Legionella bacteria are naturally occurring organisms that become a public health concern when they colonize and multiply in human-made water systems. These bacteria thrive in warm, stagnant water, often found in plumbing sections that are rarely used or have low flow. Flushing is the primary mechanical measure used to mitigate this risk by replacing “aged” water with fresh, disinfected water from the main supply. This proactive maintenance step removes the conditions that promote bacterial growth within a building’s water supply lines.
Understanding the Need for Water System Flushing
The necessity of flushing is directly tied to the conditions that allow Legionella to multiply to harmful levels. The bacteria flourish in tepid water, specifically between 20° Celsius (68° Fahrenheit) and 45° Celsius (113° Fahrenheit). When water sits unused, it enters this thermal danger zone, particularly in cooled hot water systems or cold lines exposed to warm ambient temperatures.
The lack of water flow promotes the development of biofilm, a slimy layer on pipe surfaces. Biofilm is a protective matrix for Legionella, shielding the bacteria from disinfectants like chlorine in the municipal water supply. Flushing physically removes stagnant water and associated nutrients, introducing a fresh supply that is typically colder, hotter, and contains a higher level of disinfectant. Regular water movement is a fundamental strategy for disrupting this bacterial colonization process.
The Complete Safe Flushing Procedure
A thorough flushing procedure must prioritize safety to prevent the inhalation of water droplets, which is how Legionella causes illness. Before starting, anyone at increased risk of infection should avoid the immediate area. The person flushing should wear appropriate personal protective equipment (PPE), such as a properly fitted N95 or N100 respirator and gloves, especially if a known contamination risk exists.
The goal is to minimize the creation of aerosols, which are fine water mists that can be inhaled. For showerheads, which are high-risk sources, remove the head and submerge the hose end in a bucket of water before turning on the flow. For faucets, running the water gently and placing a cloth or sponge over the stream helps minimize splash-back and misting.
The flushing sequence should begin with fixtures farthest from the main water entry point and move progressively closer, ensuring the entire pipe length is cleared. Both the cold and hot water lines must be flushed independently at each outlet. After flushing, clean and descale any removed showerheads or faucet aerators, as these components can harbor biofilm and scale.
Calculating the Necessary Flushing Duration
The core principle for flushing duration is running the water until the stagnant volume has been completely exchanged with fresh supply water, not a fixed time. The most reliable metric for determining this exchange is temperature stabilization. For cold water, the tap should run until the temperature reaches its minimum stable temperature, ideally 20° Celsius (68° Fahrenheit) or below.
For hot water, the tap should run until the water reaches its maximum maintained temperature, which should be at least 50° Celsius (122° Fahrenheit) at the outlet. This temperature threshold is necessary because sustained heat above 50° Celsius begins to kill Legionella bacteria. The time required varies significantly based on the length and diameter of the pipe run from the water heater or main line.
In a typical residential setting, running a faucet for two to five minutes is often sufficient to exchange the water volume in the immediate branch line. Larger fixtures, such as showers or bath taps, may require longer durations to achieve temperature stability. For buildings vacant for an extended period, the water should be run for a minimum of three to five minutes at every outlet. Infrequently used outlets, such as guest room fixtures, should be flushed routinely at least weekly.