By Chad Edmondson
We interrupt this regularly scheduled series on hydronic balancing to announce that ASHRAE has officially published Standard 188-2015, Legionellosis: Risk Management for Building Water Systems.
It’s a timely bit of information given our current discussion about balancing, even though it is directed at domestic water rather than hydronic heating and cooling.
Among other things related to the prevention of Legionella, Standard 188 states:
“All water systems shall be balanced and a balance report for all water systems shall be provided to the building owner or designee.”
The keyword here is “all” water systems.
What Does Balancing Have To Do With Legionella?
Why has ASHRAE decided to address balancing in a standard that is written for the purpose of Legionella prevention? The reason has to do with domestic hot water recirculation systems – particularly large systems with multiple returns coming back to the boiler.
If these return lines are not balanced it is possible that a period of “no flow” might occur in one or more of the return lines. Often referred to as “dead legs”, these stagnant areas in the pipe increase the risk for Legionella growth because scale and biofilm tend to collect there, creating a safe haven for Legionella to grow. Remember -- Legionella can grow and multiply in water temperatures beyond its typical survival range if it happens to be residing in a cozy bit of scale. That’s why it is important to keep the water moving – even during periods of no demand.
Dead legs can be avoided by installing an automatic balancing valve on each return line to ensure that some amount of flow is always maintained through each line—under all demand conditions.
Time to Get Serious about Domestic Water Balancing
Now that Standard 188 has been passed, it is likely to become an ANSI standard, which will no doubt be accepted into local codes. It’s just a matter of time.
So if you are designing or installing any kind of domestic recirculation line now or in the near future don’t forget to balance. As we have discussed in the past, Standard 188 shifts the responsibility of Legionella prevention to building owners and operators. As such, it will leave them more vulnerable to lawsuits resulting from a Legionella related incident.