand to bring the level to approximately two-thirds full.

Correct sand: What Type of Sand for Pool Filter

If sand started appearing after winter or an impact

Cause: Cracked lateral (most common overall)

Laterals are the plastic arms at the bottom of the filter tank that connect to the standpipe. They have small slots that allow filtered water through but are sized to hold #20 sand. When a lateral cracks - from freeze damage, impact, or age - the slot widens enough to pass sand.

How to diagnose:

1. Turn off the pump and release pressure
2. Remove the multiport valve from the top of the filter
3. Pull out the standpipe and lateral assembly
4. Inspect each lateral arm by bending it slightly and looking for cracks along the length and at the base where it connects to the manifold

Even hairline cracks that look minor will allow significant sand to pass under operating pressure.

Fix: Replace the cracked laterals. Individual laterals can be replaced without replacing the entire assembly. Match to your filter model.

Pool filter lateral replacement parts on Amazon

If sand appeared after normal operation (no recent changes)

Cause: Aged or worn lateral slots

Over many years, the plastic lateral slots can widen slightly from wear or chemical exposure. This is more common in filters over 10 years old. The fix is the same as a cracked lateral - replace the laterals.

Also check the standpipe itself. If the standpipe has cracked lengthwise, water can carry sand through the crack into the return line. Hold the standpipe up to light after removing it - cracks are visible.

If sand only appears during backwashing

Cause: Normal carryover (not a problem)

A small amount of fine sand or DE returning to the pool during or immediately after the RINSE step is normal. This is why the RINSE step exists - to re-settle the sand bed before returning to FILTER mode.

If you are seeing sand only during the first minute of operation after backwashing and then it stops, this is normal. Run the RINSE step for a full 30 seconds and the problem resolves.

If sand appears continuously during normal FILTER operation, the causes above apply.

How to Replace Pool Filter Laterals

What you need:

• Replacement laterals (matched to your filter model)
• A plastic cup or scoop
• A bucket or tarp for the sand

Procedure:

1. Turn off the pump, release pressure via the air relief valve
2. Remove the multiport valve from the top of the filter
3. Tape over the standpipe opening with duct tape (prevents sand from falling in)
4. Scoop out the sand and save it in a bucket (if it is less than 5 years old, it can be reused)
5. Remove the standpipe and lateral assembly from the tank
6. Unclip or unscrew the damaged laterals and install new ones
7. Reinstall the assembly and standpipe
8. Remove the duct tape from the standpipe
9. Fill the tank halfway with water (cushion for the sand)
10. Pour the sand back in slowly around the standpipe
11. Reassemble the valve and run a full backwash cycle before using the pool

Related Guides

• How to Change Pool Filter Sand - if the sand also needs replacing
• What Type of Sand for Pool Filter? - confirming you have the correct grade
• Pool Filter Pressure Too High - diagnosing pressure problems
• Pool Filter Maintenance Guide - all maintenance topics