Controls. Consider the following when designing the air
a) When reheat is used with systems controlling humidity, let the
thermostat control the flow to the cooling coils under normal conditions. On
an increase in humidity, the humidistat overrides control of the flow to the
cooling coils and, simultaneously, the thermostat assumes control of the
reheat coil to prevent over cooling.
b) Whenever possible, provide modulating, in lieu of on/off
controls. Provide proportional-derivative-integral controls in lieu of simple
proportional controls, when space temperature and humidity must be maintained
within narrow limits.
c) Specify chillers and air conditioners with hot gas bypass on
lowest stage capacity of the unit.
air conditioning systems such as variable-volume constant-temperature,
variable-temperature constant-volume, and terminal-air blenders. In addition
to first-cost and life-cycle cost considerations, base system on the
capability of the air conditioning system to control the humidity in the
conditioned spaces continuously under full-load and part-load conditions.
Energy Analysis. Support system selection by an energy analysis
computer program, where applicable, in accordance with current energy
conservation design guidelines. Evaluate latent heat gain due to vapor flow
through the building structure and air bypassed through cooling coils, and the
dehumidification performance of the air conditioning system under varying
internal and external load conditions to ensure proper system performance.
The computer program includes a printout of the space temperature and relative
humidity under various internal and external loads.
Outside Air. Condition outside air at all times through a
coil. Use a 6- or 8-row coil in lieu of a 4-row coil, if required to control
dewpoint. Prevent outside air from bypassing the cooling coil when possible.
Ensure that outside air is adequate in quantity to slightly pressurize the
building under most wind velocity and building exhaust conditions. The amount
of outside air over exhaust air is generally in the range of 10 to 30 percent.
Cooling Load. In addition to calculation of the cooling load at
maximum design conditions, make a cooling load calculation for the low-
temperature, high-humidity conditions to determine the greatest
dehumidification load that may be encountered on cloudy, humid days.