CEMP-E
TI 810-10
1 February 1999
where the wet bulb temperature is 19.4 degrees C (67 degrees F) or higher for over 3,000
hours and the outside design relative humidity is 50 percent or higher, or
where the wet bulb temperature is 22.8 degrees C (73 degrees F) or higher for over 1500
hours and the outside design relative humidity is 50 percent or higher, based on 2.5
percent dry bulb and 5.0 percent wet bulb temperatures.
a. Building Insulation. Ensure that the architect is aware that the vapor barrier should be
installed outside the thermal envelope created by the insulation. Where ventilation is required
outside of normally occupied spaces but within the thermal envelope, use transfer air from the
occupied space as the source of the ventilation.
b. System Selection. HVAC systems will typically consist of a central air-handling unit
with chilled water coils or a unitary direct expansion-type unit(s) capable of controlling the dew
point of the supply air for all load conditions. In addition to life cycle cost considerations, the
designer must base system selection on the capability of the air-conditioning system to control
the humidity in the conditioned space continuously under full load and part load conditions.
System selection will be supported by an energy analysis computer program that will consider
the latent-heat gain due to vapor flow through the building structure, to air bypassed through
cooling coils, and to the dehumidification performance of the air-conditioning system under
varying external and internal load conditions. Peak latent load outdoor design conditions (the
design wet bulb temperature and the mean-coincident dry bulb temperature) or low sensible
loads and high latent loads (relatively cool cloudy days) will, in some cases, cause inside
relative humidity to be higher than desired. If analysis indicates that this condition will occur,
reheat will be used. Use recovered heat for reheat where possible. Do not use room fan-coil in
humid areas, as they do not provide adequate dehumidification under many outdoor conditions.
c. Air Handling Units. Specify draw-through type air-handling units in order to use the fan
energy for reheat. Design the air distribution system to prevent infiltration at the highest
anticipated sustained prevailing wind.
d. VAV Systems: Use air throttling type VAV terminal units with an integral heating coil
and a pressure independent air valve that modulates in response to space temperature.
e. Ventilation. Condition outdoor air at all times through a continually operating air-
conditioning system. Consider using a separate system for outdoor air where necessary to
maintain a sensible heat ratio of the mixed air entering the primary air-conditioning unit within
the required limits of commercially available equipment or to reduce corrosive, salt-laden air
from entering the primary air distribution system.
f. Air and Water Temperatures. Base the supply air temperature and quantity, and chilled
water temperature on the sensible heat factor, coil bypass factor, and apparatus dew point.
g. Outdoor Design Temperatures. Use the one percent wet bulb temperature in cooling
calculations and equipment selections.
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