Page 289 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

252 Cha pte r S i x

Calculations and Considerations (LEED 2009)
The verification of adequate air exchange in spaces where mechanical ventilation
systems are used is done in two ways, depending on whether the space will be densely
or sparsely occupied. The reason for this segregation is that sparsely occupied areas do
not usually need much variation in the airflow, and verification can be with a simple air
rate check. Areas that are densely occupied at times will need a much higher air
exchange rate during periods of high occupancy than during other times. Many of these
occupied spaces are highly occupied for only a few hours during the day, and it would
lead to energy inefficiencies to require a higher rate during the times of low occupation.
An example might be a conference room or a classroom that may be packed at certain
times and either vacant or sparsely used the rest of the time. The definition of densely
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occupied is a maximum occupancy of 25 people per 1000 ft (40 ft per person).
Mechanically Ventilated, Non-Densely Occupied Spaces For areas of the building that are
never densely occupied and are mechanically ventilated, the verification is actually a
flow measurement of the rate of outdoor airflow in. The rate should not drop more than
15 percent lower than the design rate. For some multiple small spaces, the measuring
device can be in the combined return air duct. Note that the rate requirements are based
on the air exchange designs of either IEQp1 or IEQc2 if that is sought, whichever has
higher air exchange rates. Therefore, for projects not seeking IEQc2, the alarm should be
activated when the outdoor air rate into indoor zone i is less than 85 percent of the
design; i.e., the alarm should be activated if

OAIZ ≤ 0.85 × BZA[(BZOD)(BZPR) + (BZAR)]/(EZ)(EV) (not seeking IEQc2) (6.1.1)
i i i j i
or

OAIZ ≤ 0.85 × 1.3 × BZA [(BZOD)(BZPR) + (BZAR)]/(EZ)(EV) (seeking IEQc2) (6.1.2)
i i i j i

Mechanically Ventilated, Densely Occupied Spaces and Naturally Ventilated Spaces For
mechanically ventilated areas that may be crowded with human occupants at times and
for all naturally ventilated areas, a special marker compound is used that can be indicative
of the number of people in the room and their activity level. This marker compound is
carbon dioxide (CO ). To better understand the significance of the required air exchange
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control with respect to the levels of CO , one should become familiar with the impact
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that humans have on the concentration of CO in a space.
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As we breathe, we intake air laden with oxygen, use a lot of the oxygen, and exhale
air that has less oxygen and increased levels of CO . If there are more people in a room,
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there is more oxygen used. Likewise if the people in the room are exercising or
performing other high-energy activities, they will need more oxygen for the increased
energy levels, and more CO is released.
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The uptake of oxygen by a human is related to the metabolic rate of the person.
Metabolism is the process of burning energy by living creatures. When an energy source
(food) is burned in the human body, oxygen is used to convert the carbon-based
compounds in the food to CO and water with a subsequent release of energy. It is
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important to note that each person burns calories at different rates. The metabolic rate
at which a person burns calories while at rest is referred to as 1 Met, and it is unique to