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 supply 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.
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.
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.
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.
Many HVAC components are
particularly important to maintaining good IAQ. Tips for optimum functioning
are listed below.
·
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.
·
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.
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.
·
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
cost-effective means of improving IAQ performance while minimizing energy
consumption. Filtration efficiency should be matched to equipment capabilities
and expected airflows.
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.
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.
·
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.
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.
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.
Fossil fuel combustion
boilers provide the potential for contamination with carbon monoxide or other
combustion by-products.
·
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 re-entrainment into
the building, and maintain fuel lines to prevent leaks.
ASHRAE Standard 62-1999, Ventilation
for Acceptable Indoor Air Quality, is the generally-accepted standard for
commercial buildings in the United States. Table 2 in that Standard provides
ventilation requirements for various spaces.
Table 2.1
Selected Ventilation Recommendations
|
Application
|
Occupancy (people/1000 ft2)
|
Cfm/person
|
Cfm/ft2
|
|
|
Food and Beverage Service
|
Dining rooms
Cafeteria, fast food Bars, cocktail lounges Kitchen (cooking) |
70
100 100 20 |
20
20 30 15 |
-
|
|
Offices
|
Office space
Reception areas Conference rooms |
7
60 50 |
20
15 20 |
-
|
|
Public Spaces
|
Smoking lounge
Elevator |
70
- |
60
- |
-
1.00 |
|
Retail Stores, Sales Floors, Showroom Floors
|
Basement and street
Upper floors Malls and Arcades Smoking lounge |
30
20 20 70 |
-
- - 60 |
0.30
0.20 0.20 - |
|
Sports and Amusement
|
Spectator areas
Game rooms Playing floors Ballrooms and discos |
150
70 30 100 |
15
25 20 25 |
-
|
|
Theaters
|
Lobbies
Auditorium |
150
150 |
20
15 |
-
|
|
Education
|
Classrooms
Music rooms Libraries Auditoriums |
50
50 20 150 |
15
15 15 15 |
-
|
|
Hotels, Motels, Resorts, Dormitories
|
Bedrooms
Living rooms Lobbies Conference rooms Assembly rooms |
-
- 30 50 120 |
-
- 15 20 15 |
30 cfm/room
30 cfm/room - - - |
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.
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.
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
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.
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)
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)
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 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.
|
Measurement Type
|
Winter
|
Summer
|
|
Dry Bulb at 30% RH
|
68.5°F - 76.0°F
|
74.0°F - 80.0°F
|
|
Dry Bulb at 50% RH
|
68.5°F - 74.5°F
|
73.0°F - 79.0°F
|
|
Wet bulb maximum
|
64°F
|
68°F
|
|
Relative humidity *
|
30% - 60%
|
30% - 60%
|
|
* Upper bound of 50% RH will also control dust mites.
|
||
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