How to Install an Under Sink Water Filter (Easy Step-by-Step)

Keen to learn the process of installing an under sink water filter?

Wondering whether you can install this type of filtration system yourself?

We’ve answered all your questions in this guide.

🤔 Can Anyone Install an Under-Sink Water Filter System?

First off, you might be wondering whether you’re capable of installing an under-sink water filter system, or whether you’re best leaving the job to an expert.

If you’ve clicked on this guide, that means you’re at least toying with the idea of installing the filter system yourself, which is good. Installing a filter can be complex, so you need to be keen to give it a go.

You should be able to install an under-sink filter system if:

  • You’re capable of following instructions.
  • You’re a handy person.
  • You have DIY or plumbing experience for small jobs, like changing faucets.
  • You’re not familiar with plumbing work, but you’re willing to learn.

Different types of under-sink filters have varying levels of complexity. For instance, an under-sink reverse osmosis system has a water storage tank and a drain line that require additional installation.

We recommend reviewing your user manual and breaking down the steps involved in installation before deciding whether or not to hire a plumber. You might find that the process is easier than you expected, and you can save some money and do it yourself.

Installing an under-sink water filter

🧰 Tools and Supplies for Installing Under-Sink Water Filters

Ready to get started with installing an under-sink water filtration system? Here are the tools and supplies you’ll need:

  • An under-sink water filter
  • The cartridges for the filtration system
  • Scissors or cutters for plastic tubing
  • Plumber’s tape
  • An adjustable wrench
  • A drill
  • A screwdriver
  • A bucket

You can buy these supplies from any hardware store, like Home Depot or Lowe’s, or online marketplaces like Amazon.

📖 Step by Step Process: Installing an Under-Sink Water Filter

Under-sink water filters are designed to be hooked up to your cold water supply underneath your kitchen sink. The installation process involves intercepting the cold water line and installing the filter. If your water filtration system comes with an included faucet, you’ll also need to drill a hole in your kitchen sink, or use an existing hole from a soap dispenser.

Follow these steps to install an under-sink water filter at your cold water line:

Step 1: Switch Off the Cold Water Supply

To prevent leaking or flooding while you install the filter, turn the cold water supply valve, or shutoff valve, to stop water from flowing into the system.

The cold water supply valve is usually on a vertical pipe at the back of your under-sink counter. Feel the pipes they’re attached to if you can’t tell your hot shutoff valve from your cold shutoff valve. The cold water valve will be on a pipe that’s significantly colder.

Shutting off the cold water supply

Step 2: Disconnect the Tubing

The under-sink system needs to intercept your water line, so your cold water tubing needs to be disconnected from the kitchen faucet. Loosen the nut and remove the tubing from the supply line using a wrench.

Step 3: Install a T-Adapter

Your under-sink system should have come with a copper T-adapter to install the system at your water line. Install this adapter at the feed water line.

To make sure water can’t escape from this connection, wrap Teflon tape around the adapter threads, then use a wrench to tighten the fit. Just be sure not to overtighten.

Disconnect the tubing and connect the t-adapter

Step 4: Connect the Filter’s Tubing to the T-Adapter

Next, connect the under-sink water filter’s tubing to the copper T-adapter.

First, install a shut-off valve at this tubing, which will make it easy to replace the filter cartridges. Then, attach the nut and copper insert from the T-fitting, then connect and tighten the fitting to keep it in place.

Step 5: Drill a Hole For the Faucet

This step is only applicable to an under-sink filtration system that comes with a dedicated faucet, like a reverse osmosis system. Check your user manual and the contents of your box to determine whether or not your system comes with a faucet.

To drill a hole in your countertop, check the size of the faucet, then drill the hole to snugly fit the faucet. Most standard-sized faucets fit in a 1⅜ inch hole. Alternatively, you might be able to install the faucet at an existing hole from a soap dispenser.

Different filtration systems may have their own unique installation processes and requirements. Consult your user manual and follow the steps outlined by the manufacturer, and contact customer service if you’re unsure.

Step 6: Install the Faucet

The new water filter faucet should be installed near to, but not as a replacement to, your existing faucet. That’s why you’ve drilled an extra hole – to install the new faucet separately. It’s handy to keep your existing faucet for hot water, or to divert the cold water supply back to that faucet if there’s an issue with your under-sink water filter.

To install the new faucet, hold the tap in position above the surface of the countertop while you tighten the fitting underneath. Fill any empty space around the faucet’s gasket to prevent water from leaking underneath the sink.

Connect the faucet to the filter tubing using nuts, washers, and any other supplies outlined by the manufacturer.

Installing the faucet

Step 7: Mount the Filtration Unit on the Wall

Most filters come with a mounting bracket that lets you mount them against the wall, preventing them from taking up too much under-sink space.

Mark where you plan to install the mounting bracket and drill the holes for the system, making sure there’s enough space to easily perform filter changes and other maintenance. Then, slot the unit onto the bracket.

Mounting the filtration system on the wall

Step 8: Connect the System to your Plumbing

You can now connect the filter stages to the plumbing connections. Examine the system and identify the inflow and outflow ports on either side (it’s important not to get these ports confused).

Connect the faucet’s tubing to the outflow line, and connect the cold water feed line to the inflow line. This means that water will flow in the correct direction from the cold water pipe, through the filter, and out of the faucet.

Step 9: Switch On the Water

The unit should now be completely installed. Switch on your water supply by turning the valve back on, moving the valve slowly to prevent damage from sudden increased water pressure.

If your filter system has its own shut-off valve, slide this to allow water into the unit. Relieve any trapped air by pressing the pressure release valve. If you have a filter with a separate faucet slowly open to relieve the pressure and flow the water.

Switching on the water supply and faucet

Step 10: Check the System for Leaks

If you don’t adequately connect tubing, the fittings need to be tightened, or the system’s o-rings are out of place, you may notice a leak. It’s important to fix leaks as soon as you see them to prevent the problem from getting worse.

If you notice a leak, switch off your water supply and use more Teflon tape or hand-tighten your connections again. If this doesn’t stop the leak, open the filter housing and check that all o-rings are correctly slotted in place.

Step 11: Flush the Filters

You’ll now have access to filtered water – but it’s not quite ready for drinking. Most filters need to be flushed before they can be used. This washes away any carbon flecks or anything else that may be lingering in the cartridges from the manufacturing process.

Flush your filters according to your manufacturer’s instructions. Hold a bucket under your faucet to catch the water, and use it to water your plants or wash your dishes. You’ll then have drinking water that looks and tastes great – perhaps even better than bottled water.

🧠 How to Install an Under-Sink Filtration System FAQs

How do you install an under-sink reverse osmosis system?

Under-sink reverse osmosis systems can be installed similarly to standard water filters – but they often have additional components to install, like water storage tanks and drain lines. If you’re looking for step-by-step instructions on how to install an RO system, use this guide.

How do you change a single under sink water filter?

To change a filter in your under-sink filtration system, unscrew the filter housing and slide the old filter out of place. Wash the housing to remove contaminants, then slide the new filter into the housing and screw the housing back onto the system. Flush the filter as advised by the manufacturer.

What order do water filters go in?

If your system has multiple stages, the first stage is usually a sediment filter, followed by a carbon, KDF, or ion exchange filter. RO systems then have a semi-permeable membrane stage, and most systems finish with a post-filter or polishing filter. This order ensures you can enjoy the highest-quality filtered water and extends the lifespan of each filter stage.

Should I get a plumber to install my filtered water system?

If you’re not confident with DIY or the installation process looks challenging, you can consider hiring a plumber to install your under-sink system. After all, the money spent on a plumber might just save you the money spent on fixing the potential damage you could make from installing a filtered water system incorrectly.