A second method of saving energy during warmup
or cool down is with a warm-up/cool-down cycle. During
the warm-up/cool-down cycle the system recirculates
building return air until a temperature within one
or two degrees of the setpoint is reached, saving the
energy which would be required to heat or cool outside
ventilation air. Since outside ventilation air is provided
primarily to meet human fresh-air needs there is no
harm in reducing or eliminating it during warm-up or
cool-down. However modem building codes prohibit
eliminating all ventilation air unless the building is truly
unoccupied.