ASHRAE Journal - March 2022 - 74

ENGINEER'S NOTEBOOK
COLUMN
Water Treatment 101
BY JEFF BOLDT, P.E., HBDP, FELLOW ASHRAE; JOSHUA INCE, P.ENG., MEMBER ASHRAE
Water treatment is a mystery to many consulting engineers, but is required for many
reasons. In HVAC systems the most common goals of water treatment are to maintain
peak performance, reduce water consumption and minimize corrosion, scale, and
fouling while controlling microbial growth. This column summarizes the most important
items for the HVAC designer to know and improve their master specifications for
water treatment.
Different methods of water treatment are required for
each HVAC system due to the many variables of design
such as different materials of construction, various
water temperatures, water versus glycol systems, steam
versus hot water systems, makeup water sources and, of
course, whether the system is open loop or closed loop.
Closed Water Systems
Closed systems are isolated from the atmosphere,
minimizing the introduction of oxygen. These systems
typically require small amounts of makeup water for
times when maintenance is performed on pumps,
strainers or valves. Treatment design depends on the
materials of construction, the makeup water quality and
the maximum water temperature.
For heating water systems, if they deliver >150°F
(65.7°C) water, biocidal treatment may not be needed.
But for the newer low-temperature heating water
systems that use condensing boilers and/or energy
recovery chillers, a biocide is required, and we (those
involved in the design of systems) generally avoid corrosion*
inhibitors that add nutrients to the water, i.e.,
molybdate is preferred over nitrite. We normally allow
*Corrosion: the deterioration of materials, usually metals, by reaction
with their environment.
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ASHRAE JOURNAL ashrae.o rg M A R CH 2022
both sodium nitrite (aka nitrite) and molybdate as corrosion
inhibitors in systems that occasionally reach
180°F (82.2°C), but only allow molybdate in systems
that never get above approximately 140°F (60°C). This
is because nitrite is a close cousin to the nitrogen that
farmers inject into their fields, and it can be a nutrient
for nitrogen-reducing microbes.
On the flip side, some municipalities may restrict or
prohibit discharge of molybdate. In these cases, alternate
forms of corrosion control may need to be used
such as orthophosphate, zinc phosphate, or sodium
silicates. These alternate forms are safer to discharge
but can be harder to control and may be less effective.
Nitrite is also preferred for heating water systems
because heat drives oxygen out of the system, and
nitrite can donate its own oxygen to form an inhibition
layer on metal surfaces.
For chilled water systems, treatment design will be like
hot water systems being dependent on materials of construction,
the makeup water quality, and design water
temperature. Due to the lower temperatures compared
to hot water systems, the water treatment program
Jeff Boldt, P.E., HBDP, is managing principal at IMEG Corp. He is SSPC 90.1
mechanical subcommittee chair. Joshua Ince, P.Eng., is a regional sales manager at
DuBois Chemicals. He is vice chair of TC 3.6, Water Treatment.
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