Stop the flow first
Do this before you diagnose anything. The tank is filling faster than the valve can shut it off, and every minute it runs is more water out of the overflow.
- Lay something rigid across the tank. A batten, a broom handle, a spirit level — anything that spans it and will take a few kilos of pull.
- Tie the float arm up to it. String, cable tie, wire, a belt. Lift the arm so the float sits well above the water and hold it there. The valve closes, the inflow stops. That is the whole trick.
- Then close the inlet properly. There should be a small servicing valve on the pipe feeding the tank, within a foot or so of it, turned with a screwdriver slot or a small lever. Quarter-turn it closed. If there is not one, turn off the main stopcock.
- Leave the overflow alone. Do not block it, plug it, or tape over it. It is the only thing standing between a failed valve and a soaked ceiling.
While the arm is tied up the tank is not refilling, so the cold taps and the hot water it feeds will run down and stop — do not start a bath. And if a shower pump is fed from that tank, do not run it dry; pumps are damaged by running without water in a way tanks are not.
What is actually happening up there
Two tanks live in London lofts and they fail the same ways. The cold water storage cistern is the big one, a few hundred litres, filled from the mains through a float valve and gravity-feeding the cold taps and the cold feed to your hot water cylinder. The feed and expansion cistern is the small one beside it, topping up the heating circuit and giving the water somewhere to expand as it heats. Both have a float valve. Both overflow.
The overflow pipe doing its job is not the fault. It is the symptom. The regulations require every storage cistern to have a valve that shuts the inflow off below the overflowing level, and an overflow pipe with a means of warning you the level has been exceeded — which is why it is usually run outside where you can see it dripping. The pipe is a warning device. You are being warned.
The four ways a float valve fails
Float valves are simple, and simple things fail in a short list of ways. One of these four is behind almost every overflowing tank.
1. The washer or seat has perished
The valve closes by pressing a rubber washer against a brass seat. The washer hardens, splits or picks up grit, or the seat scores. The valve almost closes, and passes a trickle forever. This is the most common failure by a distance, and it gives the classic symptom: the overflow never gushes, it just drips, and it has probably been doing it for months.
2. The float has taken on water
Plastic floats crack at the seam or the threaded boss and fill with water. A waterlogged float never rises far enough to close the valve, so the tank fills until the overflow takes the surplus. Lift the float out and shake it. If you hear water inside, that is your answer, and a new float is a few pounds.
3. The arm or the piston has seized
London water is hard. Scale builds on the piston of an older Portsmouth-pattern valve, or on the arm pivot, until the mechanism stops moving freely. The float rises, the arm does not follow, the valve stays open. You can usually free it, and it usually seizes again.
4. The valve is rated for the wrong pressure
This one catches people out because the valve is new and still fails. Float valves come with different orifice sizes for different supply pressures. A low-pressure valve on a strong mains supply cannot generate enough leverage to hold itself shut — it seals, then weeps, then passes. A high-pressure valve on a weak or tank-fed supply refills painfully slowly instead. If the overflow started just after someone changed the valve, check the rating first.
| What you see | Most likely cause | What it needs |
|---|
| Steady drip from the overflow, never stops | Perished washer or scored seat | New washer, or a new valve |
| Tank level sits high, overflow runs properly | Waterlogged float, or the arm set too high | New float, or re-set the level |
| Stops and starts for no reason | Scale on the piston or pivot | Strip and clean, then replace |
| Overflow started after a recent valve change | Wrong pressure rating fitted | Correct valve for the supply |
| Valve shuts fine, tank still fills | Not the valve — see below | Diagnosis, not a part |
The permanent fix
Cleaning the piston and re-bending the arm both work. Neither lasts. The labour is the same whether you strip the valve or swap it, so on a tank that has already overflowed once the answer is a new valve.
A proper job means: isolate at the servicing valve, drop the level, fit a new float valve rated for the actual supply pressure, fit a new float, and set the shut-off so the water settles a clear inch or so below the overflow — not just under it. Modern diaphragm valves shut off harder and quieter than the old brass piston pattern, which also cures the water hammer some tanks produce on every refill. If there is no servicing valve on the inlet, add one.
While the lid is off, look at the rest. A missing lid, no insulation, an overflow with no insect screen, or a galvanised tank shedding rust are all cheaper to sort while someone is already up there.
Discharging outside: it is not harmless, it is billed
An overflow running to a pipe over the garden feels like a non-event, so it gets left. On a metered supply it is a continuous, round-the-clock draw of mains water you are paying for, and unlike a burst it never announces itself — no damage to prompt the call, just a bill that has quietly grown.
It is also one of the standard explanations for a meter that keeps turning with every tap shut off, and worth ruling out before anyone mentions a buried supply pipe. Go outside on a dry day and look at the overflow outlet. A dripping pipe is a diagnosis you just made for free — see a water meter turning with everything off for how to rule the rest in or out.
Discharging inside: stop reading and deal with it
An overflow that goes nowhere is a different problem entirely. Mains water is filling an open tank with no escape route, and the surplus goes into the loft — onto the insulation, through the ceiling plaster, and down. It happens when the overflow pipe was never connected, has come apart, discharges into the loft void instead of outside, is blocked with debris, or froze solid in the last cold snap while the valve kept passing.
Signs the water is going into the loft rather than out of the wall: a damp or sagging ceiling under the tank with no leak anywhere else, insulation matted and heavy around the tank, a musty smell up there, or a tank overflowing at the rim with no drip outside.
Isolate the supply immediately, using the steps at the top of this page. Then get the ceiling looked at: saturated plasterboard holds a surprising amount of water before it fails, and it fails all at once. Water coming through a ceiling and ceiling leak repair cover what happens next.
When it is not the float valve at all
Three cases look identical from the ground and are not a valve problem. Replacing a perfectly good valve fixes none of them.
- Pumping over. On an open-vented system the vent pipe terminates over the feed and expansion cistern. If the pump is badly positioned or the system is airlocked, heating water is pushed up the vent into the tank, which overflows. The tell: it happens when the heating runs, and the tank water is warm or discoloured.
- A failed cylinder coil. A perforated coil in an indirect cylinder lets one circuit pressurise the other and push water back into the feed tank. Rare, and it is a cylinder job, not a valve.
- It is rain. A wet patch near the tank that comes and goes with the weather is a roof problem. We will tell you that rather than charge you to find out.
Who you actually need
Most overflowing tanks are an hour of ordinary plumbing and a cheap part. You do not need a leak detection survey for a fault you can stand over and watch, and we would rather say so than sell you one.
We are the right call when the tank has already done damage — a ceiling down, insulation soaked, the floor below stained — or when the tank is not the cause and nobody can work out what is. Our rate is £150 per hour, agreed before we attend, no call-out fee, and under no find, no fee you do not pay for the looking time if we do not find it. Where a claim is involved, a detection visit includes a written report prepared to support a trace-and-access claim; what the policy covers is the insurer's decision, not ours. We cover London, and our own handymen and builders put the ceiling, plaster and decoration back afterwards. Pricing sets out the lot.
