Water Filter Installation: Faucet, Under-Sink and RO Diagrams (DIY)

Installing a water filter depends first and foremost on the type of filter you choose, not on how handy you are. A faucet-mounted filter screws directly onto the outlet of your existing faucet. An inline under-sink filter connects to the cold-water line before the water reaches the regular faucet. A tankless reverse osmosis system, such as Hydrion Purity RO600, also requires a feed tee on the cold-water line, a drain saddle for wastewater and a dedicated faucet installed separately on the countertop or sink. Each setup requires different tools and a different amount of time, and the checks you make before buying — the thread on the faucet or feed connection, the clearance under the sink, and the water pressure and temperature — determine whether installation remains a quick project or ends up requiring a plumber.

• A faucet-mounted filter is usually installed without special tools, by screwing it onto the faucet outlet, but you need to check the existing thread before buying because it varies by manufacturer and faucet age.
• An inline under-sink filter connects to the cold-water line, usually after the angle valve, and sends all the filtered water directly to the regular kitchen faucet.
• Tankless reverse osmosis, such as RO600, has the most complete setup: a feed tee, a drain saddle and a dedicated faucet; the manufacturer estimates installation takes about 30 minutes for someone doing it themselves.
• DIY installation has clear limits: an old or non-compliant plumbing system, a missing or faulty angle valve, or any work near the electrical system are situations where you should call a qualified plumber, not improvise.

Which installation setup suits you, depending on the filter you choose

Before buying any water filter, it’s worth knowing which installation category it belongs to, because that determines the tools and space you’ll need, not just how the filtration itself works. Faucet-mounted filters are the simplest type: they attach to the faucet outlet, in place of or over the existing aerator, and can be moved from one faucet to another without leaving a trace. Inline under-sink filters connect to the cold-water pipe, usually just after the angle valve, and stay fixed under the sink, filtering only the cold water that passes through them, without a separate countertop faucet. Tankless reverse osmosis systems, such as RO600, are the most complex: they need their own feed from a tee installed on the cold-water pipe, a drain for the water produced by the filtration process, and a dedicated faucet installed separately, because water filtered by reverse osmosis does not mix with untreated water from the regular faucet. Reading the installation diagram before buying saves you from an unwelcome surprise once the box arrives, because each category requires at least one extra part that you won’t see on the box: a fitting compatible with the angle valve, or an available hole for the dedicated faucet.

Installing a faucet-mounted filter: setup, tools and time required

A faucet-mounted filter screws onto the kitchen faucet outlet, either in place of the existing aerator or over it, depending on the model. You don’t need special tools: remove the old aerator by hand or, if it’s stuck, with pliers wrapped in a cloth so you don’t scratch the faucet, then screw the filter onto the same thread. Installation usually takes a few minutes, and most of that time goes into checking the thread, not the actual screwing on.

What to check before buying: common aerators on kitchen faucets have metric threads, usually in the M22 or M24 sizes, either internal or external, but the exact size varies by manufacturer and faucet age, so you need to check the existing thread rather than assume there’s one universal size. If the filter doesn’t fit straight away, the problem is usually a missing thread adapter, not a genuine incompatibility — the guide to removing the aerator and thread adapters shows you step by step how to identify the existing thread and which adapter you need. If you want to compare exactly which thread sizes each faucet-mounted filter model in the range supports, thread compatibility by model goes into that detail.

Inline under-sink filter: setup and steps

An inline under-sink filter connects to the cold-water line, usually just after the angle valve and before the water reaches the regular kitchen faucet. In practice, you disconnect the existing hose between the angle valve and the kitchen faucet, install the filter between the two ends using fittings suited to the hose diameter, and reconnect everything. The space required under the sink depends on the filter housing’s dimensions, so it’s worth measuring the available height and width under the sink before buying.

This type of setup sends all the filtered water to the regular faucet, unlike reverse osmosis, which separates the water into purified water and wastewater and therefore needs its own faucet. This simplifies installation because you don’t need a new hole in the countertop or sink, but it also means that the number of filtration stages is generally lower than in a reverse osmosis system. If you’re still deciding between the two types, the guide to inline under-sink filters versus reverse osmosis compares how each one works.

Tankless reverse osmosis (RO600): complete installation setup

A tankless reverse osmosis system, such as Hydrion Purity RO600, has the most complete installation setup of the three because, in addition to the filtration unit itself, it adds three new components the other filter types don’t require: a feed tee, a drain saddle and a dedicated faucet.

The feed tee is installed on the cold-water pipe that already supplies the kitchen faucet, usually right at the angle valve; RO600 comes with this tee and a 1/2-to-3/8-inch reducer for cases where the existing connection has a larger diameter. The drain saddle attaches to a nearby drainpipe, usually the sink’s, and carries away the water produced by the reverse osmosis filtration process at a 2:1 ratio of purified water to wastewater — that is, for every unit of filtered water produced, the system sends twice as much water down the drain. The full explanation of this ratio and daily capacity (GPD) explains why wastewater is part of how the membrane works, not pointless waste.

The dedicated faucet is installed separately from the regular kitchen faucet because water filtered by reverse osmosis does not mix with untreated mains water. For RO600, this faucet requires a 12 mm hole in the sink or countertop, and the countertop must be no more than 3.85 cm thick for the faucet to be secured properly. The filtration unit itself stays under the sink and has no storage tank — it supplies the faucet directly through an internal pump instead of holding filtered water in a pressurized tank, as conventional reverse osmosis systems do. The process by which the membrane separates purified water from wastewater is explained in detail in the complete guide to reverse osmosis, if you want to understand why the setup looks exactly like this.

The manufacturer estimates that complete installation, including the feed tee, drain saddle and dedicated faucet, takes about 30 minutes for someone doing it themselves, using the parts included in the box. If you want detailed steps, with each connection explained in order and more photos of the actual installation, the step-by-step guide to installing RO600 goes into that level of detail; this article shows you only the general setup, so you know what to expect before you begin.

What to do if you don’t have an available hole for the dedicated faucet

Many sinks already have an unused hole, left for a soap dispenser or a secondary faucet that was never installed; if that hole is close to 12 mm in diameter, it can usually be used directly for the RO600 faucet. If there’s no available hole, the countertop or sink needs to be drilled, and the material matters here: a wooden or laminate countertop can be drilled with a regular drill bit, but granite, engineered quartz or porcelain requires a special drill bit and an adapted technique; otherwise, you risk cracking the countertop. A new hole in a hard material or in a thick stainless-steel sink is generally a job for a plumber or a tradesperson with the right tools.

What to check before buying: thread, clearance, pressure and temperature

Some delayed installations and water filter returns aren’t caused by incorrect installation, but by a check someone skipped before buying. Four things are always worth checking:

• The thread on the existing feed connection: angle valves in European plumbing systems often have a 1/2-inch G thread, meaning a parallel thread standardized under ISO 228 (source: British Standard Pipe, Wikipedia), but you should still check the actual thread on your valve because differences between manufacturers and the age of the plumbing can change things.
• Clearance under the sink: RO600 needs approximately 13 by 38 by 43 centimeters of space for the unit, so measure the available clearance, taking into account any other items already stored there.
• Feed-water pressure and temperature: RO600 is designed for a pressure of 1.4 to 5.5 bar and a temperature of 4 to 38 degrees Celsius, using treated cold mains water, not untreated well water or hot water.
• An available hole in the sink or countertop for the dedicated faucet, or a willingness to make a new one, as described above.

Leak test and first start-up

After any installation involving new connections, whether for an inline filter or a reverse osmosis system, turning on the water for the first time is when you check for leaks, not when you walk away from the kitchen satisfied. Let the water run for a few minutes and carefully watch each newly installed connection: the feed tee, the tubing joints and the drain saddle. A drop appearing at a joint almost always means the nut isn’t tight enough or the seal isn’t sitting correctly, not that the part is defective; a little extra tightening, without overdoing it, solves most cases.

For RO600, the manufacturer recommends letting the dedicated faucet run for about 20 minutes before your first glass of water, to flush the new membrane and cartridges. The system also has a programmed automatic flush, triggered after start-up, after a longer period of operation and every 24 hours if it remains in standby, as well as a safety shutoff after 30 minutes of continuous operation and leak detection, which shuts off the feed if a problem occurs. These are useful mechanisms, but they don’t replace a visual check after installation: a sensor responds to a leak that has already occurred; it doesn’t prevent one from happening.

When to call a plumber and when you don’t need to

For most kitchens with modern plumbing and a working angle valve, the installation described above, whether it’s a faucet-mounted filter, an inline filter or RO600, can be done with the parts included in the box and the steps in the manual, by anyone with a little patience. That’s what these products are designed for.

There are, however, a few situations where DIY installation reaches its limits and a qualified plumber is the right choice: old plumbing with corroded pipes or angle valves that no longer shut off tightly; a missing angle valve on a line with no water shutoff point; a sink or countertop made from a material that’s difficult to drill without the right tools; or any work that would bring water close to the existing electrical system. This article does not provide electrical advice. Electrical safety depends on each home’s installation, so checking it is the responsibility of a qualified professional, not a generic diagram in an article.

If the checks above — compatible thread, sufficient clearance, pressure and temperature within the accepted limits, and a working angle valve — all check out, installing any of the three setups is generally a project you can do yourself in an hour, not a reason to call someone. If the tankless option, with a dedicated faucet and feed tee, seems right for your kitchen, the RO600 page shows exactly which installation parts are included, so you know in advance whether you need anything else.