How to Replace Reverse Osmosis Pre- and Post-Filters: A Step-by-Step Guide

Why Replacing Your RO Filters Matters

Regular RO filter replacement is essential to protect your reverse osmosis system and extend the life of your RO membrane. Whether you have a whole house system with Big Blue housings or a point-of-use (POU) system with inline filters, replacing pre-filters and post-filters ensures optimal water filtration performance, prevents membrane damage, and maintains pure, high-quality drinking water.

Most RO sediment and carbon filters require replacement every 12–24 months, depending on water quality and usage. This comprehensive guide covers both system types, showing you exactly how to change filters for whole house and point-of-use reverse osmosis systems.

When Should You Replace RO Pre- and Post-Filters?

Most Crystal Quest® sediment and carbon filters last 12–24 months, with 18 months being average under normal use. Replace your filters if you notice any of the following:

• Slower water flow from your RO faucet
• Strange tastes or odors in filtered water
• Discoloration in the sediment filter cartridge
• High TDS readings despite recent membrane replacement

Timely filter changes help extend membrane life and keep your system running efficiently.

Before Replacing Carbon and Sediment Filters

Before you begin, gather your replacement cartridges and any necessary tools to access the filter housings. This is also a great time to inspect the O-rings and sanitize housings if buildup is present.

Tools & Supplies You May Need:

• New sediment and carbon pre-filters
• Post-filter (e.g., carbon block, polishing, or remineralization)
• Filter housing wrench
• Clean gloves
• Food-grade silicone lubricant (for O-rings)
• Mild soap or sanitizer for cleaning housings

Replacing Pre-Filters for Whole House Reverse Osmosis Systems

Whole house RO systems typically use Big Blue or standard-sized filter housings that require a filter wrench to open. These systems often have multiple pre-filters designed to protect the RO membrane from sediment and chlorine.

1. Shut off the main water supply

Turn off the main water valve feeding your whole house system. Open a downstream faucet to relieve pressure.

2. Unscrew the filter housings

Use the filter housing wrench to carefully unscrew each Big Blue housing, starting with the first pre-filter (usually sediment). Place a large bucket underneath to catch any water spills.

3. Remove and discard old filters

Pull out used cartridges and dispose of them properly. Note the order and types of filters for correct replacement.

4. Clean the filter housings

Rinse each housing with warm water and mild dish soap. Scrub any sediment buildup or biofilm gently with a soft brush. Rinse thoroughly.

5. Inspect and lubricate O-rings

Check each O-ring for cracks or flattening. Replace if damaged. Apply food-grade silicone lubricant to ensure a good seal.

6. Insert new filters

Install new filters in the correct order—typically sediment first, then carbon. Observe flow direction arrows if present.

7. Reattach and tighten housings

Screw housings back by hand until the O-ring contacts the head. Use the wrench to tighten only an additional quarter turn to avoid damage.

8. Restore water and check for leaks

Slowly turn on the main water supply. Inspect housings for leaks. Run water for 5–10 minutes to flush new filters before the RO membrane.

Replacing Pre- and Post-Filters for Point-of-Use RO Systems

Point-of-use (POU) RO systems use two main filter types: cartridge filters housed in sumps (similar to whole house systems but smaller) and inline filters with quick-connect fittings. We'll cover both replacement procedures below.

Replacing Cartridge Filters in POU Housing Sumps

Many under-sink RO systems use standard 10" filter cartridges in compact housing sumps. These require opening the housing to access the filter inside.

1. Shut off water and depressurize

Turn off the angle stop valve under the sink and close the RO storage tank valve. Open the RO faucet to release system pressure.

2. Remove filter housings

Use the filter wrench to unscrew each housing. POU systems typically have 2-3 vertical housings. Place a towel underneath to catch drips.

3. Remove old cartridges

Pull out the used filter cartridges and dispose of them. Note which filter goes where - typically sediment first, then carbon.

4. Clean housings

Rinse housings with warm water. Check for sediment buildup or slime. Use mild dish soap if needed, then rinse thoroughly.

5. Check O-rings

Inspect the O-ring in each housing. Replace if cracked or flattened. Apply a thin layer of food-grade silicone lubricant.

6. Install new cartridges

Drop new filter cartridges into housings. Ensure they seat properly at the bottom. Most 10" cartridges fit standard POU housings.

7. Reattach housings

Screw housings back on hand-tight, then use the wrench for an additional 1/4 turn. Don't overtighten or you may crack the housing.

8. Test and flush

Turn water back on slowly. Check for leaks at housing seals. Run the faucet for 5-10 minutes to flush carbon fines before use.

Replacing Inline Filters with Quick-Connect Fittings

Some POU systems use inline filters with JACO or similar quick-connect fittings. These don't require opening housings but need careful handling of tubing connections.

1. Shut Off the Water Supply and Tank Valve

Turn off the angle stop valve under the sink and close the RO storage tank valve. Open the RO faucet to depressurize the system.

2. Disconnect the Inline Filters

For JACO or other quick-connect fittings:
Push in the collet while pulling the tubing straight out to release it. For threaded connections: unscrew carefully. Label or photograph connections before removal to ensure correct reassembly.

3. Remove Old Inline Filters

Note the flow direction arrows on each filter. Remove filters by disconnecting both ends. Keep towels handy for any water drips.

4. Install New Inline Filters

Check flow direction arrows on new filters—they must point toward the water flow. For sediment and carbon pre-filters, flow goes toward the RO membrane. For post-filters, flow goes toward the faucet.

5. Connect the Tubing

For JACO and other quick-connect fittings:
Push tubing firmly into the fitting until it stops (about ¾"). Gently pull back to ensure it's locked. For threaded fittings: hand-tighten, then add a ¼ turn.

6. Check All Connections

Ensure all tubing is fully seated. Quick-connect fittings should show tubing visible through the collet window. Threaded fittings should be snug but not over-tightened.

7. Restore Water and Test

Slowly open the angle stop valve and tank valve. Check all connections for leaks. Let the system pressurize for about 5 minutes before opening the faucet.

8. Flush the New Filters

Run the RO faucet for 5–10 minutes to flush carbon fines from new filters. The water may appear cloudy initially—this is normal and will clear with flushing.

Keep Your RO System Running Like New

Regular replacement of your reverse osmosis pre-filters and post-filters is a simple yet vital maintenance task that protects your investment and ensures consistent water quality. By following these steps and adhering to a regular replacement schedule, you'll extend the life of your RO membrane, maintain optimal system performance, and continue enjoying pure, great-tasting water.

Remember, replacing pre- and post-filters is just one part of comprehensive RO system maintenance. Click any of the links below to explore other essential tasks, such as replacing your RO membrane every 2–4 years, cleaning the membrane to restore flow rates, and sanitizing your storage tank to prevent bacterial growth. For complete maintenance schedules and best practices, be sure to check out our comprehensive RO maintenance guide.

Shop Crystal Quest® Replacement Cartridges

Need replacements? Browse our selection of Crystal Quest® pre- and post-filter replacement cartridges designed to deliver long-lasting performance and maximum contaminant reduction for your RO system.