Routine Flushing Protocols for Legionella Control

Overview

Routine flushing eliminates stagnant water conditions that promote Legionella proliferation in domestic hot water systems. Stagnation allows water temperatures to drift into the growth range (77-108°F) and depletes residual disinfectants. Systematic flushing protocols form a critical operational component of ASHRAE 188 water management programs.

Physical Basis for Flushing

Legionella bacteria replicate in biofilm and stagnant water where:

1. Temperature conditions favor growth: Water temperatures between 77°F and 108°F provide optimal proliferation rates
2. Nutrient accumulation occurs: Stagnant water accumulates organic matter and corrosion products
3. Disinfectant residuals decay: Chlorine or other biocides degrade over time without replenishment
4. Biofilm development accelerates: Stationary water facilitates bacterial attachment and biofilm formation

Flushing displaces stagnant water with fresh supply water, resets temperature profiles, and introduces residual disinfectant.

Flush Volume Calculations

The volume required to fully exchange stagnant water depends on pipe geometry and fixture characteristics.

Pipe Volume Formula

$$V_{pipe} = \frac{\pi D^2 L}{4}$$

Where:

• $V_{pipe}$ = pipe volume (in³)
• $D$ = internal pipe diameter (in)
• $L$ = pipe length from circulation loop to fixture (in)

Flush Time Calculation

$$t_{flush} = \frac{V_{pipe}}{Q_{fixture}}$$

Where:

• $t_{flush}$ = required flush duration (min)
• $V_{pipe}$ = pipe volume (gal)
• $Q_{fixture}$ = fixture flow rate (gpm)

Temperature Stabilization Criterion

Flushing continues until outlet temperature stabilizes:

$$\Delta T = T_{outlet} - T_{supply} \leq 2°F$$

Where:

• $T_{outlet}$ = measured fixture outlet temperature (°F)
• $T_{supply}$ = supply temperature at circulation loop (°F)
• $\Delta T$ = acceptable temperature deviation (°F)

Flushing Frequency by Building Type

Low-use fixtures: Used less than twice per week High-risk areas: Immunocompromised patients, ICU, transplant units

Flushing Procedure Workflow

flowchart TD
    A[Start Flushing Protocol] --> B{Identify Fixture Type}
    B -->|Low-Use Fixture| C[Hot Water First]
    B -->|Standard Fixture| D[Check Last Flush Date]

    C --> E[Open Hot Water Fully]
    E --> F[Measure Initial Temperature]
    F --> G[Flush Until T > 120°F]
    G --> H{Temperature Stable?}
    H -->|No| G
    H -->|Yes| I[Maintain 120°F for 5 min]

    I --> J[Switch to Cold Water]
    J --> K[Flush Until T < 68°F]
    K --> L{Temperature Stable?}
    L -->|No| K
    L -->|Yes| M[Document Results]

    D -->|Due for Flush| C
    D -->|Not Due| N[Skip - Update Log]

    M --> O{More Fixtures?}
    N --> O
    O -->|Yes| B
    O -->|No| P[Submit Weekly Report]
    P --> Q[End Protocol]

Standard Flushing Procedures

Weekly Low-Use Fixture Flushing

1. Identification Phase

   • Identify all fixtures used less than twice per week
   • Include guest rooms, emergency showers, utility sinks, break room fixtures

2. Hot Water Flush Sequence

   • Open hot water valve fully
   • Measure initial outlet temperature
   • Flush until outlet temperature exceeds 120°F
   • Maintain 120°F for minimum 5 minutes
   • Verify temperature stability (< 2°F variation over 1 minute)

3. Cold Water Flush Sequence

   • Switch to cold water only
   • Flush until outlet temperature below 68°F
   • Maintain temperature for minimum 2 minutes
   • Verify temperature stability

4. Documentation Requirements

   • Fixture identifier and location
   • Start and end temperatures (hot and cold)
   • Flush duration for each phase
   • Date, time, and operator name

Monthly System Flushing

Extended flushing for fixtures in regular use but with potential dead-leg stagnation:

1. Flush all fixtures for minimum 10 minutes (hot water)
2. Target outlets: 124°F minimum at fixture
3. Verify recirculation system maintains 122°F return temperature
4. Document temperature profile across building zones

Thermal Shock Procedure

Applied to fixtures with confirmed or suspected Legionella presence:

1. Preparation

   • Increase water heater setpoint to 140-150°F
   • Verify scald protection devices are temporarily bypassed or removed
   • Post signage warning occupants

2. Execution

   • Flush each fixture with 140°F water for 30 minutes minimum
   • Maintain outlet temperature above 131°F (10-minute Legionella kill time)
   • Monitor for consistent temperature throughout flush

3. Cool-Down and Restoration

   • Reduce water heater setpoint to operational temperature
   • Reinstall scald protection devices
   • Resume normal operations after system stabilizes

Critical Considerations for Seldom-Used Fixtures

Priority fixtures requiring weekly attention:

• Emergency eyewash and safety showers
• Vacant guest rooms or patient rooms
• Seasonal-use facilities (closed wings, summer camps)
• Redundant or backup fixtures
• Dead-end branch lines with minimal flow

Extended absence protocols:

• Buildings closed more than 1 week: full system flush before reopening
• Individual fixtures unused more than 2 weeks: thermal shock treatment recommended

Documentation and Record Keeping

ASHRAE 188 requires documented evidence of flushing activities:

Minimum documentation elements:

• Fixture identification system (room number, tag, or unique identifier)
• Scheduled frequency vs. actual completion date
• Temperature measurements (initial and final, hot and cold)
• Flush duration
• Operator performing procedure
• Anomalies or deviations from protocol

Record retention:

• Maintain flushing logs for minimum 3 years
• Electronic logging systems preferred for trend analysis
• Corrective actions documented for fixtures failing temperature criteria

Integration with Water Management Programs

Routine flushing operates as one control measure within comprehensive ASHRAE 188 water management programs:

1. Hazard analysis identifies stagnation-prone fixtures and system segments
2. Control limits establish acceptable temperature ranges and maximum stagnation time
3. Monitoring procedures verify flushing effectiveness through temperature measurement
4. Corrective actions address fixtures consistently failing to achieve target temperatures
5. Program validation confirms Legionella control through periodic sampling

Flushing protocols must be integrated with temperature maintenance, disinfectant residual monitoring, and system design modifications to address persistent stagnation issues.

Effectiveness Verification

Temperature alone does not confirm Legionella elimination. Verification requires:

• Periodic microbiological sampling (quarterly or as specified by water management team)
• Trend analysis of flush temperatures across building zones
• Investigation of fixtures requiring extended flush times
• Correlation of flushing compliance with Legionella detection rates

Facilities with consistent flushing protocol compliance demonstrate significantly reduced Legionella colonization compared to buildings with sporadic or absent flushing programs.