DIY vs Buying Water Filtration

Water Filtration  · 

What An Under-Sink Kit Puts In The Cabinet

An under-sink reverse osmosis kit arrives as parts, not as an appliance. A typical box holds a sediment prefilter, one or two carbon prefilters, the RO membrane in its housing, a post-carbon polishing cartridge, a pressurised storage tank, a dedicated drinking faucet, a feed valve that tees off the cold angle stop, a drain saddle, and coils of quarter-inch and three-eighths tubing with push-to-connect fittings on the ends. Assembly means mounting a bracket under the cabinet, running tube between five or six housings in series, and making three connections to the house: cold supply in, drain out, and the faucet through the counter.

Buying the install instead means a plumber or water treatment dealer supplies the same components and does the same work, usually with their own fitting choices and their own warranty on the labour. The gap in price between the two is mostly time, plus the cost of carrying liability for a leak. Everything below is about where that time actually goes and which parts a careful homeowner can do without.

Drilling The Faucet Hole

The drinking faucet needs a hole, and where that hole goes decides how hard the job is. The easiest case is an existing hole from a soap dispenser or a spare accessory that already has the right diameter. Otherwise you drill, and the sink material governs the tool: stainless steel wants a step drill or a hole saw with cutting oil and a slow speed, cast iron enamel needs a carbide or diamond bit with light pressure so the enamel does not chip away from the edge, and composite or quartz wants a diamond core bit kept wet.

Start with a small pilot, keep the bit square to the surface, mask the area to catch chips, and avoid drilling near a weld, a drain rib or the edge of the sink where the surface is unsupported. A standard drinking faucet shank wants a hole sized to its base — smaller than the opening a main kitchen faucet needs — and the shank has to be long enough to pass through the deck thickness plus the counter, which is where thick stone counters surprise people. If you are not willing to drill the counter, a filter that mounts at the existing faucet or a countertop unit avoids the question entirely.

The Drain Saddle, Traps And Air Gaps

RO membranes need a path for the water they reject, and that path is a drain saddle clamped around the sink’s drain pipe. It goes on the vertical tailpiece above the trap, never on the horizontal run after it, and above the point where waste water stands, so the reject line is not sitting in dirty water. The saddle wraps the pipe, a gasket seals it, and a small hole is made through the gasket and pipe wall for the tubing fitting. Do not tighten it so hard that the pipe deforms, because a distorted tailpiece leaks at its slip joint.

Where local plumbing code requires an air gap, the drinking faucet is an air-gap design with two tubes running to it: one carries reject water down to the saddle, the other vents to the sink basin so a backed-up drain cannot siphon into the filtered supply. An air-gap faucet is noisier and needs a second hole only if it is a deck-mounted gap; most units bundle both tubes through the single faucet hole. Backflow protection at the feed also matters — the supply must come off the cold angle stop through a valve, never from a fixture that could draw waste back into the drinking line.

PEX, Push-To-Connect And Compression

Three fitting families show up in these installs and they fail for different reasons. PEX with crimp or cinch rings is the most forgiving in a wall, because the ring makes a mechanical, permanent joint on the tube, but you need the crimp tool and the correct rings rather than a pair of pliers. Push-to-connect fittings seat on an O-ring and grip with a collet: fast, no tools, and reliable if the tube end is cut square with no score marks, pushed fully home, and left with no side load. Pull the tube to confirm it is captured, and use a locking clip. Compression fittings with a brass ferrule suit copper: cut square, deburr, slide the nut and ferrule on, and hold back-up with two wrenches so the joint does not twist the pipe.

The RO plumbing itself is a different world from house plumbing. Quarter-inch and three-eighths tubing and their fittings are not compatible with half-inch supply lines, so the feed valve adapts at the angle stop and everything downstream is small-bore tube. Threaded connections get PTFE tape or pipe dope, push fittings get neither, and every joint you make should be reachable later rather than buried behind the tank.

Storage Tank, Pressure And Waste Ratio

Filtered water from the membrane fills a bladder tank, and the tank’s empty pressure is set at the factory to be lower than the household’s supply pressure so incoming water can compress the air charge. If the pre-charge is wrong, or if the supply pressure is low, the tank holds less than its nominal volume and the faucet runs to a dribble sooner. The tank also sets the wait: draw a full glass and you can hear the unit refill, which takes a while because the membrane passes water slowly by design.

Rejection is a property of the membrane, the feed pressure and the incoming total dissolved solids, which is why a system plumbed to weak pressure performs worse than the same parts on strong pressure. The waste ratio comes from a flow restrictor matched to the membrane, which sends part of the feed water to drain instead of through the membrane. Restricting waste harder makes the system look efficient on paper but leaves concentrated water sitting against the membrane, which scales it and shortens its life. A permeate pump or a booster pump changes both ends of that trade, at the cost of noise and another part to fail.

Water Damage, Warranty And Liability

The risk in this job is not the water you drink, it is the water under the cabinet. A push fitting that is not fully seated weeps slowly, and a slow weep behind a tank ruins a cabinet floor, warps a shelf and eventually reaches a neighbour through the slab. Fit a drip tray under the unit, keep a leak sensor in the cabinet, and shut the feed valve off when you leave for an extended trip. Test every joint with the system pressurised and paper towel wrapped around each fitting so the first sign of a weep is obvious.

Warranty follows the same split. The kit maker’s warranty covers the parts it shipped, and a failure caused by a fitting that is not theirs, or by an assembly mistake, is generally excluded. A plumber’s work usually carries its own labour warranty and their insurance, which is the part you are buying when you hire one. Homeowner insurance will typically cover a sudden leak, but a carrier or an adjuster may ask who installed the system and when, so keep the paperwork either way.

Permits, Landlord Approval And When To Call A Professional

Most jurisdictions let a homeowner work on plumbing in their own single-family home, but many require a permit for a new fixture or a new drain connection, and an RO reject line tied into the drain is exactly that. Rules vary enough that the safe move is to ask the local building department what triggers a permit before drilling, because an unpermitted alteration can surface at a resale inspection. In a rental, drilling the counter and clamping the drain are permanent modifications: you need the landlord’s written approval, and you should expect to restore the sink at move-out. Condominium and apartment buildings may also restrict alterations to shared drainage stacks.

Hire the work out when the angle stop will not close and the water cannot be isolated, when the kitchen sink drains have already shown signs of clogging, when the counter is a material you are not prepared to drill or repair, or when building rules require a licensed plumber for a permit. Those are the cases where the money saved on labour is smaller than the cost of a repair and a claim.

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FAQ

Can I install an under-sink reverse osmosis system in a rental?

Only with your landlord’s approval, because the install drills the counter or sink deck and clamps a saddle onto the drain pipe. Both are permanent modifications a landlord can reasonably object to. A countertop or faucet-mounted filter gets the same drinking water without touching the plumbing.

Why does an RO system keep producing water after the tank is full?

It stops when the tank pressure matches the supply pressure, which is what the shutoff valve in the system senses. If it never stops, the tank’s air charge is low, the supply pressure is high, or the shutoff valve is not seating, and one of those has to be fixed before the drain keeps running.

Are push-to-connect fittings reliable enough to trust under a sink?

Yes, when the tube is cut clean and square, pushed fully into the fitting until it seats, and not side-loaded by the way the tubing is routed. Score marks, a tube that was pushed only part way, or a hard bend at the fitting are what cause the slow leaks people blame on the fitting design.

Do I need a permit to add an under-sink filter?

It depends on the local code and on what you are connecting to. A cartridge filter that tees into the cold supply usually does not need one; a reverse osmosis drain line tied into the waste system often does. Ask the building department before you drill, not after.

Is the storage tank optional?

Some systems run tankless, drawing water straight from the membrane as you use it, which removes the tank and the wait for a refill but makes the flow at the faucet very slow. A tank stores the trickle so you can draw a glass or a pot at a usable rate.