INTRODUCTION TO Heating, Ventilation, and Air Conditioning (HVAC) System For Architects and

Ventilation Systems
There are significant spatial and seasonal variations in the volume of air
delivered by most HVAC systems. HVAC Operators must understand the
variations to know how to provide occupants with adequate outdoor air in all
spaces throughout the year. The ventilation features most important to IAQ
are the way in which supply air volume is controlled, and the way in which
outdoor air delivery is controlled.
In most HVAC systems a portion of ventilation air supplied to occupied spaces
is outdoor air and a portion is recirculated air. The total volume of air is
important for two reasons:
_ Air movement contributes to thermal comfort. The lack of air movement
can create a sensation of hot/stuffy air.
_ In many VAV systems (see below), outdoor air is a constant fraction of
the total suppl air. Thus, the total volume of outdoor air depends on
both the outdoor air fraction, and the supply air volume.
There are two major types of HVAC systems based upon the use of airflow to
control temperature--the Constant Volume (CV) system, and the Variable Air
Volume (VAV) system.
Constant Volume (CV) Systems
In a Constant Volume (CV) ventilation system, variations in the
thermal requirements of a space are satisfied by varying the
temperature of a constant volume of air delivered to the space. A
constant fraction of outdoor air will mean that a constant volume
of outdoor air will be delivered to occupied spaces. This volume
can be set to satisfy applicable ventilation standards. CV
systems are less energy efficient than VAV systems, but controls
for outdoor air delivery are simpler to manage.
Variable Air Volume (VAV) Systems
In a Variable Air Volume (VAV) ventilation system, variations in
the thermal requirements of a space are satisfied by varying the
volume of air that is delivered to the space at a constant
temperature. VAV systems reduce HVAC energy cost by 10-
20% over CV systems but complicate the delivery of outdoor air.
If the fraction of outdoor air is constant, the total volume of
outdoor air will be reduced as the supply air volume is reduced.
An inadequate outdoor air fraction, combined with an inadequate
VAV box minimum setting, may result in inadequate outdoor air
flow to occupant spaces. This would occur during part-load
conditions. VAV systems also complicate pressure relationships
in the building and make testing, adjusting, and balancing more
difficult.
Most of the year, the volume of outside air may be reduced to
about a third of the outdoor air volume at design load. This could
result in indoor air quality problems. Separate controls to insure
adequate outside air year round do not increase energy costs.
Some new VAV systems incorporate these controls.
Economizer
Economizers are controls of the outdoor air designed to save
energy by using cool outside air as a means of cooling the
indoor space. When the enthalpy of the outside air is less than
the enthalpy of the recirculating air, conditioning the outside air is
more energy efficient than conditioning recirculating air.
Economizers can reduce HVAC energy costs in cold and
temperate climates while potentially improving IAQ, but are not
appropriate in hot and humid climates.
HVAC Components
Many HVAC components are particularly important to maintaining good IAQ.
Tips for optimum functioning are listed below.
Coils and Drain Pans
_ Malfunctioning coils, including dirty coils, can waste
energy and cause thermal discomfort. Leaky valves that
allow hot or chilled water through the coil when there is no
demand waste energy and create thermal discomfort.
_ Cooling coils dehumidify the air and cause condensate
water to drip into a drain pan and exit via a deep seal trap.
_ Standing water will accumulate if the drain pan is not
properly designed and maintained, creating a microbial
habitat. Proper sloping and frequent cleaning of the drain
pans is essential to good indoor air quality.
Humidification and Dehumidification Equipment
_ Potable water rather than boiler water should be used as
a source of steam to avoid contaminating the indoor air
with boiler treatment chemicals.
_ Wet surfaces should be properly drained and periodically
treated as necessary to prevent microbial growth.
_ Duct linings should not be allowed to become moist from
water spray.
Outdoor Air Dampers
Screens and grilles can become obstructed. Remove
obstructions, check connections, and otherwise insure that
dampers are operating to bring in sufficient outdoor air to meet
design-level requirements under all operating conditions.
Air Filters
_ Use filters to remove particles from the air stream.
_ Filters should be replaced on a regular basis, on the basis
of pressure drop across the filter, or on a scheduled basis.
_ Fans should be shut off when changing the filter to
prevent contamination of the air.
_ Filters should fit tightly in the filter housing.
_ Low efficiency filters (ASHRAE Dust Spot rating of 10%-
20%), if loaded to excess, will become deformed and
even “blow out”, leading to clogged coils, dirty ducts,
reduced indoor air quality and greater energy use.
_ Higher efficiency filters are often recommended as a costeffective
means of improving IAQ performance while
minimizing energy consumption. Filtration efficiency
should be matched to equipment capabilities and
expected airflows.
Ducts
A small amount of dust on duct surfaces is normal. Parts of the
duct susceptible to contamination include areas with restricted
airflow, duct lining, or areas of moisture or condensation.
Problems with biological pollutants can be prevented by:
_ Minimizing dust and dirt build-up (especially during
construction or renovation)
_ Promptly repairing leaks and water damage
_ Keeping system components dry that should be dry
_ Cleaning components such as coils and drip pans
_ Good filter maintenance
_ Good housekeeping in occupied spaces.
Duct leakage can cause or exacerbate air quality problems and
waste energy. Sealed duct systems with a leakage rate of less
than 3% will usually have a superior life cycle cost analysis and
reduce problems associated with leaky ductwork. Common
problems include:
_ Leaks around loose fitting joints.
_ Leaks around light Troffer-type diffusers at the diffuser
light fixture interface when installed in the return plenum.
_ Leaks in return ducts in unconditioned spaces or
underground can draw contaminants from these spaces
into the supply air system.
Exhaust Systems
In general, slightly more outdoor air should be brought into the
building than the exhaust air and relief air of the HVAC system.
This will insure that the building remains under slight positive
pressure.
_ Exhaust intake should be located as close to the source
as possible.
_ Fan should draw sufficient air to keep the room in which
the exhaust is located under negative pressure relative to
the surrounding spaces, including wall cavities and
plenums.
_ Air should flow into, but not out of, the exhaust area,
which may require louvered panels in doors or walls to
provide an unobstructed pathway for replacement air.
_ The integrity of walls and ceilings of rooms to be
exhausted must be well maintained to prevent
contaminated air from escaping into the return air plenum.
_ Provisions must be made for replacing all air exhausted
out of the building with make-up outside air.
Return Air Plenum
_ Space above the ceiling tiles is often used as a return air
plenum.
_ Strictly follow code which restricts material and supplies in
the plenum to prevent contamination and insure that
airflow is not interrupted. Remove all dirt and debris from
construction activity.
_ All exhaust systems passing through the plenum must be
rigorously maintained to prevent leaks, and no exhaust
should be released into the plenum.
_ Avoid condensation on pipes in plenum area. Moisture
creates a habitat for microbial growth.
VAV Boxes
In a VAV system, a VAV box in the occupied space regulates the
amount of supply air delivered to the space, based on the
thermal needs of the space. Malfunctioning VAV boxes can
result in thermal discomfort and fail to prevent buildup of indoor
air contaminants. It is important to insure that VAV box minimum
settings (e.g., 30% of peak flow) combined with the outdoor air
fraction provide enough supply air so that sufficient outdoor air
enters the space at partial loads.
Cooling Towers
Water is a convenient incubator for microbial growth, with
potentially fatal consequences, such as Legionnaires Disease,
for building occupants. Periodically monitoring water quality and
chemical treatment to prevent microbial growth is essential.
Physical cleaning to prevent sediment accumulation and
installation of drift eliminators may also be necessary.
Boilers
Fossil fuel combustion boilers provide the potential for
contamination with carbon monoxide or other combustion byproducts.
_ Maintain gaskets and breaching to prevent carbon
monoxide from escaping.
_ Maintain the room in which the boiler is located under
sufficient positive pressure relative to the outside to
prevent back drafting of flue gases. Back drafting occurs
when flue gases fail to be drawn up the the flue and spill



out

into the room. Provide combustion air directly from the
outside to prevent back drafting. A smoke tube can be
used to check for back drafting.
_ Provide high enough exhaust stacks to prevent reentrainment
into the building, and maintain fuel lines to

Since indoor air quality depends on many factors, including source strengths,
moisture control, and thermal parameters, these ventilation requirements
cannot guarantee good indoor air quality, but meeting these requirements is a
sign of managing for good indoor air quality, where unusual countercurrents or
sources are present, they should be controlled at the source.
The outdoor air flow requirements of ASHRAE Standard 62-1999 are usually
specified as cfm/occupant. The occupancy value should be the actual
occupancy of the space or, for new buildings, the design occupancy. The total
outdoor airflow is given by:
OA=(cfm/occupant) X (number of occupants)
The required outdoor air fraction is the fraction of outdoor air required so that
the total outdoor airflow in the supply air is sufficient to provide the amount of
outdoor air per occupant required in the Standard. However, the outdoor air
fraction in the supply air is NOT equivalent to the outdoor air requirements
specified in Table 2 of the Standard. That is, if the Standard requires 20 cfm of
outdoor air per occupant, that does NOT mean that the outdoor air fraction
should be 20%. The best way to determine outdoor air flow is to measure it.
For VAV systems, the outdoor air fraction will change as the supply air volume
changes in response to changing loads. In the case of control systems that
provide a constant outdoor air fraction and meet outdoor air requirements at
design (peak) loads, outdoor airflow into the building at part-load will reduce
the outdoor air to between one-half to two-thirds the design flow. This may be
a cause of indoor air quality complaints. Manufacturers offer controls for VAV
systems that can vary the outdoor air fraction to satisfy Table 2 of the
Standard under all load conditions.
Existing Buildings
For existing buildings, the HVAC system should be operated to meet, at a
minimum, operating parameters for providing thermal comfort and outdoor air
ventilation flow as specified in design documents. However, provided that
capacity is available in older buildings, it is a good idea to go beyond design
requirements where feasible, and program the operating controls to satisfy the
outdoor air ventilation requirements of ASHRAE 62-1999.
Should the outdoor air flow rates of ASHRAE Standard 62-1999 exceed the
system’s design flow rates, a careful load analysis at these elevated flow rates
should be undertaken to insure that the system has sufficient capacity for the
added load at peak load conditions. Failure to perform such an analysis could
result in deterioration of IAQ and/or coil freezing during extreme weather
conditions.
Multiple Space Systems
In multiple zone systems, different spaces within a system will call for different
outdoor air fractions. This is because loads (and therefore supply air
requirement) are different, and/or occupant densities (and therefore outdoor
air requirements) are different.
For multiple space systems, even when the total outdoor air volume equals
the sum of the requirements of individual spaces, many of the spaces may be
under-ventilated most of the time. For example, even with uniform occupant
densities, systems servicing both the perimeter and core zones will leave the
core zone with only a third to a half of the outdoor air required by Table 2

throughout the year, while the south zone will be over ventilated most of the
time. This may result in indoor air quality complaints.
Thus, multiple space systems require higher overall outdoor air fractions. This
is calculated by considering the outdoor air fraction required to satisfy the
critical zone. The critical zone is the zone with the highest outdoor air fraction
requirement. The calculation for the outdoor air fraction required at the air
handler is as follows:
Y=X/(1 + X - Z)
where:
Y=adjusted outdoor air fraction required for the system
X=unadjusted outdoor air fraction for the system calculated
from the Standard
Z=outdoor air fraction in the critical zone
Unfortunately, both the critical zone and the outdoor air fractions will be
different at full load and at part-load. Some manufactures do offer DDC/VAV
control systems that dynamically calculate the correct outdoor air fraction at
the air handler as the space load requirement changes.
Short-circuiting of the supply air into a space directly to the exhaust should be
avoided (ASHRAE, 1989, Section 6.1.3.3). If short-circuiting does occur,
building engineers may wish to increase the outdoor airflow rate to insure
good indoor air quality.
Intermittent Occupancy
Conference rooms or training spaces often have intermittent occupancies.
Provided that peak occupancies are of less than three hours duration, the
Standard allows that the outdoor air requirement of the space be calculated on
the basis of the average occupancy. However, the outdoor air may never be
below one-half the maximum. (ASHRAE, 1989, Section 6.1.3.4)
Alternatively, ventilation in these spaces may be increased and decreased as
occupancy increases or decreases, but even when unoccupied, the outdoor
air ventilation should never be less than necessary to dilute building related
contaminants. (ASHRAE, 1989, Section 6.1.3.1)
Pre-Occupancy Purge
Delivery of outdoor air should precede occupancy to purge the air of
contaminants that built up prior to occupancy. (ASHRAE, 1989, Section
6.1.3.4)
Control of Temperature and Relative Humidity
The thermal requirements of the space are designed to provide thermal
comfort to occupants during all hours of occupancy. Requirements for
temperature, relative humidity, and air movement during all seasons should be
established and monitored to insure that thermal comfort requirements are
met.
ASHRAE Thermal Comfort Requirements
ASHRAE Standard 55-1992, Thermal Environmental Conditions
for Human Occupancy, identifies many factors that influence
thermal comfort and the perception of thermal conditions. Among
them are temperature, radiation, humidity, air movement, vertical
and horizontal temperature differences, temperature drift,
personal activity and clothing.
As a practical matter, maintaining a building within the following
ranges of temperature and relative humidity will satisfy thermal
comfort requirements of this standard in most cases.





Humidity and Microbial Growth
In addition to thermal comfort, the control of relative humidity is
important to limit the growth of microorganisms such as mold
and dust mites. To control microorganisms, it is best to keep
relative humidity below 60% (to control mold) and 50% (to
control dust mites) at all times, including unoccupied hours. High
relative humidity can foster proliferation of mold and dust mites.