Cooling Tower Cleaning: Look to ASHRAE For Guidelines On Reducing Legionella

The sunny skies over much of the country recently remind us that summer is just around the corner.  Ahhh – sunny days, blue skies, gentle breezes and balmy temperatures – what’s not to love about that?

So in the spirit of spoiling the good mood we just created, we remind you that there is also a dark side to the light days of spring and summer –  Legionella Pneumophila.  The U.S. Centers for Disease Control (CDC) receives more than 1,000 reports of Legionnaire’s disease every year.   The CDC has been aware for many years that there is a connection between evaporative devices (specifically cooling towers) and the transmission of L. pneumophila.

The American Society of Heating, Refrigerating and Air Conditioning Engineers, Inc. (ASHRAE), created Guideline 12 in 2000, setting forth ways to manage the risks posed by cooling towers.  It remains a definitive document for anyone responsible for the management and operation of HVAC systems.  According to the prologue to Guideline 12, several factors can contribute to the rapid growth (amplification) of legionella in water: temperatures between 77-108 degrees, stagnation of water, the presence of scale or sediment, biofilms and the presence of amoebas.

ASHRAE’s Guideline 12 addresses how cooling towers and evaporative condensers can become incubators for the L. pneumophila bacterium, but also speaks to what you can do to help prevent that from happening.  The “key” recommendations are to keep your system clean and use a biocide – and carefully watch your water chemistry.

Legionella pneumophila requires that amoebas or a biofilm be present in the water for it to multiply – it doesn’t seem to want to grow as rapidly in just  plain water.  Since there isn’t much you can do about the water temperature in most systems — it will be what it will be — the object of maintenance and cleaning programs is to deprive the bacteria of the nutrients and environment it needs to thrive.

Another issue to consider is restarting a cooling tower after it has been offline for any length of time.  Since Legionella likes the stagnant, warm water of a system in shutdown, it is extremely important to clean your cooling tower before you start it back up.

According to ASHRAE’s Guideline 12,  you should drain your cooling tower water for any shutdown of more than three days.   Before you restart it, fill the tower back up and treat with a biocide such as Goodway’s Pancare and with chlorine to kill any bacteria that may be present.  Bacteria like to grow in stagnant water that you might find in drain pans, dead legs of piping runs and little-used storage containers.  Use of a biocide here will help control and eliminate bacteria.

(Disclaimer:  We at Just Venting are not professional biologists, so do not consider what we say here to be more than a basic overview.  For full details, consult with ASHRAE or with your own scientific experts.)

Rich Silverman
Goodway Blogging Team