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Inline Filter Guide: Installation, Use and Testing
For every 10″ × 2″ inline filter we sell: sediment, carbon, specialty and remineralizing, with 1/4″ threaded, 1/4″ Quick Connect or 3/8″ Quick Connect ports.
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All sections: Before you start · Filter types · Where to use · Connections · Cutting and C-clips · Water line · Under-sink RO · Countertop RO · Filter order · TDS testing · pH testing · Replacement · Troubleshooting · Parts · Video
Before you start: check these first
- Check your tubing size. Two sizes are in common use: 1/4″ OD and 3/8″ OD (OD means outside diameter). The size is usually printed on the tubing.
- Check that the filter fits your tubing before you cut anything. Compare the filter ports and any adapters in the bag with your tubing size.
- Find the flow arrow on the filter label. Water must go in the direction of the arrow.
- Get your tools ready: a tubing cutter (recommended; a sharp utility knife may also work), a towel and a small bucket or bowl.
- Check that you have mounting clips. Some filters are sold without clips. In the long run it is better to secure an inline filter than to leave it hanging on the tubing. See parts and accessories.
- Check that you have enough tubing. Some installations need an extra length to reach the filter. See parts and accessories.
- Choose your installation below: water line, under-sink RO system or countertop RO system.
Cold water only
Use inline filters on cold water lines, 36–100°F, at normal household pressure (up to 90 PSI). Keep them indoors and protect them from freezing.
Filter types
Find your filter on the label, then read what it does and where it goes.
Sediment
First stage
A 5-micron sediment filter. Reduces dirt, sand, silt and rust particles, and protects the filters and appliances after it.
Carbon
Taste and odor
Granular carbon (GAC), coconut carbon and carbon block filters. Reduce chlorine taste and odor. Used on their own or as a stage in an RO system.
Specialty
Specific jobs
DI (deionization), bone char, calcite, GAC + KDF-55 and softening resin filters. Each one does a different job; see the notes below.
Remineralizing
After RO
Alkaline and mineral filters. Add minerals back to RO water and raise its pH. Always the last stage, after the RO membrane.
Specialty filters in more detail
| Filter | What it does | Good to know |
|---|---|---|
| DI (deionization) | Mixed-bed resin that reduces dissolved solids (TDS) in RO water to a very low level. | Use it after the RO membrane. Common uses are aquariums, lab work, hydroponics, humidifiers and any need for water close to 0 ppm. |
| GAC + KDF-55 | Granular carbon with KDF-55 media. Reduces chlorine taste and odor. | Can be used on a water line or as a stage in an RO system. |
| Bone char | Bone char with coconut carbon, used as a polishing stage. | Works best at a slow flow rate. |
| Calcite acid neutralizer | Calcite (crushed marble) that raises the pH of acidic water and adds a little calcium. | Use it after an RO system. Check your pH first; it is meant for water that is acidic. |
| Softening resin | Ion-exchange resin that reduces hardness (calcium and magnesium) to limit scale. | Releases a small amount of sodium. It cannot be regenerated; replace it when scale returns. Works best at a slow flow rate. |
Where to use each filter
| Filter | Water line | Fridge line | Ice maker line | Air humidifier | Traditional RO (under sink) | Countertop RO |
|---|---|---|---|---|---|---|
| Sediment, 5 micron | Yes | Yes | Yes | Yes | No It has its own larger sediment stage |
Yes |
| Carbon (GAC, coconut, carbon block) | Yes | Yes | Yes | Yes | Yes As the post filter |
Yes Second stage, or as an extra stage |
| GAC + KDF-55 | Yes | Yes | Yes | Yes | Yes | Yes |
| DI (deionization) | No Tap water uses it up too fast |
No | Yes* | Yes After RO |
Yes After RO |
Yes After RO |
| Alkaline, mineral, remineralizing | No | Only on RO water | Only on RO water | No It adds minerals |
Yes Last stage |
Yes Last stage |
| Bone char | Yes* | Yes* | Yes* | No | Yes After RO |
Yes |
| Calcite acid neutralizer | No | No | No | No | Yes After RO |
Yes |
| Softening resin | Yes* | Yes* | Yes* | Yes* | Yes | Yes |
* Slow the water flow for better results. The longer the water stays in contact with the filter media, the better the filter works.
Softening resin on a water line: we also recommend adding a carbon filter on the same line.
“Water line” means a cold water line to a faucet or an appliance, such as a coffee maker.
Connections
Our inline filters come with one of three port types. The label and the product page show which one you have.
| Port type | How to connect | Avoid |
|---|---|---|
| 1/4″ threaded (FPT) | Wrap the male threads of your fitting with 3–4 turns of PTFE thread tape, clockwise. Screw it in by hand, then tighten a further quarter to half turn with a wrench. | Do not overtighten. The filter body is plastic and can crack. |
| 1/4″ Quick Connect | Cut the tubing straight, at 90°. Remove the blue C-clip if one is fitted. Push it into the port until it stops, about 5/8″ deep, then pull back gently to check it holds. Then fit the blue C-clip. | Do not use tubing with a scratched or angled end; it will leak. |
| 3/8″ Quick Connect | Same as 1/4″ Quick Connect, with 3/8″ tubing. Filters supplied with elbow adapters: push the adapter stem into the filter port first, then the tubing into the adapter. | Do not force 1/4″ tubing into a 3/8″ port, or the reverse. |
To remove tubing from a Quick Connect port
Turn off the water and release the pressure. Remove the blue C-clip, push the collar (the ring around the tube) in toward the filter, and pull the tube out while holding the collar in.
Cutting tubing and using the blue C-clips
Most leaks come from a bad cut or a missing clip, so these two steps matter more than any other.
Cut the tubing straight
- Cut at a 90° angle, straight across the tube. A cut that is off by about 15° or more will leak.
- Use a tubing cutter if you can. It gives a clean, square cut every time. A sharp utility knife may also work if you cut carefully and straight. Our tubing cutter cuts both 1/4″ and 3/8″ tubing and also releases Quick Connect fittings.
- Check the end. It should be round, smooth and free of scratches for the last inch. If it is not, cut again.
Blue C-clips (locking clips)
- The blue C-clip sits behind the collar of a Quick Connect port and locks the tubing in place.
- If your filter arrives with blue C-clips already fitted, remove them before you push the tubing in. The tubing cannot seat fully with the clip in place.
- After the tubing is in, put the C-clip back on every Quick Connect port.
Installation on a water line
For a fridge, ice maker, coffee maker or other cold water line. This is the simpler installation.
- Turn off the water to the line and open a faucet or dispenser to release pressure.
- Choose the place for the filter: a straight section of tubing with enough room for the filter and for a gentle bend on each side.
- Put a towel and a bucket under the tubing. Expect a small amount of water when you cut into an existing line.
- Cut the tubing straight, at 90°. See the section above.
- Remove the blue C-clips from the filter ports if they are fitted.
- Connect the supply side to the filter inlet. Push the tubing in until it stops, then pull back gently to check it holds.
- Flush the filter for at least 5 minutes into the bucket or a sink. Turn the water on slowly.
- Connect the outlet side to the tubing that goes to the appliance.
- Fit the blue C-clips on both ports.
- Turn the water on and check for leaks now, after 10 minutes, and the next day. Dry the fittings with a paper towel first so a fresh drop is easy to see.
- Secure the filter so its weight does not pull on the tubing. Inside a cabinet, use 2″ screw-mount clips. Then write the install date on the label.
Installation on a traditional under-sink RO system
First time adding a filter to an RO system?
Watch an installation video on YouTube before you start. There is a short example at the end of this page, or search YouTube for “add inline filter to RO system”. RO systems differ, and seeing it done once makes the job much easier.
- Shut off the water supply to the RO system. Close the tank valve as well, then open the RO faucet to release pressure.
- Work out where the filter goes. Carbon post filters, alkaline, mineral and DI filters all go after the RO membrane and the storage tank. An alkaline or mineral filter is the last stage before the faucet. See the right order below.
- Check the tubing size at that point (1/4″ or 3/8″ OD) and confirm that the filter ports or adapters match it. Do this before you cut.
- Cut the tubing straight, at 90°, with a towel underneath.
- Remove the blue C-clips from the filter ports if they are fitted.
- Connect the filter with the flow arrow pointing toward the faucet. Push each tube in until it stops.
- Fit the blue C-clips on both ports.
- Secure the filter. Use a 2.5″ × 2″ clip to hold it on the RO membrane housing, or a 2″ × 2″ clip to hold it on another inline filter.
- Open the water supply and the tank valve, and check every connection for leaks now, after 10 minutes, and the next day.
- Flush the new filter for at least 5 minutes, with the RO faucet open. Then let the tank fill and discard the first tank of water.
- Write the install date on the filter label.
Installation on a countertop RO system
Follow the instructions from the manufacturer of your countertop system. Each system has its own layout, and the manual shows where each stage sits and how to release it.
- Check the tubing size and the filter ports before you start, as above.
- Keep the stage order: sediment first, carbon second, the RO membrane, then the carbon post filter, and an alkaline or mineral filter last.
- Remove the blue C-clips before pushing the tubing in, and put them back afterwards.
- Flush the new filter for at least 5 minutes before you drink the water.
The right order when you use more than one filter
Filters work in a fixed order: sediment first, carbon next, and any mineral or DI filter last.
Fridge, ice maker or coffee maker line
Cold water line→Sediment→Carbon→Appliance
Sediment protects the carbon filter from clogging. Use one or both.
Traditional under-sink RO system
Pre-filters in housings→RO membrane→Storage tank→Carbon post filter→Alkaline or mineral→Faucet
The system already has its own sediment stage. Inline filters go after the membrane and tank.
Traditional under-sink RO system with a DI filter
Pre-filters in housings→RO membrane→Storage tank→DI filter→Carbon post filter→Faucet
The storage tank sits after the RO membrane. The DI filter comes next, and the carbon post filter is last.
Countertop RO system
Faucet adapter→Sediment→Carbon→RO membrane→Carbon post filter→Alkaline or mineral
An alkaline or mineral filter is always the last stage.
Countertop RO system with a DI filter
Faucet adapter→Sediment→Carbon→RO membrane→DI filter→Carbon post filter
The same order as under the sink: RO membrane, then DI, then the carbon post filter.
Aquarium, lab, hydroponics or humidifier water
RO system→DI filter→Container
Use DI on RO water only. On tap water the resin is used up very quickly.
Testing with a TDS meter
A TDS meter measures total dissolved solids in parts per million (ppm). Use it to check the RO membrane and each stage after it.
Wait one week after installation before you test
New filters hold carbon dust for the first few days, and that dust gives false TDS and pH readings. Example: tap water reads 200 ppm, and water from an RO system with new filters can read 250 ppm at first. After the water has run for 5–10 minutes it drops to 20–50 ppm or lower. For readings you can trust, test one week after installing new filters.
Which stages to test
- Sediment and carbon filters: there is no point in testing them with a TDS meter. They do not reduce dissolved solids, so the reading tells you nothing about how well they work.
- Test each stage after the RO membrane. That is where a TDS meter gives useful numbers.
What reading to expect
| Where you test | What to expect |
|---|---|
| After the RO membrane | Much lower than your tap water. Example: with tap water at 200 ppm, expect about 20–50 ppm or lower. |
| After a DI filter | It depends on the water coming from the RO membrane. With RO water around 20 ppm, expect 0–1 ppm. With higher TDS after the RO, the result can be anywhere from 0 to 15 ppm. |
| After a carbon post filter | The same as before it, or 2–6 ppm higher. Carbon filters release a little carbon dust. This is normal. |
| After an alkaline or mineral filter | Higher than before the filter. The filter adds minerals, so this is normal. |
How to test correctly
- Wait one week after installing new filters.
- Let the water run for 5–10 minutes before you take a sample. Water that has been sitting in the system reads higher than freshly made water.
- Rinse the cup with the water you are testing, then fill it. Soap or tap water left in the cup changes the reading.
- Dip the meter to the fill line, stir gently to release air bubbles, and wait until the number is steady.
- Compare two readings. One number alone tells you little. Test your tap water and the filtered water at the same time, with the same meter.
- Rinse the meter with clean water and replace the cap.
When to test
- One week after you install a new RO membrane or new filters, to record your starting numbers.
- Once a month for a DI filter. Replace it when the reading rises well above your starting number.
- Every few months for an RO system, or whenever the taste of the water changes.
To work out how well an RO membrane is performing
Subtract the RO reading from the tap reading, divide by the tap reading, and multiply by 100. Example: tap 200 ppm, RO 20 ppm. (200 − 20) ÷ 200 × 100 = 90% reduction.
Testing pH
pH shows how acidic or alkaline water is, on a scale from 0 to 14. Pure RO water is slightly acidic, often between 6 and 7. An alkaline or mineral filter raises it; check your filter’s product page for its expected range.
Wait one week here too
Carbon dust from new filters affects pH readings as well as TDS readings. Test pH one week after installation.
How to test correctly
- Wait one week after installing a new filter. The first water through an alkaline or mineral filter also reads much higher than normal.
- Run the water for a minute before taking a sample. Water that has sat inside the filter for hours picks up more minerals and reads higher.
- Use a calibrated pH meter or pH test drops. Calibrate a meter with buffer solutions (pH 7 and pH 4 or 10) as its instructions describe.
- Give the meter time. RO water has very few minerals, so meters respond slowly in it and the number drifts. Stir gently and wait for it to settle.
- Test straight away. RO water absorbs carbon dioxide from the air and its pH drops within minutes in an open cup.
Low-cost pH meters and RO water
Pen-type pH meters are made for water with normal mineral content. In RO or DI water they can show unstable or wrong numbers. If the readings jump around, pH test drops made for low-mineral water are usually more reliable.
When to replace
| Filter | Typical life | Replace sooner if |
|---|---|---|
| Sediment | Up to 12 months | Water flow drops, or the filter looks dark. In water with a lot of sediment it can clog much sooner. |
| Carbon (GAC, coconut, carbon block) | 6–12 months, or about 1,500 gallons | Chlorine taste or odor returns, or flow drops. |
| GAC + KDF-55 | 6–12 months, or about 1,500 gallons | Taste or odor returns. |
| DI (deionization) | Depends on the TDS of the water going in | The TDS reading after the filter rises well above its starting number. |
| Alkaline | 3–12 months, or 300–500 gallons | pH falls back toward the level of your RO water. |
| Mineral, remineralizing | 6–12 months, or about 400 gallons | Taste changes, or pH falls back. |
| Bone char | 6–12 months | Flow drops or taste changes. |
| Calcite acid neutralizer | 6–12 months | pH improvement fades. |
| Softening resin | About 6 months | Scale starts to appear again. |
These are typical figures. Your water quality and how much water you use set the real schedule.
Troubleshooting
| Problem | Likely cause | What to do |
|---|---|---|
| Black specks or grey water from a new carbon filter | Carbon dust. Normal for a new filter. | Flush for at least 5 minutes, until the water runs clear. |
| Cloudy or milky water from a new mineral filter | Fine mineral particles or trapped air. Normal for a new filter. | Flush for at least 5 minutes. Air bubbles clear by themselves within a day or two. |
| Drip at a Quick Connect port | Tubing not pushed in fully, the end is scratched or cut at an angle, or the blue C-clip was left in place while the tubing was pushed in. | Release the tubing, cut the end straight at 90°, and push it in until it stops. Fit the blue C-clip. |
| Drip at a threaded port | Too little thread tape, or the fitting is cross-threaded. | Remove the fitting, apply fresh PTFE tape, and refit it by hand before tightening. |
| Low water flow | Filter installed backwards, kinked tubing, or a clogged filter. | Check the flow arrow and the tubing. Replace the filter if it is due. |
| TDS is not lower after a sediment or carbon filter | These filters do not reduce dissolved solids. | Nothing is wrong. See the TDS section above. |
| TDS after the RO system is higher than tap water, right after new filters were fitted | Carbon dust from the new filters. | Run the water for 5–10 minutes and the reading will drop. For a reliable reading, test again one week after installation. |
| TDS after a DI filter is higher than expected | High TDS in the water going in, water flowing too fast, or resin that is used up. | Check the RO water before the DI filter, slow the flow, and replace the DI filter if the reading stays high. |
| pH readings jump around | Low-mineral water and a pen-type meter. | Use pH test drops, or wait longer for the meter to settle. |
Parts and accessories
Clips, tools, tubing and fittings that are often needed when adding an inline filter.
| Type | Part | Use it to |
|---|---|---|
| Mounting clips | 2″ screw-mount clips, 2-pack | Fix a 2″ inline filter to a cabinet wall or panel. |
| Mounting clips | 2.5″ × 2″ clips, 2-pack | Hold a 2″ inline filter on an RO membrane housing. |
| Mounting clips | 2″ × 2″ clips, 2-pack | Hold a 2″ inline filter on another inline filter. |
| Tools | Tubing cutter with release tool | Cuts 1/4″ and 3/8″ tubing square, and releases Quick Connect fittings. |
| Tools | Collet release tool | Disconnects 1/4″ and 3/8″ Quick Connect tubing. |
| Locking clips | Blue C-clips for 3/8″, 10-pack | Spare locking clips for 3/8″ Quick Connect ports. |
| Locking clips | Blue C-clips, 10-pack | Spare locking clips for 1/4″ Quick Connect ports. |
| Tubing | 1/4″ OD tubing, clear | Extra length for longer runs. Choose the length. |
| Tubing | 1/4″ OD tubing, white | Extra length for longer runs. Choose the length. |
| Tubing | 3/8″ OD tubing, white | Extra length for 3/8″ lines. Choose the length. |
| Fittings | 1/4″ tubing union | Joins two 1/4″ tubes. |
| Fittings | 1/4″ elbow stem connector, 2-pack | A 90° turn at a 1/4″ Quick Connect port, for tight spaces. |
| Fittings | 1/4″ Quick Connect elbow, threaded, 2-pack | Connects 1/4″ tubing to a 1/4″ threaded filter port, at 90°. |
| Fittings | 1/4″ Quick Connect straight fitting, threaded, 2-pack | Connects 1/4″ tubing to a 1/4″ threaded filter port, straight. |
| Adapters 1/4″ to 3/8″ | 1/4″ thread × 3/8″ Quick Connect elbow, 2-pack | Connects 3/8″ tubing to a 1/4″ threaded filter port, at 90°. |
| Adapters 1/4″ to 3/8″ | 1/4″ thread × 3/8″ Quick Connect straight fitting, 2-pack | Connects 3/8″ tubing to a 1/4″ threaded filter port, straight. |
| Adapters 1/4″ to 3/8″ | 1/4″ stem × 3/8″ Quick Connect elbow, 2-pack | Connects 3/8″ tubing to a 1/4″ Quick Connect filter port, at 90°. |
| Adapters 1/4″ to 3/8″ | 1/4″ × 3/8″ elbow reducer, 2-pack | Joins 1/4″ tubing to 3/8″ tubing, at 90°. |
More parts: all tubing, clips and fittings · faucet adapters for countertop RO
Video: replacing an inline filter
A short general example of replacing an inline filter. Your filter and fittings may look different, so follow the steps in this guide for your own setup.
Video by Water eStore on YouTube, shown as a general example.
Not sure which filter you have, or where it goes?
Send us a photo of the filter label and your setup, and we will tell you where it fits. Contact us →
Please note: Filter life and performance depend on your water chemistry, pressure and usage, so results vary from home to home. These filters are for use on municipally treated or otherwise microbiologically safe water. They are not a substitute for testing your water.