Float Valves (Ball Cock) in India: How They Work, Sizing, Fitting & Fixing Overflow

A float valve — the fitting most Indian plumbers and hardware shops call a ball cock — is the small self-acting valve that fills a water tank and shuts the supply off the moment it is full, with no electricity, sensor or attention from you. A hollow ball floats on the water surface; as the level rises the ball lifts a lever that presses a washer against the inlet seat and stops the flow. When you draw water and the level drops, the ball falls, the valve reopens, and the tank tops itself up. It is one of the oldest and most reliable pieces of plumbing in any home.

This guide sits under the Studio Matrx plumbing valves guide and is a companion to the guide on ball valves, which despite the similar name is a completely different device — a quarter-turn on/off tap, not an automatic filler. For how the tank itself is sized and specified, see the forthcoming water storage tanks guide and, for bathrooms specifically, the bathroom water tank calculator.

What a float valve does and how it works

A float valve is an automatic, level-controlled shut-off. Its job is not to throttle flow or control pressure — it is fully open or fully closed depending on one thing: the water level under the float.

The mechanism is simple and mechanical:

• A hollow float (ball or oblong) sits on the water and rises and falls with the level.
• The float is fixed to a lever arm, pivoted at the valve body.
• As the water rises, the arm swings up and drives a plunger and washer onto the inlet seat.
• When the washer seals the seat, inflow stops. As water is drawn off and the level falls, the arm drops, the washer lifts off the seat, and water flows again.

Because it needs no power and no external control, a float valve is what makes an overhead tank fed from a pump or the municipal main fill and stop by itself.

Think of the float valve as the tank's own brain. The pump or the mains delivers water; the float valve decides when the tank has had enough. If it fails, the tank has no way of knowing it is full — which is exactly why a failed float valve causes an overflow.

Where float valves are used

The same basic device appears wherever a vessel needs to be kept topped up automatically:

• Overhead (terrace) tanks — the classic use. The pump or municipal line feeds the tank; the float valve stops it overflowing. On pumped systems the float valve is the last line of defence, and a separate float switch usually cuts the pump (more on that below).
• Underground sumps — a large float valve on the municipal inlet keeps the sump filled without manual intervention, so the transfer pump always has water.
• WC cisterns — every flush tank has a small float valve (often a compact side-inlet or bottom-inlet type) that refills the cistern after each flush and shuts off silently.
• Cooling towers, RO feed tanks, cattle troughs, industrial process tanks — anywhere a level has to be held automatically, a heavier-duty float valve does the job.

Float valve vs float switch — a common confusion

These two things are often on the same tank and are easy to mix up:

• A float valve (ball cock) is a purely mechanical valve on the water pipe. It physically opens and closes the inlet. No wiring.
• A float switch is an electrical sensor with no valve at all. It floats up and down and switches a circuit — typically to start and stop the pump, or to sound a full-tank alarm.

A well-set-up overhead tank often has both: the float switch turns the pump off when full (saving energy and pump wear), while the float valve is the mechanical backup that stops an overflow if the switch sticks. Motorised or sensor-based smart shut-off and leak-detection valves are a separate topic — see the Studio Matrx smart-home material rather than relying on a ball cock for that role.

Materials: brass vs plastic float valves

Two families dominate the Indian market.

• Brass float valves — body, arm and internals in brass, usually with a copper or plastic float. Robust, tolerant of pressure and heat, long-lived, and repairable (you can replace the washer). Preferred for overhead tanks, sumps and anything on a pumped or high-pressure line. Costlier.
• Plastic (PP / nylon / PVC) float valves — light, cheap, corrosion-proof and fine for low-pressure WC cisterns and small tanks. The float is usually a sealed plastic ball. Cheaper and adequate, but the arm and clip can fatigue and the seat can wear faster under grit.

For a terrace tank on a booster pump, brass is the safer specification; for a flush cistern, a good plastic unit is normal and perfectly serviceable.

Sizes and a selection guide

Float valves are sized by their inlet connection (the thread that screws into the tank wall or the supply pipe), commonly 15 mm (½"), 20 mm (¾"), 25 mm (1") and up. Bigger inlets pass more water and are used where fast filling or higher flow is needed.

Prices are indicative retail ranges (2026) and vary by brand and city — verify locally.

Many modern valves are adjustable: you set the shut-off level either by bending the metal arm slightly, by an adjustment screw at the pivot, or (on cistern types) by sliding a clip on the float rod. Setting the level a little below the overflow gives you a safety gap.

Fitting a float valve

• Mount the valve near the top of the tank, threaded through the wall with the float swinging freely on the inside — nothing (pipe, wall, another fitting) must obstruct the ball's travel.
• Set the shut-off level below the overflow/warning pipe, so that even if the valve is slow to seat, water leaves via the overflow, not over the tank rim.
• Keep the supply isolated by a separate stop valve (a ball valve or gate valve) upstream, so you can shut off water to service the float valve without draining the system. See the plumbing valves guide for how these fit together.
• On pumped systems, don't rely on the float valve alone — pair it with a float switch so the pump actually stops, rather than dead-heading against a closed float valve.
• Ensure the tank has a correctly sized overflow pipe routed to a safe, visible discharge, so a stuck valve announces itself instead of soaking the slab.

Common failures and how to spot them

Nearly every float valve complaint reduces to one symptom — the tank overflows or the cistern keeps running — and a handful of causes:

The costliest of these is a stuck-open valve on a terrace tank: it wastes water continuously and, if the overflow is undersized or blocked, floods the roof and the walls below. This is why the overflow pipe and, on pumped systems, a working float switch matter — they contain a float valve failure instead of letting it run for hours. A slow overflow can quietly waste thousands of litres a month, so a dripping float valve is worth fixing promptly.

Hard water is the biggest enemy: scale builds on the seat and washer and stops a clean seal. In hard-water areas, expect to replace the washer periodically as routine maintenance.

Pros and cons

Pros

• Fully automatic, needs no power, sensor or attention.
• Simple, cheap and near-universally available in Indian hardware shops.
• Easily repaired — usually just a washer — and long-lived in brass.
• Provides a mechanical overflow backstop even where a float switch controls the pump.

Cons

• Wears out silently; a failed valve overflows rather than warning you.
• Sensitive to grit and hard-water scale on the seat.
• Purely mechanical — it cannot turn a pump off; that needs a float switch.
• Cheap plastic units can fatigue, crack or sink over a few years.

Indicative cost

A domestic ¾" brass float valve is roughly ₹600 - ₹1,500; a plastic cistern valve ₹120 - ₹350; a large brass or gunmetal valve for a sump or cooling tower runs ₹3,000 - ₹12,000+. A replacement washer is a few rupees. Fitting is minor labour, usually bundled into a plumber's visit. These are indicative 2026 retail figures — confirm current local prices.

References

• National Building Code of India (NBC) — Part 9, Plumbing Services, for water supply and storage tank provisions.
• Uniform Plumbing Code of India (published by the Indian Plumbing Association).
• Bureau of Indian Standards specifications for float-operated (ball) valves for water supply fittings — confirm the current IS number and edition with a BIS source before quoting it.