Flush tank running, weak or not filling: what each fault means

A flush tank has two main parts, and almost every fault is one of them. The inlet — a float valve or fill valve — lets water in and stops when the tank is full. The outlet — a flush valve with a flapper or a siphon — dumps the water into the bowl when you press the button and must seal again afterwards. A tank that runs all the time has a problem with one or the other; a tank that flushes weakly or fills slowly has a problem with the inlet or with how much water it is being allowed to hold.

Here is how to tell which, in the order we would check it.

Tank running or hissing all the time

Lift the lid and look. Two possibilities:

• Water level is above the overflow pipe and draining down it. The inlet is not shutting off. The float is set too high, is stuck on the tank wall, has a split and filled with water, or the float valve washer has worn. Bend or screw the float down a little and see if it stops. If the float itself is waterlogged, it needs replacing.
• Water level is below the overflow, yet water is trickling into the bowl. The outlet seal is not seating. Usually a flapper that has hardened or scaled, a chain that is too short and holding it slightly open, or grit under the seal. Check the chain has a little slack with the button up. Wipe the flapper and the seat. If the rubber is stiff or cracked, it is a replacement.

A quick test for a silent leak: put a few drops of food colouring or a pinch of turmeric in the tank and wait fifteen minutes without flushing. Colour in the bowl means the flapper is passing.

Weak flush

A weak flush is not enough water or not enough drop.

• Not enough water. The float is set too low, or someone has put a brick or bottle in the tank to "save water" — on a modern 6-litre cistern that just means a flush that does not clear. Raise the float to the marked line. Remove the brick.
• Water is there but does not rush. The flapper is opening and closing too fast (chain too long), the flush valve is partly scaled, or the rim holes under the bowl edge are blocked with scale — very common on borewell water. A wire or a pointed plastic tool clears rim holes.
• Dual-flush button sticks or only half-presses. The button mechanism or the cable to it. Often just needs cleaning and re-seating.

Not filling, or filling very slowly

• Inlet filter blocked. Most float valves have a small mesh filter at the inlet. Scale, sand and sediment from borewell water pack it. Close the angle valve, undo the inlet nut, pull the filter, rinse, refit. This fixes most slow-fill complaints on its own.
• Angle valve not fully open after someone closed it for an earlier repair.
• Low pressure at the tank. If the basin tap next to it is also weak, the problem is upstream — the overhead tank is low or the line is scaled — not the cistern.
• Float valve worn out. Plunger-type valves wear; newer tower fill valves are quieter and faster. Replacing one is a standard job.

Noisy filling, banging pipes

A loud hiss that changes pitch as the tank fills is usually a worn float valve diaphragm. A bang when the valve shuts (water hammer) means the valve is closing too sharply — often a cheap replacement valve on a high-pressure line. Both are fixable; neither is dangerous, but the hammer will loosen joints over time.

What you can do and what to leave

Safe to do: lift the lid, free a stuck float, adjust the float height, slacken or shorten the chain, clean the inlet filter, clear rim holes, test for a flapper leak. Leave to a technician: replacing the flush valve or float valve (the tank comes off or fittings under it are undone — easy to crack ceramic), anything on a concealed cistern behind a tiled wall, and any job where a plastic fitting is already cracked.

Concealed cisterns

Wall-hung toilets with the tank inside the wall use a small access plate behind the button. Everything is reachable through it if you know the mechanism; not every part is stocked locally, so tell us the brand when booking so the technician comes with the right kit.

What it costs

A flush tank fault on our rate card is ₹350 for the labour — float valve, flush mechanism, flapper, button — with parts at MRP and shown to you before fitting. If the whole cistern is cracked, a replacement tank is quoted separately before any work starts.

When to call

A running tank you have already tried adjusting; a slow fill that does not improve after the filter is cleaned; a flush that stays weak with the tank full; any concealed cistern; and anything cracked. A running flush is the single biggest silent water waster in a home, so it is worth booking rather than living with the hiss.