How do I find dead legs in my water system?

You find dead legs by surveying the system, not by looking for something obviously wrong, because a dead leg usually looks like perfectly ordinary pipework. The reliable method is a walk of the whole system, tracing each run from the incoming main or storage vessel to every outlet, combined with a temperature survey that reveals the branches water is not actually moving through. What follows is that survey as a procedure, in the order that works. For the background on what counts as a dead leg and why it matters, see what are dead legs; this page is the field guide.

Before you start: gather the paperwork and the kit

Two things make the survey faster and more complete. First, any drawings of the water system: as-fitted plans, plumber's sketches, or the schematic from the last risk assessment. Pipe runs on paper tell you where to look, and where a drawing shows a branch you cannot find, that is a lead, not a dead end. Second, the building's history. Ask what has been altered: removed water heaters, taken-out tanks, refitted bathrooms, a dishwasher or drinks machine that used to be somewhere. Dead legs are made by building work, so the alteration history is a map of where they hide.

For kit you need a thermometer with a surface probe for pipework and an immersion probe for outlets, a torch, and a way to record findings, a notebook, a marked-up floor plan, or the risk assessment builder on a phone. A step ladder for ceiling tiles, if you have them, covers most of the rest.

Step 1: walk the system source to outlet

Start at the incoming main or the cold water storage tank, and at the calorifier or water heater, and trace each run physically as far as you can. At every tee and branch, ask where it goes and what it serves. You are looking for three signatures:

• Capped and blanked ends. Any pipe that stops at a cap, blank flange, or closed valve and serves nothing. Check above ceilings, behind bath panels and boxing, in plant rooms, and at the old positions of removed cylinders, tanks, and appliances.
• Branches to little-used outlets. Anything used less than about weekly: guest or accessible toilets, cleaners' sinks, outside taps, showers in disused rooms. These are live dead legs.
• Orphaned pipework. Runs that fed equipment now gone: a removed dishwasher, water cooler, or process line. The equipment left; the branch stayed; the water in it stopped moving that day.

Step 2: run the temperature survey

Temperature is what turns suspicion into identification, because a dead leg is thermally disconnected from the flow. Work outlet by outlet. Run each hot tap and confirm it reaches at least 50°C within a minute; run each cold tap and confirm it drops below 20°C within two. An outlet that fails either test is at the end of a branch that is too long, too still, or both. The temperature checker logs each reading against the correct limit so the survey doubles as a monitoring record.

Then go along the pipework itself with the surface probe. On a hot run, live pipe sits hot; a branch that has drifted down towards room temperature is not being flushed by flow. On a cold run the pattern reverses: live pipe stays cool, and a section that has warmed into the 20°C to 45°C band is sitting stagnant. A length of pipe in the growth range while everything connected to it is in range is a dead leg, whether or not you have yet found where it ends. Our temperature chart gives the target figures for each part of the system.

Step 3: account for everything you cannot see

You will be left with branches the drawings show but you cannot trace, and temperature anomalies with no visible cause. Do not write these off. Check the accessible voids first: lift ceiling tiles along the suspected run, open inspection panels, look above sinks and behind boxing. Where the building's alteration history says something was removed on the other side of a wall, treat the unaccounted branch as a dead leg until proven otherwise, and either open up or have a plumber trace and cap it back at the live run. A suspected dead leg you record and resolve is a finding; one you record and ignore is a liability.

Step 4: record before you fix

Log every find the moment you make it: location, approximate length, what the branch serves or used to serve, and its temperature. Photograph the capped stubs. This record is not admin for its own sake. It is what lets a plumber price the removal without a second visit, what populates the dead-leg section of the risk assessment, and what demonstrates, later, that you surveyed the system properly rather than glanced at it.

Step 5: remove or manage

Every dead leg ends one of two ways. Remove it: cut the redundant pipe back to the tee on the live run, so no stub remains for water to sit in. This is the right answer for every blind end and every outlet that is not genuinely needed, and note the trap: capping the pipe near its end leaves the whole stagnant length in place. Manage it: where the outlet must stay but is rarely used, put the branch on a documented weekly flush to temperature, as set out in flushing little-used outlets, and name it in the written scheme of control. Whichever route, the action goes in the log book with the date, and the dead leg goes into the risk assessment with its control. The survey records; it does not certify, and a system with its dead legs removed today grows new ones with the next alteration, which is why the finding belongs in the review trigger list too.