Under-Sink Water Purifiers: Complete Guide to Choosing, Installing and Maintaining Them

Why Under-Sink Purifiers Are the Premium Choice for Most Homes

The countertop purifier that sits beside your sink is the most common purifier format in Bangladesh — and for good reason. It is easy to install, easy to service, and highly visible (which helps you remember to maintain it). But it takes up counter space, has visible tubes and filter housings, and the tank sits on your counter or in your cabinet in an ad hoc arrangement.

Under-sink purifiers solve all of these aesthetic and space problems. The entire system — pre-filters, RO membrane housing, post-filters, and storage tank — installs inside your sink cabinet. Only a single dedicated faucet is visible on the sink deck or countertop. The result is a clean kitchen with professional-grade water purification hidden from view.

What an Under-Sink System Looks Like

A complete under-sink RO system occupies a cabinet space of roughly 50cm wide × 40cm deep × 45cm tall. The components are:

• Three to four filter housings mounted vertically or horizontally on a wall bracket inside the cabinet
• RO membrane housing — a longer cylinder, typically 30–35cm
• Pressurised storage tank — the largest component, typically 8–12 litres
• UV module (in systems with UV) — a small inline cylinder with power lead
• Auto shut-off valve — small inline component
• Check valve and flow restrictor — small inline components
• Dedicated tap faucet — the only visible part, mounted through the sink deck

The system connects to your existing cold water supply line using a T-connector or saddle valve and drains reject water into the sink drain.

Types of Under-Sink Purifiers

Standard Under-Sink RO

The most common type. Multi-stage filtration ending with an RO membrane and UV stage, with a pressurised storage tank. This is the direct under-sink equivalent of countertop RO purifiers.

Best for: Households with high TDS, arsenic concerns, or those wanting comprehensive protection

Tankless (Instant) Under-Sink RO

A newer design that produces purified water on demand without a storage tank — using a higher-capacity membrane and a booster pump that maintains adequate pressure. The purifier produces water fast enough to supply the tap directly.

Pros: No tank means no stagnant stored water; more compact; constant fresh purified water

Cons: Requires reliable electricity at all times; higher cost; some models have minimum pressure requirements

Best for: Modern kitchens where space is at a premium and power supply is reliable

Under-Sink Carbon Filter Only (No RO)

For households with low TDS source water that only need chlorine and taste/odour improvement — not full TDS reduction. A single or dual under-sink carbon block filter system connects to the existing hot or cold water line and requires no electricity, no drain connection, and no storage tank.

Best for: Municipal supply areas with low TDS (below 150 ppm) and good microbial safety

Choosing the Right Under-Sink Purifier

Step 1 — Check Your Cabinet Dimensions

Measure your sink cabinet interior carefully before purchasing. Standard under-sink RO systems require:

• Height: Minimum 40cm from shelf to cabinet ceiling
• Width: Minimum 45cm
• Depth: Minimum 35cm

Tankless systems are more compact. If your cabinet is too small, a tankless system or a wall-mounted unit (mounted on the wall beside the under-sink cabinet) may be the solution.

Step 2 — Confirm Your Inlet Pressure

Under-sink RO systems with booster pumps work at any pressure above 15 PSI. Systems without booster pumps require a minimum of 40 PSI. Check your inlet pressure — for low-pressure situations common in Bangladesh's high-rise flats, ensure you choose a system with a built-in booster pump.

Step 3 — Match Capacity to Usage

Step 4 — Consider the Faucet Style

The dedicated purified water faucet is the only visible component of the entire system. Choose one that complements your kitchen design. Options include:

• Standard chrome tall faucet — classic, widely available, easy to replace
• Designer brushed nickel or matte black — matches modern kitchen aesthetics
• Dual-lever hot/cold faucet (for systems with a heater module) — dispenses both ambient and hot purified water from a single fixture

Installation: What a Plumber Will Do

A competent plumber or purifier technician installs an under-sink system in 2–3 hours. The steps are identical to countertop installation except that everything is contained inside the cabinet:

1. Mount the filter bracket on the cabinet wall
2. Install the filter housings and connect tubes between stages
3. Install the saddle valve on the cold water supply pipe
4. Drill the faucet hole in the sink deck (if not already present)
5. Mount the faucet from above, secure from below
6. Connect the drain saddle to the drain pipe
7. Position the storage tank and connect
8. Plug in UV and pump power
9. Flush system (2 full tank volumes) before first use

The entire system is invisible from the kitchen. Only the small dedicated faucet tells you the system exists.

Maintenance Differences From Countertop Systems

Under-sink systems require the same filter changes as countertop units but accessing the filters requires opening the cabinet and reaching inside — slightly more awkward than a countertop unit where everything is at eye level.

Best practice: Replace filters while kneeling in front of the open cabinet, with a towel on the floor and a small bucket for any residual water. Label each filter housing with a permanent marker indicating which filter stage it contains (sediment, carbon, post-carbon) to avoid confusion during replacement.

Filter replacement costs and intervals are identical to countertop RO systems:

Top Under-Sink Purifier Brands Available in Bangladesh