Dead legs and little-used outlets: a practical flushing guide

Stagnant water is where Legionella gets its foothold. Two of the most common culprits we find are dead legs and outlets that hardly anyone uses. Here's how to spot them, how to flush them properly and when it makes more sense to take them out altogether.

December's a good time to think about this. Offices empty out for Christmas, guest bathrooms sit unused until the in-laws arrive, and lots of buildings close for a week or two. All of that means water standing still in pipes.

What's a dead leg?

A dead leg is a length of pipe where water can't flow through, or rarely does. Common examples:

• a pipe that used to feed a sink, washing machine or shower that's since been removed, and was just capped off
• a branch to an outlet that's been disconnected but left in the wall
• oversized or redundant pipework left over from earlier alterations
• a long run to a single tap that's hardly ever used, which behaves much like a dead leg

The water inside sits at room temperature, which often falls somewhere in the 20°C to 45°C range Legionella likes. Over time biofilm builds up on the pipe walls. Then every so often that water mixes back into the system, carrying whatever's grown with it.

They're very common in older buildings and conversions, where kitchens and bathrooms have moved around over the years. Often nobody even knows they're there until someone traces the pipework.

What counts as a little-used outlet?

Any tap, shower or other outlet that isn't used regularly. The usual suspects:

• showers in spare rooms, guest bathrooms or empty HMO rooms
• accessible toilets and showers that are only occasionally used
• cleaners' sinks and outside taps
• staff showers that people stopped using
• taps and showers on a floor that's only half occupied
• emergency showers and eyewash stations

The weekly flushing rule

HSG274 Part 2, the HSE's guidance for hot and cold water systems, says little-used outlets should be flushed at least once a week. That's the one to remember. The aim is to draw fresh water through the pipe and the outlet so nothing sits still for long.

Flushing works, but only if it's actually done every week and written down. If you know you'll struggle to keep it up, read the section further down on removing outlets.

How to flush properly

1. Run both hot and cold. Turn each one on and let it run for a few minutes, until the water reaches its normal temperature. Hot should get to at least 50°C and cold should drop below 20°C.
2. Keep spray down. Open the tap gently rather than at full blast. For showers, run the water into the tray with the head held low, or remove the head first. Keep your face away.
3. Include toilets. Flush cisterns in unused toilets, with the lid down.
4. Don't forget mixer showers and TMVs. Run them on full hot and full cold if the controls allow, so both supply pipes get turned over.
5. Write it down. Date, outlet, who did it. A simple table on a clipboard in the plant room is fine. Without a record, it didn't happen as far as an inspector is concerned.

Making it workable

Flushing a handful of outlets is easy. Flushing forty across a big building every week gets forgotten. A few things help:

• Keep a list of exactly which outlets need flushing, with a floor plan if the building's large.
• Give the job to a named person, with a named deputy.
• Build it into an existing routine, such as the cleaners' weekly round.
• Ask people to use the quiet outlets. Sometimes just putting a little-used shower back into normal service solves the problem.

Your written scheme of control should spell all this out. If it doesn't, have a look at our guide on what's in a written scheme of control.

When to remove instead of flush

Flushing is a control. Removal is a fix. If an outlet genuinely isn't needed, the better option is usually to take it out, and to cut the pipework back to the main run so no dead leg is left behind. Simply capping it off at the wall just creates a brand new dead leg.

Dead legs themselves can't really be flushed because there's no outlet on the end. The answer there is removal, cutting the redundant section out as close to the live pipe as practical.

This is everyday work for our remedial works team. It's normally quick, and once it's done there's one less thing to remember every week.

A typical scenario

Imagine an office with a basement shower block that nobody's used since bike racks were moved outside. Upstairs, a kitchenette was taken out during a refit but its pipes were capped above the ceiling. The office manager flushes the showers when they remember, which isn't often.

A sensible plan would be to decide whether the showers are still wanted. If yes, put them on the weekly flushing list and record it. If no, remove them properly. Either way, the capped kitchenette pipes should be traced and cut back to the main run. That's a morning's work and it takes two risks off the register for good.

Christmas closures

If your building's shutting for the holidays, plan for it now. Ideally someone comes in to flush during the closure. If that isn't possible, flush every outlet before staff return and check temperatures once the hot water's back up to speed. If your system's been shut for longer, our guide on reopening a building after closure goes into more detail.

Don't fancy doing the flushing yourself? Our Legionella maintenance visits can take care of it and keep the log up to date, and we can quote to remove any dead legs we find along the way.