How To Install Under Sink Water Filter?

Under sink water filters are units that are installed under the sink of your  kitchen to get clean drinking water out of your tap.

An under-sink filtration system intercepts your existing cold water line. Water flows through the filter media, and contaminants become trapped inside the filter. When water leaves the system, it has been filtered – so any harmful contaminants have been removed.

Under-sink filtration systems rely on water pressure to send water through the filter media. This means they don’t need to be connected to an electricity supply, and they provide almost instant access to filtered water.

3 Different Under-Sink Water Filter Models

The three most common under-sink filtration models are single-stage units, multi-stage systems, and reverse osmosis systems.

Single-Stage Units

As the name suggests, single-stage units are filtration systems that consist of a single filter stage. These tend to be the most affordable water filter systems for under-sink use, and offer targeted filtration that focuses on a select type of contaminant or group of contaminants.

Single-stage units are beneficial for anyone dealing with a specific water quality issue, but they’re not as thorough as multi-stage systems.

Multi-Stage Systems

Multi-stage systems are under-sink filters that consist of two or more filter stages. Most multi-stage filters have three separate filtration stages, each targeting a different set or size of contaminants.

Multi-stage systems are a good option for people who want to remove more than just a handful of contaminants from their water.

Reverse Osmosis Systems

Reverse osmosis systems are multi-stage units that feature a semi-permeable membrane. A reverse osmosis system purifies water and eliminates virtually all total dissolved solids. This system is the most expensive type of under-sink filter available.

RO systems are ideal for people who want to remove as many contaminants as possible from their tap water. Typically, reverse osmosis filters have three filter stages and their own separate faucet.

How to Install a Reverse Osmosis System

So, you’ve been looking at purified water solutions for a while, and after plenty of research and consideration, you’ve purchased a reverse osmosis system for your home.

You might think the hard part is over – but not so fast! You still have to install the system, and that can pose quite a challenge.

If you’ve found this guide, I’m going to assume that you’ve bought an under-sink RO filter. This is the most popular type of reverse osmosis system available, and, unlike countertop alternatives, requires a somewhat involved installation.

Don’t feel too daunted though, when you know what you’re doing you’ll realize that installing a reverse osmosis system is actually pretty straightforward. I’ll be sharing the step-by-step installation process for the average RO system below. Be sure to check your user manual for clarification, as some systems may vary.

What You’ll Need | How To Install Under Sink Water Filter?

To install a reverse osmosis system, you’ll require a few tools and materials:


  • Electric drill (preferably variable speed)
  • Metal-cutting drill bits: 1/2″, 1/4″ and 1/8″
  • Concrete drill bits: 1/2″, 1/4″ and 1/8″
  • Adjustable wrench
  • Screwdriver
  • Hammer
  • Cutting tool (for plastic tubing)
  • Level


  • Plumbers/teflon tape
  • Reverse osmosis system
  • Adapter tee (only if needed – see instructions below)

How to Install a Reverse Osmosis System

How To Install Under Sink Water Filter (2)

Step 1: Prepare for the Installation

Before you even buy a reverse osmosis system, you must be sure it’ll fit underneath your kitchen sink.

Use a tape measure to get measurements of your available space, then compare the figures with the dimensions of the system you’re interested in. You may want to think about moving some of the larger items under your sink to a new location if possible.

You should also be certain that you have enough space to connect the unit to a dedicated faucet. Most reverse osmosis water filters come with their own faucet, which may require drilling and mounting onto your sink or countertop.

When looking at RO systems to buy, you may notice that some products come with an installation kit. This kit includes the tools and components you’ll normally require for assembly and installation of your system, which may save you a trip to your local hardware store later on down the line.

Step 2: Set Up The RO System

When your reverse osmosis water filter arrives, remove it from its box and unwrap the packaging. Before doing anything, check over the system and ensure that the filter connections and components are compatible with the plumbing beneath your sink.

If you need an adapter tee to connect the unit to your cold water supply, pick one up from your local hardware store before you begin the installation process. Ensure that the tee is compatible with your particular under sink supply source and the R.O. feed line.

Check that your drain pipes and water supply lines don’t need to be moved – if they do, you might have to call in an expert.

If your under-sink space isn’t suitable, don’t despair. You can usually install the system in another location, like your basement or garage, or in an adjacent cabinet, and connect it up to the waterline that will eventually end up underneath your sink.

Of course, this job will require more work, and you may need extra tubing to reach due to the additional distance now involved.

Step 3: Install the Faucet

If your reverse osmosis unit comes with a faucet (it may be called a “spigot” in your user manual), your first task is to install it. It’s best to do this job first, as it’ll mean you will be able to more easily access the piping beneath your countertop before you crowd up the space with the RO unit.

You might find that your sink already has a knock-out opening, which will make it easier to install the faucet. Otherwise, you’ll just need to drill a new hole in the sink.

Use a 1/4″ drill bit to make the initial hole, then switch to a 1/2″ drill bit to make the hole wider. Or use a step bit. Drilling through granite, porcelain/cast iron, stainless steel or man made substances will require specialized bits and skills.

This may be a good time to consult a professional if you are not comfortable with your knowledge or skillset. Mistakes can be very costly. Make sure you choose a suitable spot for your faucet installation, preferably near your existing faucet, allowing it to empty into the sink and providing adequate room to swivel.

Take the waterline (included in your RO setup) and connect it to the faucet’s air gap, making sure it passes through the faucet opening, then attach the faucet base to your counter.

Step 4: Set Up The Storage Tank

If your system has a water storage tank, follow this step next.

Install the tank connector first, using plumber’s tape to seal the connections and prevent leaks. Then screw the connector for your faucet onto the storage tank, hand-tightening it only.

Place your storage tank in a suitable under-sink location.

Because drinking water will be stored in the tank after it has passed through the RO system, it makes sense to install the tank close to your faucet, preferably directly underneath.

Step 5: Mount The RO System

If you haven’t already, measure the reverse osmosis unit’s wall mount and mark a location for it underneath your kitchen sink, being sure that the markings are level.

Keep in mind that most systems need to be mounted above the cabinet floor to allow you to unscrew and change the filters without needing to remove the entire unit from beneath the sink.

Screw the wall mount into place and attach the reverse osmosis water filters directly to the mount.

Step 6: Pre-Fill The Storage Tank

One option is to pre-fill your reverse osmosis water tank to ensure there’s enough pressure when you’re checking for leaks. It’ll also make it easier to flush your post-filter before use.

To pre-fill your tank, simply connect the feed line that will eventually attach to the entrance to your RO system directly to your tank.

Let the tank fill with drinking water, then shut off your pressure by closing off the tank valve.

You can then remove the feed line from the tank valve.

Step 7: Connect The Water Line

Now you have your main system installed, it’s just a case of connecting everything together.

The water line that feeds your reverse osmosis filtration system needs to be connected to your cold water supply line.

The process of connecting the tubing depends on the situation beneath your kitchen sink. The majority of reverse osmosis systems include a half-inch adapter that will fit with your faucet’s half-inch flex line.

Here’s where you might have to use an adapter tee, however, if the water supply line and the filter’s tubing aren’t compatible. Check your manual for more info.

Step 8: Install The Drain Saddle

A drain saddle connects the reverse osmosis filtration system to the drain. It allows wastewater to flow into the drain line already installed underneath the sink.

You will want to avoid installing the drain saddle valve above the P- or J-trap in the drain line. It is better to install this component on a horizontal waterline to minimize noise during performance.

While you’re down there, check out the quality of your drain pipe. If it’s corroded, I would strongly recommend buying a new one before you connect any tubing.

When you’ve chosen your installation spot, drill a hole into the top of the pipe using a 1/4″ drill bit.

Attach the saddle to the pipe, bolting it in place. Ensure that the drilled hole and drain saddle hole are aligned properly.

Water will now be able to flow through the drain saddle into the drain line when the reverse osmosis system is in operation.

Step 9: Connect Additional Tubing

Once your system is connected to the cold water line, you’ll need to install the remaining tubing, including those that run from the reverse osmosis filtration system to the storage tank and from the tank to your faucet.

Tubing can vary in size from product to product, so read your manufacturer’s user manual carefully to make sure you’re properly connecting yours.

Step 10: The Finishing Touches

Your RO system is now technically ready to use, but there are still a few small things you’ll have to do before you can access drinking water from the unit:

  • First off, check all your connections and fittings for leaks.
  • Let the system build pressure by opening the ball valve, and again, check for leaks.
  • You can then open the valve on the tank, let water flow from your faucet for 5 minutes, and – you guessed it – check for leaks.
  • Finally, leave the RO tank to fill with water – generally around 2-3 hours. Drain, but don’t drink this water. If you don’t want to waste it, save it for cleaning or watering your plants.
  • You should repeat the process once more, letting the tank become full and empty, again, not using the water produced.
  • The purified water that fills your tank for the third time is safe and ready to be enjoyed.


Under-Sink Water Filters Lifespan

Different under-sink filters have different lifespans, depending on their capacity, filtration methods, surface area, pore size, and quality.

Your water quality and water usage also affect the lifespan of an under-sink unit. You might need to buy replacement filters for your system every 6 months, while somebody else might only buy a filter replacement filter every 9 months.

Typically, the lifespan of sediment filter cartridges is 6 months, while carbon cartridges have a filter life of 9 months, and post or polishing filters last for 12 months.

What do Under-Sink Water Filters Remove?

The contaminants targeted by an under-the-sink filtration system depends on the media used in the filtration process.

Common contaminants removed by the best under-sink water filter systems are:

  • Chlorine taste and odor
  • Chloramine
  • Lead and other heavy metals
  • Pesticides
  • Herbicides
  • Disinfection byproducts
  • Nitrite
  • Nitrate

A specialty under-sink filter system might be capable of removing additional contaminants, like:

  • Bacteria
  • Viruses
  • VOCs
  • Pharmaceuticals
  • Radon
  • Fluoride
  • Arsenic

Under-sink RO systems also remove beneficial minerals and salts, like calcium and magnesium, from water.

When to Call a Professional

If you don’t consider yourself particularly handy, the instructions in your user manual don’t make much sense, or you just don’t have the time or patience to install a reverse osmosis filtration system, you can always consider hiring an expert for installation.

Plumbers and professional handymen are well-versed in installing RO filters, and they should be able to get the job done in half the time it’d take you.

Some reverse osmosis systems can be more difficult to install than others.

Systems with more than 4 filtration stages, those that use more space, and units that are purchased alongside additional filters, like UV purifiers and remineralization filters, may pose more of a challenge. Again, if you don’t think you’re up for the task, you should consider calling a water treatment specialist or plumber instead.

You can find plenty of trustworthy plumbers in your local area whose services will include installation of RO systems. Handing the job over to an expert will take the pressure off you, as you won’t be concerned about the quality of the assembly.

Make sure to read reviews and ask your plumber for photos of their previous projects if you want to know more about what they’re capable of.

Remember, installation by a plumber or a handyman will add extra money to your upfront investment. Usually, a plumber will charge up to $300 for the job, depending on the complexity of your situation.

However, you may prefer to spend that little bit extra on getting the job done right, rather than potentially having to spend even more money on fixing your own mistakes.

The long-term benefits of a reverse osmosis system generally mean it pays for itself, too, so you might think that adding a little extra to your initial investment is worth it.


This is my blog about the ways and solutions that can help you improve your health by taking more value from drinking water. As improving health means a lot to me, I decided to create the Water On Top project with the purpose to reveal the benefits of water, the finest products to take our daily water to another level, and much more great stuff about water that I believe is on top of our nutrition.

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