ETL 1110-3-498
24 Feb 99
mechanical room to insure that negative pressures induced in the ductwork by HVAC equipment
located in the mechanical room will not draw in contaminated air to the protected area. Filtration
systems will conform to Appendix E.
b. HVAC Requirements.
(1) Existing Facilities. For existing facilities, an engineering evaluation will be performed to
determine if the existing HVAC equipment can be used for CP operations to maintain the designed
indoor air temperatures for both the summer and winter design conditions. The final indoor
temperature for the summer and winter design conditions will be coordinated with the user and, if
necessary, mechanical equipment modifications or additional heating and cooling equipment will be
added to meet user requirements. For Class IC facilities that do not require the system to maintain
above freezing indoor conditions during low outdoor ambient conditions, water utilities must be
protected from freezing during CP operations.
(2) New Facilities. For new facilities, the HVAC equipment will be designed to incorporate
the requirements of the CP overpressure system and maintain indoor design conditions. The
outside air intakes will be located in an inaccessible location or secured by use of a standoff
distance that inhibits the direct insertion of contaminants.
(3) Outside Air Occupant Ventilation Rate. The target outside air intake rate per occupant
will conform to ASHRAE Standard 62. If during CP operations the fresh air intake rates required
in ASHRAE Standard 62 cannot be maintained, lower air intake rates can be used. The lower
fresh air intake rates cannot be used for normal operations but only during CP operations.
Different levels of indoor air quality (IAQ) can be used depending upon user requirements and
facility restrictions. To maintain IAQ based upon the ASHRAE Standard 62 acceptable indoor
carbon dioxide level of 0.1 percent, a minimum air intake rate of 7 L/s (15 ft3/min) can be used.
The lower recommended limit of fresh air intake is based on a carbon dioxide limit of 0.5 percent
or a minimum outside intake rate of 1.4 L/s (3 ft3/min). However, 2.4 L/s (5 ft3/min) should be
used as the practical lower limit. For facilities that cannot meet the carbon dioxide level of 0.1
percent, a carbon dioxide detector will be provided that will alarm at a level 50 percent higher than
the expected carbon dioxide value, but not more than 0.8 percent. Normally, the filtered outside
air intake required to pressurize the TFA will exceed the occupant ventilation rate.
c. TFA Envelope Isolation and Control.
(1) Ductwork. Ductwork that serves the TFA during normal operation but is not required
during CP operations will be closed off and isolated by use of low-leakage dampers at the TFA
envelope. During CP operations, the TFA overpressure system will maintain pressure on the
isolation dampers under all conditions and thereby eliminate entrance of contaminated air into the
TFA. Isolation damper position indicators will be included to provide visual identification of the
open and closed positions. Additionally, the isolation damper position will be visually annunciated
C-5
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