The constraint that decides everything else
In a house, the first instruction is turn the water off. In a block of sixty flats nobody can give that instruction. You cannot take hot water away from sixty households while somebody walks a floor with a probe, and the residents who would complain loudest are the ones already sitting under the stain.
That changes the job. Most published leak detection advice assumes the system can be isolated, drained and charged with gas. On a communal riser or a live secondary return, it cannot. What is left is the smaller set of methods that work on a pressurised system in use, and a much heavier reliance on the data the plant already produces.
It also changes when we attend. A leak in an occupied block is easier to find at two in the morning, because the draw-off stops. Every tap in the building is noise; when it goes quiet, what is left on the acoustic kit is the leak and the pumps.
Start with the numbers the plant room already keeps
Three readings usually narrow the problem more than an hour of walking will.
- The pressurisation unit. On a sealed primary circuit the make-up counter is the most useful number in a plant room. Firing once a month is normal. Filling nightly is a measured, dated loss, and it puts the leak on the primary side rather than the domestic side.
- The bulk meter, read overnight. Midnight and again at five, with consumption at its lowest. A block never reads zero, but a steady overnight flow that does not change when you isolate branches is a leak, not residents.
- The cold feed and F&E tank. On an open-vented system, a ball valve that never stops trickling is feeding a loss downstream. Tie the float up for an hour. If the level drops with nothing drawing, water is leaving the system.
Site staff can take all three before we arrive, which shortens a chargeable visit.
What actually fails in a hot water plant room
The calorifier or storage vessel
Stored vessels fail in two directions. Externally, at the immersion boss, the inspection flange gasket, the drain cock or the connections — usually slowly, and usually hidden under insulation that soaks it up and holds it against the metal. Take the jacket off before you call a vessel dry.
Internally, a failed heating coil misleads people. Domestic hot water sits at a higher pressure than the low temperature hot water heating it, so when the coil splits, DHW crosses into the primary circuit rather than the other way round. What you see is a heating circuit whose pressure keeps climbing and a relief valve discharging. Nobody reports a hot water leak. Somebody reports the boiler behaving oddly.
The plate heat exchanger
A gasketed plate pack weeps at the edge of the pack, visibly — a line of scale or a drip track down the frame between two plates. On most units the pack has a manufacturer's compressed dimension it can be re-torqued back to. Tightening past that figure crushes gaskets and turns one weep into several. A brazed plate unit has no gaskets and fails internally instead, with the same signature as a split coil: cross-flow between circuits, pressure moving where it should not, and no water on the floor at all.
The secondary return
This is the run that leaks quietly for longest. Secondary return pipework is small bore, hot twenty-four hours a day, and routed through ceiling voids and risers where nobody looks. It is also the circuit most likely to have been altered during a refurbishment, which means dissimilar metals, an improvised reducer, and a joint nobody can now find a drawing for. A weep on a 22mm return in a ceiling void can run for a year before it reaches a light fitting.
Two things that are not leaks
An older gland-packed circulator is supposed to weep slightly; a bone-dry one is usually over-tightened. A mechanical-seal pump is not, and water at its head is a failed seal. Water from a tundish or relief discharge is not automatically a leak either: a potable expansion vessel that has lost its air charge leaves nowhere for the expansion to go, so it lifts the valve every cycle. Eliminate both before anyone opens a duct.
Which methods survive a live system
| Method | On a live communal system |
|---|
| Tracer gas | Usually unavailable — the circuit has to be drained, and a communal riser cannot be. Viable on a branch you can isolate out of hours, and on the primary side when the heating is off. |
| Acoustic and correlation | The workhorse here. It needs pressure, which a live system has in abundance. Its enemy is noise, which a plant room also has — so the sweep happens overnight, or with the pumps briefly stopped. |
| Thermal imaging | Good on secondary returns and hot risers behind plasterboard, because the pipework is permanently hot. The trap is that same heat: a whole duct reads warm, so a patch is only evidence when it is warmer than the run beside it. |
| Moisture mapping | Rarely finds the leak. Frequently decides which four flats we need access to, which on a block is most of the work. |
| Sectional isolation | Suits a live building best. Close one branch at a time for ten minutes and watch the meter or the make-up counter. Nobody loses hot water for a day; one stack loses it for the length of a coffee. |
The order is: numbers, then isolation to reach a stack, then acoustic and thermal to reach a length of pipe, then one opening. The value is not in the kit — it is in narrowing sixty flats to one duct before anything is cut. Our tracer gas page is blunt about the drain-down requirement for the same reason.
Why it is almost never the flat that reported it
Water leaving a hot pipe in a riser does not fall onto the nearest ceiling. It runs along the duct, along a joist, down the back of a party wall, and appears at the first place the ceiling is penetrated: a downlighter, a light rose, an extract grille, a badly sealed hatch. Those can be two or three floors below the fault and one flat sideways.
So the flat that calls you is telling you where the ceiling has a hole in it, not where the leak is. That is worth saying to residents early, because the alternative is a leaseholder convinced the flat above is at fault, a flat above that has been dry for months, and a dispute that costs the block more than the repair. The same reasoning we set out for residents on a leak from the flat above applies at building scale.
Access follows from that. Once the trace points at a riser we need entry to cupboards in several flats, at a time when people are in, and twenty-four hours of notice from the agent is the difference between one visit and two.
Who pays, and why that gets settled before we attend
We are not lawyers and we do not read leases. But the question is always structured the same way, and answering it on day one saves more than anything technical we do.
- Communal or within a demise? Leases usually retain the structure and communal services with the freeholder and demise what sits inside the flat. Plant equipment, risers and secondary returns normally fall on the retained side; a branch inside a flat usually does not.
- Who is instructing us? Whoever instructs gets the invoice and the report. If a leaseholder calls us and the fault turns out to be communal plant, that is a conversation they then have with the agent — so on a block it is cleaner if the agent instructs from the start.
- Which policy is in play? Communal damage generally goes to the block buildings policy. Trace and access cover, where the policy includes it, pays for finding the leak and making good the damage caused by getting to it. The insurer decides what their policy covers; nobody at this end can promise you a payout.
The failure mode we see most in London blocks is not a hard leak. It is six weeks of three parties establishing whose problem it is while the water keeps running and the claim gets harder to evidence. Settle the instruction, then argue apportionment. Our insurance claims pages set out what a report must contain to survive a loss adjuster.
What a visit costs and what you get
£150 per hour, agreed before we attend, with no call-out fee. If we do not find the leak, you do not pay for the time we spent looking. Out-of-hours attendance is quoted the same way, before anyone gets in a van.
The visit produces a written report prepared to support a trace and access claim: what was tested, what was eliminated, how the leak was located and what had to be opened to reach it. That is the document the loss adjuster and the freeholder both want.
We also finish the job. The pipe is repaired, and our own handymen and builders put back the ceiling, plaster, tiling and decoration, so the block is not left with an open duct and a second contractor to find. See commercial leak repair for how we work on managed buildings.
One honest limit: if a thirty-year-old calorifier is at the end of its life, or a return has been patched four times, we will say so rather than sell a fifth patch. Replacement plant is a different job.
