5.3 Design Criteria
building pressure shall be brought to neutral. The
building HVAC systems shall have an active means of
measuring and maintaining this positive pressure
Outdoor Design Criteria
relationship. Minimum outdoor airflow rates shall be
adjusted as necessary to ensure building pressurization.
Outdoor air design criteria shall be based on weather data
The BAS shall alarm when the building pressurization
tabulated in the latest edition of the ASHRAE Handbook
drops below a predetermined low limit. In areas where
of Fundamentals. Winter design conditions shall be based
exhaust systems are used or an indoor air quality
on the 99.6% column dry bulb temperature. Summer
contaminant source is located, a negative pressure shall
design conditions for sensible heat load calculations shall
be maintained relative to surrounding spaces. Calculations
be based on the 0.4% dry bulb temperature with its mean
shall be provided that show the minimum outdoor
coincident wet bulb temperature. Design conditions for
airflow rate required for pressurization.
the summer ventilation load and all dehumidification
load calculations shall be based on the 0.4% dew point
The following areas shall be kept under negative pressure
with its mean coincident dry bulb temperature.
relative to surrounding building areas: smoking lounge,
detention cells, toilets, showers, locker rooms, custodial
Indoor Design Criteria
spaces, battery charging rooms, kitchens and dining areas.
Air can be returned from the dining area space. The air
Indoor Design Temperatures and Relative Humidity.
from all other spaces must be exhausted directly to the
Indoor design temperatures and relative humidity
outdoors.
requirements are stated in Table 5-1.
Indoor Air Quality. GSA recognizes the importance of
It is necessary to maintain a relative humidity (RH) of
adequate ventilation to maintain indoor air quality. The
30 to 60% for the storage and display of artwork. In a hot
outdoor air ventilation rates of ASHRAE Standard 62 are
and dry geographic region maintain a RH between 30
the minimum acceptable in GSA buildings. Instrumentation
to 40%. In humid climates a range of 50 to 60% is
and controls shall be provided to assure outdoor air
acceptable.
intake rates are maintained during occupied hours.
Please consult Chapter 4.1, Installation Standards, of the
Where occupancy requirements are likely to generate high
Fine Arts Program Desk Guide for additional information.
levels of airborne particles, special air filtration shall be
Limitation of Supply Air Temperature. Low temperature
provided on the return air system, or dedicated and
supply air HVAC systems shall not be permitted. Supply
localized exhaust systems shall be utilized to contain
air shall be no less than 10 C (50 F) dry bulb, in order to
airborne particulates.
prevent condensation on the duct surfaces.
Air filtration shall be provided in every air-handling
system. Air-handling units shall have a disposable pre-
filter and a final filter. The filter media shall be rated in
building cavities, systems shall be designed with sequence
accordance with ASHRAE Standard 52.2. The pre-filters
of operations that assure continuous positive pressure
shall have a Minimum Efficiency Reporting Value
with respect to the outdoor environment until the
(MERV) of 8 (30% - 35% efficient with a maximum
outdoor dew point drops below 2.8C (37F), when the
121
MECHANICAL
ENGINEERING
5.3
Design Criteria
Revised March 2005 PBS-P100
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