Dead Legs and Legionella: Identification, Risk, and HSG274 Requirements

Dead legs are one of the most frequently identified risk factors in legionella risk assessments. They are also one of the most frequently misunderstood. Many assessors note their presence but fail to adequately assess the risk they pose or provide specific enough recommendations for their management.

This article explains what dead legs are, why they matter, how to identify them, and what HSG274 Part 2 requires.

What is a dead leg?

A dead leg is a section of pipework that is connected to the water system but through which water does not flow regularly. The water in a dead leg is effectively stagnant -- it sits at ambient temperature, providing ideal conditions for legionella growth.

Common examples include:

• Capped-off pipe branches where an outlet has been removed but the pipework was not cut back to the main distribution pipe
• Redundant pipework from decommissioned equipment (old water heaters, removed sinks, disconnected machinery)
• Long branch connections to infrequently used outlets
• Bypassed pipework around water treatment equipment or meters
• Temporary connections that were never removed

Why dead legs matter

Dead legs create three conditions that favour legionella growth:

Stagnation. Water in a dead leg does not flow. Without flow, there is no replacement of water, no maintenance of temperature, and no flushing of nutrients and bacteria. Stagnant water is the primary risk.

Temperature. Water in a dead leg will eventually reach ambient temperature -- which in most buildings falls within the 20-45 degrees C danger zone. Even in a well-controlled system, a dead leg undermines temperature management because the stagnant water is neither heated nor cooled.

Biofilm. Stagnant water allows biofilm to develop on the internal pipe surfaces. Biofilm provides nutrients and a protected environment for legionella bacteria, making them harder to kill through chemical treatment or thermal disinfection.

A dead leg does not need to be long to be a risk. HSG274 Part 2 historically defined a dead leg as any branch connection longer than 1.5 times the pipe diameter, though the 2024 edition takes a more risk-based approach.

How to identify dead legs

During a site assessment, dead legs can be identified through:

Visual inspection. Look for capped-off pipe branches, blanked-off tee connections, and pipework that leads to nowhere. Check behind panels, above ceiling tiles, and in service ducts where modifications are commonly made without full decommissioning.

System schematic review. Compare the system schematic (if one exists) with the physical installation. Any discrepancy may indicate modifications where pipework was not fully removed.

Conversation with site staff. Ask whether any outlets, equipment, or areas of the building have been decommissioned or taken out of use. Site staff often know about historical changes that are not documented.

Temperature checks. An outlet that takes an unusually long time to reach temperature may be served by a long branch connection that is effectively acting as a dead leg.

What HSG274 requires

HSG274 Part 2 is clear that dead legs should be avoided in water system design and removed where they are identified in existing systems. Specifically:

• New installations should be designed to eliminate dead legs. Pipework should be configured so that water flows through all sections of the system during normal use.
• Existing dead legs should be removed by cutting back redundant pipework to the main distribution pipe and capping at the tee connection.
• Where removal is not immediately practical, a flushing regime should be implemented to ensure water in the dead leg is replaced regularly. However, this is a temporary measure -- removal should be planned.

The 2024 edition takes a more nuanced, risk-based approach to dead legs. Rather than applying a rigid length threshold, it asks assessors to consider:

• The length of the dead leg relative to the pipe diameter
• The temperature conditions in the surrounding area
• The frequency of any flow through the dead leg
• Whether the dead leg is on a hot or cold water supply
• The presence of biofilm or sediment

Recommendations for assessors

When you identify a dead leg during an assessment:

1. Record its location, length, and pipe diameter. Photograph it if accessible.
2. Assess the risk. Consider the factors above. A short dead leg in a cold area with low ambient temperatures may be low risk. A long dead leg on a hot water branch in a warm plant room is high risk.
3. Recommend removal as the primary action. Specify that the pipework should be cut back to the main distribution pipe.
4. If removal cannot be immediate, recommend a flushing regime as an interim measure. Specify the frequency and volume of flushing required.
5. Rate the risk. A dead leg on a hot water system serving a shower used by vulnerable persons would typically be rated C (immediate action required). A short dead leg on a cold water supply in a ventilated area might be rated B (improvements recommended).

Dead legs vs. infrequently used outlets

Dead legs are sometimes confused with infrequently used outlets, but they are different risk conditions:

• A dead leg is a section of pipework through which water cannot or does not flow, even when the system is in use
• An infrequently used outlet is a functioning outlet that is simply not used often enough to maintain water quality

Both require management, but the solutions differ. Infrequently used outlets need a flushing regime. Dead legs need to be removed.

In L8Pro, dead legs are assessed under Condition 1.3, with the assessor recording the location, length, and recommended action. The generated report includes a specific recommendation for each dead leg identified, referenced to HSG274 Part 2.