How Pool Sand Filters Work: Step-by-Step Guide

Tools & Supplies Needed

Find on Amazon: Pool Filter Sand , Pressure Gauge , Taylor K-2006 Test Kit

Quick Answer

Pool sand filters work by using a bed of specially graded filter sand (#20 silica sand) to physically strain dirt, debris, and particles from pool water. The multiport valve directs water flow through different cycles including filtration, backwashing, and rinsing to maintain clean, clear pool water.

Step-by-Step Filtration Process

1. Water Entry
   Pool water enters the filter tank through the multiport valve when set to "Filter" position. The pump creates pressure that forces water into the top of the filter tank through the inlet pipe.
2. Distribution
   Water flows through a distributor system (spider gasket and laterals) at the top of the sand bed. This ensures even water distribution across the entire sand surface, preventing channeling and maximizing filtration efficiency.
3. Sand Filtration
   Water travels downward through approximately 18-24 inches of #20 silica sand. The sand bed acts like a physical strainer, trapping particles as small as 20-40 microns. Larger debris gets caught in the top layers while finer particles penetrate deeper before being filtered out.
4. Collection
   Clean, filtered water reaches the bottom of the tank where it's collected by lateral assemblies - slotted pipes that allow water to pass through but prevent sand from entering the return system.
5. Return to Pool
   Filtered water flows back to the pool through the return lines, completing the circulation cycle. The entire process typically takes 6-8 hours to turn over all pool water once.

Key Filter Components

Multiport Valve Functions

1. Filter Position
   Normal operation mode where water flows down through sand and back to pool. Use this position for daily filtration and when adding chemicals.
2. Backwash Position
   Reverses water flow to flush trapped dirt and debris from the sand bed. Water flows up through the laterals, loosens trapped particles, and exits through the waste line.
3. Rinse Position
   Settles the sand bed after backwashing and clears any remaining debris from the system before returning to filter mode.
4. Waste Position
   Bypasses the filter entirely, sending water directly to waste. Used for draining the pool or removing water without returning it to the pool.
5. Recirculate Position
   Circulates water without filtration, useful when the filter needs repair or during certain chemical treatments.
6. Closed Position
   Shuts off all water flow, used for equipment maintenance or winterization.

Sand Media Specifications

Pool sand filters require specific #20 silica sand with angular edges that create optimal filtration. The sand granules measure 0.45-0.55mm in diameter. Never use play sand, construction sand, or beach sand as these contain fine particles that will cloud your pool water.

A typical residential sand filter holds 150-300 pounds of sand depending on tank size. The sand bed should be 18-24 inches deep for proper filtration. Replace filter sand every 3-5 years as repeated backwashing rounds the sharp edges that provide effective filtration.

Pressure and Flow Dynamics

Sand filters operate on pressure differential. Clean sand typically shows 8-12 PSI on the pressure gauge. As debris accumulates in the sand bed, pressure increases. Backwash when pressure rises 8-10 PSI above the clean starting pressure or when water flow noticeably decreases.

Proper flow rate is crucial for effective filtration. Sand filters should operate at 15-20 gallons per minute per square foot of filter area. Higher flow rates can cause channeling where water finds easy paths through the sand without proper filtration.

Maintenance Cycle Process

1. Monitor Pressure
   Check the pressure gauge daily during swimming season. Record the clean starting pressure after backwashing for future reference.
2. Backwash When Needed
   Turn multiport valve to "Backwash" and run pump for 2-3 minutes until water in the sight glass runs clear. This typically requires 200-300 gallons of water.
3. Rinse the System
   Switch to "Rinse" position and run pump for 30-60 seconds to settle sand bed and clear any remaining debris from the system.
4. Return to Filter
   Set valve back to "Filter" position and resume normal operation. Note the new clean starting pressure.

Water Chemistry Considerations

Sand filters work most effectively with proper water chemistry. Maintain pH between 7.4-7.6 and total alkalinity at 80-120 ppm for traditional chlorine systems or 60-80 ppm for salt water generators. High pH can cause calcium precipitation that clogs sand pores, while low pH can dissolve the sand bed over time.

Use a Taylor K-2006 test kit for accurate readings. Always add chemicals with the pump running in filter mode to ensure proper mixing and distribution throughout the system.

Troubleshooting Common Issues

If your sand filter isn't clearing the pool effectively, check for channeling in the sand bed, worn laterals that allow sand into the pool, or sand that's too old and rounded. Green or cloudy water despite proper filtration often indicates the need for a SLAM (Shock Level And Maintain) process using liquid chlorine rather than relying solely on filtration.