|
HVAC Basics of Microbial Growth
Most people involved in the area of indoor air quality (IAQ) agree that it
is a complex, multifaceted issue. No "silver bullets" exist to
remedy every problem there are simply too many potential contaminant sources
and their diversity is too great. Not only do contaminants originating inside
a building threaten the quality of its environment, but those brought in from
outside through the ventilation system and infiltration must be considered as
well.
The most common indoor contaminants are the volatile organic compounds (VOC's) outgassed from interior
furnishings. Microbiological contaminants (namely bacteria, mold, mildew and
fungi) run a close second. Ventilation and filtration certainly provide a
means of combating these contaminants by diluting their concentrations within
a building. However, the basic long-term IAQ strategy for any building
should ultimately be contaminant source control.  We understand the sources and causes of
microbiological contamination within a building. Put simply, microbes need
four basic ingredients to thrive: (1) organic nutrients or "food";
(2) moisture in the form of standing water or humid air; (3) a surface on
which to grow; and (4) darkness. Therefore, if we keep the air conveyance
system clean and dry, we can significantly reduce the potential for
microbial contamination within a building. How we accomplish this objective
is determined by the manner in which air is delivered to a building's
occupied spaces.
HVAC systems, the principal means of air delivery in a building, can
be divided into two general categories: central systems and terminal
systems. A central HVAC system includes a central air-handling
unit (installed either inside or outside the building) that distributes
ventilation air and recirculated air through ducts to control terminals located
in the individual zones. By contrast, a terminal HVAC system
relies on a dedicated fan-coil unit, unit ventilator or heat pump in
each zone to handle the ventilation, filtration and comfort conditioning
duties of that space. Figure 1 depicts these two
system types.
ASHRAE Standard 62 1989, Ventilation for Acceptable Indoor Air Quality,
defines the present "standard of care" for the design of
ventilation systems used in commercial buildings. Standard 62 not only
provides guidance for determining the required amount of ventilation air, but
also cites three specific equipment design characteristics which can reduce
the HVAC system's potential for becoming a source of microbial contamination.
These basic characteristics (sloped drain pans, cleanable surfaces and
accessibility) apply to all types of equipment and can be considered
"IAQ equipment basics." Before we look at each characteristic in
detail, let's briefly review:
Why Microbes Love HVAC Systems
In the normal process of dehumidifying a building,
outdoor ventilation air and recirculated air
pass through a cooling coil that lowers the mixed air temperature below
dew point. This causes some of the water vapor held in the air to condense
on the cold coil. The condensate collects in a drain pan beneath the
coil as shown in Figure 2 and eventually flows
to the building drain. In the process, it washes much of the particulate
matter (dirt) from the passing air stream into drain pan. This wet,
dark, temperature-controlled environment creates an excellent habitat
for mold and fungal growth. The HVAC system fan circulates the bioaerosols (airborne microbial contaminants) produced by
these colonies throughout the building.
"IAQ Basic" #1:
Sloped Drain Pans
|
|
|
Figure 2: Double-Sloped, Trapped
Condensate Drain Pan
|
Until recently, air handlers and terminal units were equipped with flat
condensate drain pans. This design allows water to collect in stagnant pools,
fostering the growth of mold and slime and increasing the likelihood of
air-stream contamination. It's also promotes another less evident problem:
slime can clog the drain line and force condensate water to overflow into the
bottom of the air handler. That means more wet interior surfaces and more
potential for mold growth, not to mention possible equipment and building
damage.
ASHRAE Standard 62 1989 states that "condensate drain pans shall
be designed for self-drainage to preclude the buildup of microbial
slime." To comply with this "IAQ equipment basic,"
condensate drain pans on all new equipment with cooling coils should be sloped,
preferably in two directions, to assure positive drainage. The pans
should also be constructed of high-quality stainless steel or polymer to
resist corrosion and discourage premature leakage. Proper trapping of all
drain connections is equally important to reduce the risk of drawing sewer
gases into the air handler and distributing them throughout the building.
Carefully follow the equipment manufacturer's trapping instructions to assure
correct drainage.
"IAQ Basic" #2:
Cleanable Surfaces
Even with proper equipment design, operation and maintenance,
unexpected events like equipment malfunctions, power outages or drainage
problems can occur, raising the humidity level inside the building. Relative humidityhumidities over 60% (along with dirt, which is
almost always present in building air systems) promote microbiological
growth. Therefore, it's important to regularly inspect the entire air
handler and duct system for signs of contamination as a part of normal
HVAC system maintenance.
Equally important is the choice of materials used to construct HVAC
equipment and air delivery systems. In the past, lining the interior surfaces
of equipment and ductwork was common practice. However, this porous liner
holds dirt and moisture and is almost impossible to clean, short of removing
it altogether. Today, the accepted practice is to use double-wall ductwork
or to insulate the exterior surfaces of the ductwork.
|
|
|
Figure 3: Cleanable,
Double-Wall Construction
|
ASHRAE 62R, a revision of ASHRAE Standard 62 1989 now
available for public review, reinforces the is practice. It states that "exposed
internal insulation shall not be used from the exit of the coil to the
downstream end of the drain pan unless the insulation is impermeable to
water." This clearly precludes the use of matte-faced fiberglass
insulation in much of the air handling system.
Providing smooth, nonporous surfaces inside ducts and equipment
simplifies inspection and cleaning. The type of equipment involved will
determine how this can be accomplished. For example, terminal equipment
such as fan coils and VAV terminals can be lined with either closed-cell
foam or foil-faced insulation. Both materials are washable. Central
air handlers, however, demand a more rigid liner like sheet metal (Figure
3) because of their physical size and maintenance requirements.
The need for smooth, nonporous surfaces that permit thorough cleaning also
applies to drain pans. Only noncorrosive materials
should be used to avoid the pitting that corrosion creates. Likewise, the
sprayed foam commonly used on drain pans and other interior surfaces in the
past is no longer acceptable because it is so difficult to clean.
"IAQ Basic" #3:
Accessibility
|
|
|
Figure 4: Fully Accessible Indoor Air Handler
|
Obviously, a cleanable surface is of limited value if it
cannot be easily accessed. All areas of the HVAC equipment and duct system
should be totally accessible for inspection, cleaning and maintenance.
It's an established fact that equipment that's easy to service gets serviced
first and most often.
In central air handling equipment, the necessary access can be provided
with hinged doors and/or removable panels. Areas accessed
frequently, such as filter and cooling-coil sections, should be equipped with
access doors. At minimum, access to the ductwork interior should be provided
at areas where dirt is likely to accumulate -- that is, at turns and other
turbulent areas, as well immediately before and after any duct-mounted
devices such as humidifiers and fire dampers.
Equipment designs for terminal products should allow easy inspection and
maintenance (e.g., filter replacement) with minimal disassembly of the
unit. Easy access is especially important for fan coils and unit ventilators.
These types of systems typically consist of many units, making inspection a
time-consuming process. Although fan-powered VAV terminals do not have
cooling coils or drain pans, they must still be designed for easy filter
replacement and accessibility for inspection and other normal maintenance
within the tight confines of a ceiling plenum.
Remember the "IAQ Equipment Basics"
The key to reducing the potential for microbiological
contamination within an HVAC system is to keep it clean and dry. All
HVAC equipment must include the "IAQ equipment basics" (sloped
drain pans, cleanable surfaces and accessibility) if you
are to satisfy the equipment mandates defined in the existing ASHRAE Standard
62 1989, as well as the expanded requirements of the revised standard.
One final reminder: Consider all aspects of accessibility when
selecting and laying out equipment for a project, whether new construction or
renovation. Remember that access for inspection and possible cleaning is
required in addition to the standard service clearance requirements.
Contact info:
For additional information about Trane products
and systems capabilities, contact Gary Luepke,
Systems Marketing, at (608) 787-3366, Fax (608) 787-3005.
|