Water Tank Overflowing: Why and What to Do

An overflowing tank is one of those problems that starts as a minor annoyance — a trickle of water down the driveway or a damp patch near the tank — and quietly becomes an expensive one. If you can hear water running when nothing in the house is open, or you notice your municipal water meter ticking when demand should be zero, a stuck-open float valve is the most likely culprit.

This guide walks through why tanks overflow, what the consequences are, and exactly what to do about it.

How a float valve is supposed to work

The float valve (sometimes called a ball valve or ballcock) is the device inside your tank that controls inflow. A buoyant ball or cylinder sits on the water surface. As the tank fills, the float rises and mechanically closes the inlet. When you draw water and the level drops, the float drops with it and opens the inlet again.

It is a simple, largely passive mechanism — which is why most float valves last years without attention. But simple mechanisms fail in simple ways, and when they do, water keeps entering the tank regardless of the level.

For a closer look at how these components work and what to look for when buying a replacement, the guide on water tank float valves covers the different types available in South Africa and what each suits.

Common reasons a tank overflows

Float valve failure is the most frequent cause. The float itself can become waterlogged and sink, keeping the valve permanently open. The valve seat can wear, crack, or accumulate grit, preventing a proper seal. The arm linking float to valve can corrode or bend out of position.

Incorrect float adjustment is worth checking before assuming the valve is broken. If the float arm is set too high, the tank fills past the overflow outlet before the valve closes. A simple bend of the arm — or an adjustment screw on newer valves — often solves this without replacing anything.

Debris and scale are common in Johannesburg’s water supply. Johannesburg Water’s own figures show non-revenue water running at 44.8% in 2024/25, which reflects how much pressure variation and pipe age exists in the network. Older pipes carry sediment, and that sediment lands in your float valve. A valve that was working fine can jam open after a burst main is repaired nearby and debris flushes through.

High inlet pressure can overwhelm a float valve that is only rated to a certain pressure. If your municipal pressure is higher than the valve’s rating, it can force water past a theoretically closed seat.

Tank installed without a proper overflow outlet — or with a blocked one — means water has nowhere to go except over the rim or out of fittings. The overflow itself should be sized and positioned correctly at installation.

Why it matters: water waste and damage

Johannesburg sits in a water-stressed region, and South Africans already use around 237 litres per person per day on average — well above the global average of roughly 173 litres. A tank overflowing even slowly adds meaninglessly to that figure and directly to your water bill.

Beyond the bill, sustained overflow can:

• Saturate the ground around the tank base, undermining it over time
• Cause damp ingress into walls or foundations if the tank is positioned against a structure
• Create slip hazards on paved surfaces
• Attract mosquitoes if overflow water pools nearby

If your tank feeds a system connected to your home’s broader backup water systems setup — a pump, a pressure vessel, or a secondary storage tank — a failing float valve can also keep your pump running unnecessarily, shortening its life.

If you're not sure whether your current tank setup is installed correctly, it's worth getting a second opinion from a qualified plumber. Compare 3 free quotes — vetted installers, no obligation.

How to diagnose the problem

Before calling anyone or buying parts, run through this sequence:

1. Locate the overflow outlet on your tank. It is typically a pipe near the top of the tank wall, designed to discharge water safely if the level rises too high. If water is coming from here continuously, the float valve is not shutting off.
2. Open the tank lid and observe the float. Is it sitting on the surface or partially submerged? A waterlogged float that sits low in the water will hold the valve open.
3. Manually lift the float arm. If lifting it stops the inflow, the valve mechanism itself is functional and you likely only need to adjust the arm height or replace the float.
4. Check for visible debris around the valve seat. Sometimes a piece of grit is all that is keeping the valve from closing fully.
5. Check inlet pressure if you have a pressure gauge. Many residential float valves are rated to around 400–600 kPa; confirm your valve’s rating and compare it to your supply pressure.

Keeping a regular record of these checks is good practice. The water tank maintenance checklist sets out a practical inspection schedule that covers float valves alongside the other components worth monitoring.

What to do: fix or replace?

Adjust first. If the float arm is simply set too high, bend it slightly downward (on older brass arms) or use the adjustment screw (on newer plastic valves) so the valve closes before water reaches the overflow level. Test by watching the inflow stop as the tank approaches full.

Clean the valve seat. Turn off the supply, drain a little water so you can work on the valve, and disassemble the valve head. Wipe the rubber diaphragm or washer and the seat. Replace the washer if it is deformed or cracked — these are inexpensive and widely available.

Replace the float if it is waterlogged. Tap it: a hollow sound means it is intact; a heavy or sloshing feel means water has entered. Replacement floats are available at most plumbing merchants.

Replace the entire valve if the body is cracked, heavily corroded, or if repeated adjustments haven’t solved the problem. A like-for-like replacement is usually straightforward. Make sure the replacement is rated for your supply pressure and complies with SANS 241 (2024 edition) requirements where applicable to your installation.

Call a plumber if the problem recurs quickly after a fix, if you suspect high or fluctuating supply pressure, or if the overflow outlet itself is blocked or incorrectly positioned. Persistent overflow can also indicate a problem upstream of the tank that needs a licensed plumber to assess.

Repeated float valve failures can point to a wider installation issue worth having properly assessed. Get quotes from vetted local plumbers — free, no obligation.

Preventing it from happening again

Float valves are not fit-and-forget components. A few straightforward habits reduce the chance of a repeat:

• Flush the inlet filter (if your installation has one) every few months, particularly after any mains repairs in your street
• Inspect the float valve at least twice a year — add it to a regular tank inspection alongside cleaning and checking tank integrity
• Fit a pressure-reducing valve on the supply line if your municipal pressure is frequently high or variable
• Mark the correct water level on the inside of your tank so you can spot at a glance whether the valve is shutting off at the right point

Symptom	Most likely cause	First action
Continuous flow from overflow pipe	Float valve not closing	Check and adjust float arm
Water trickling past closed valve	Worn washer or debris on seat	Clean or replace washer
Float arm closes but water still enters	Cracked valve body or excess pressure	Replace valve; check pressure
Overflow pipe dry but tank brimming	Overflow outlet too high or blocked	Reposition or clear overflow

A well-functioning float valve is a small but important part of a reliable home water storage setup. Catching a failure early keeps your water bill in check, protects your tank installation, and avoids the kind of slow damage that is expensive to undo.