What usually gets left behind
A combi or unvented conversion changes where the water comes from. It rarely changes what is in the loft. The cylinder goes, the airing cupboard is reclaimed, the radiators run sealed, and the installer's last act is to close the stopvalve on the feed. Job done, from the point of view of the quote.
What is still up there, in a lot of London lofts, is a cistern holding roughly 115 to 230 litres, sitting on boards across the joists, with a ball float valve holding back mains pressure and a stopvalve nobody has turned since the day it was closed. Nothing draws from it. Nothing refills it, in theory. Nobody opens the hatch.
It is not an emergency. It is the kind of thing that becomes one in five or ten years, through a bedroom ceiling.
Why leaving it is a real risk, not a tidiness argument
The float valve is the part that fails
A ball float valve is a rubber washer pressed against a seat by a lever arm, designed to be exercised. In a live tank it moves several times a day and the washer stays seated. In a dead tank it never moves at all: the washer takes a permanent set and the arm seizes.
The only thing holding the mains back is then the stopvalve on the feed. Gate valves and older stopvalves are exactly the fittings that fail closed-then-weeping — the gate corrodes on the spindle, the seat scores, and the valve passes a trickle it did not pass a year ago. Now mains water is arriving in a tank whose float valve cannot shut, and whose overflow is a plastic pipe that may or may not still be clipped and clear.
This is the sequence behind a large number of loft-origin ceiling collapses, and it is slow enough that the first sign is a stain, not a bang.
The water itself goes bad
Stored water that is never drawn off is the definition of stagnation. The Water Supply (Water Fittings) Regulations 1999 are explicit that a storage cistern must be installed so as to prevent areas of stagnant water developing, and must carry a cover excluding light and insects, an overflow and insulation. A tank left in service with nothing drawing from it fails the first by design, and after a few years usually fails the rest too — covers slip, insulation is pulled about by whoever laid the loft boards, light gets in.
The Health and Safety Executive's guidance on legionella treats still water with little or no turnover as the condition that lets the bacteria multiply. In a house where the tank is genuinely dead-ended that is a low risk. Where a "redundant" tank still quietly feeds a WC cistern or an outside tap, it is not, because someone is still using that water.
The weight, stated plainly
Water weighs a kilogram per litre. A full 227-litre cistern is therefore about 230 kg of water plus the tank, on a platform built when the tank was part of the design and checked by nobody since. Loft boards get cut for cables, joists get notched for downlighters, platforms get raised to gain head for a shower. None of that matters while somebody is looking after it, and nobody is looking after this.
Weight is also why a slow overflow in a loft is worse than the same leak downstairs. Saturated mineral wool holds water against plasterboard for days before anything shows, and a ceiling that has taken on that much weight does not drip politely — it lets go in one piece.
Cutting the feed is not a decommission
The common half-job is closing the stopvalve and walking away. It leaves every problem above except the refill, and it stakes the ceiling on one old valve staying shut for a decade.
The other half-job is draining the tank and leaving it connected. Better, but a drained tank on a live feed refills the moment the valve passes, and refills unnoticed because nobody expects water in it.
A decommission means the tank can no longer receive water under the failure of any single component. That is the standard to hold anyone to.
What a proper decommission actually involves
- Confirm nothing is still fed from it. Close the outlet gate valves and run every cold tap, the WC cisterns, the outside tap and the shower. Anything that stops running is still on the tank. This is the step that gets skipped and it is the only one that matters.
- Isolate at the mains, not at the tank. The feed is cut back to a live, accessible point — usually where it tees off the rising main — and capped there with an end feed or soldered cap, or terminated at a full-bore isolating valve that is labelled. A capped stub with nothing beyond it cannot leak into the loft, whatever the old stopvalve does.
- Drain the tank properly. Siphoned or pumped down, then sponged out. Not left with 20 litres in the bottom because the outlet sits above the base.
- Disconnect and cap the distributing pipes. The outlets, the vent connection if there was an open-vented cylinder, and the overflow. Every remaining stub either removed or capped, so no open pipe ends are left in the loft for a future owner to guess at.
- Remove or deliberately retain the shell. A galvanised or large plastic cistern often will not come back through a standard hatch in one piece; cutting it up in the loft is messy but routine. If it has to stay, it stays empty, disconnected and lidless so nobody mistakes it for live storage.
- Deal with dead pipework below. Old distributing pipes running down through the airing cupboard to nothing are dead legs. Cut back and capped; left hanging is how the next person floods the place by cracking a valve open to see what it does.
- Reinstate the insulation. Loft insulation under an old tank was left thin on purpose, so house heat kept the tank from freezing. Once the tank goes, that gap is just a cold spot.
None of this is difficult work — roughly a half-day for most houses. It gets skipped because it is not part of anybody's boiler quote.
When the tank must stay
This is the part that gets people into trouble, because "we went combi" is not the same as "nothing uses the tank". Leave it in place, and keep it maintained, in any of these cases.
| Case | Why it stays |
|---|
| An open-vented hot water cylinder is still in use | The tank is the cylinder's cold feed and its vent termination. Remove it and you have no hot water and no safe expansion route. |
| The heating is still open-vented | That is a separate, smaller feed and expansion cistern, usually beside the main one. It stays unless the system is converted to sealed with an expansion vessel. |
| A gravity shower pump is still fitted | Pumps of that type need stored water and a negative-head or flooded feed. They will not run off the mains. |
| Any cold outlet still draws from it | Often one WC, a bathroom basin or an outside tap left on the old distributing pipe. Until those are changed over to the mains, the tank is a live supply. |
| It is a shared cistern in a block | Common in converted London flats: one loft tank serving several flats. It is not yours to decommission, and isolating it can leave neighbours dry. |
| It is a break cistern or a boosted storage tank | Where the incoming supply cannot deliver the flow, or where storage provides backflow separation, the cistern is doing a regulatory job. It is not redundant. |
| It is a sprinkler or fire-main tank | Never isolated or drained by a plumber on a domestic visit. That is a specialist and usually a landlord matter. |
There is an honest case in the middle. If your cold taps still run off the tank because the conversion only changed the hot side, removing it is not a tank job — it is a re-plumb of the cold distribution to the mains, with the tank removal as the last ten minutes. Anyone quoting an hour to "take the old tank out" there has not looked.
Where we come into this
We are a leak repair firm, so most redundant tanks we meet have already done their damage — a stained bedroom ceiling, a soaked loft, a neighbour's ceiling in a flat, and a tank upstairs everyone had forgotten was still connected. Finding that source is quick, because an overflowing loft tank leaves an obvious trail once the hatch is open.
What we then do is the whole of it: isolate and cap properly, decommission the tank, and put right what the water damaged. The plasterboard, plaster and decoration are handled by our own handymen and builders rather than left to you — that is leak damage repair and reinstatement, and on the pipework side, pipework repair and replacement. If water is coming through a ceiling right now, start with the first ten minutes and get the stopcock off before you read any further.
Dealing with a still-dry tank before it becomes a claim is booked work at £150 per hour, agreed before we attend, with no call-out fee — see our pricing page. If it has already come through a ceiling, the visit includes a written report setting out what was found and where, prepared to support a trace and access claim. What your policy covers is the insurer's decision, not ours.
