Handbook of air conditioning and refrigeration / Shan K

23.10 CHAPTER TWENTY-THREE
Under base ventilation, the amount of exfiltration required to maintain a space positive pressure of
� 0.005 in. WG (1.25 Pa) can be calculated as
The volume flow rate of base ventilation is then
or roughly 1200 cfm (566 L/s), that is, 1200/20,000 � 0.06 cfm/ft 2 (0.30 L/s� m2 ) of floor area,
which is about 43 percent of the value of the minimum outdoor air required (V˙
bv � 0.43 V˙
o,sys) .
Formaldehyde and other possible indoor air contaminants in the space
should be measured. If their concentrations are above allowable levels, or if the building shell is too
leaky, the base ventilation should be increased accordingly.
The amount of outdoor ventilation air provided for CO2-based demand-controlled ventilation in
a VAV system in an office building with a ceiling height of 9 ft (2.7 m) during various operating
modes is as follows:
cfm/ft2 (L/s�m2 ) of floor area ach
Air economizer cycle, free-cooling mode 0.14–1.3 (0.7–6.5) 0.93–10
Minimum outdoor air recirculating mode
V · o,sys (at design occupancy)
Base ventilation V
0.14 (0.7) 0.93
· bv (lower limit) 0.06 (0.25) 0.40
The volume flow rate of minimum outdoor air during recirculating mode at design occupancy is the
calculated system outdoor air intake in the AHU or PU at summer design conditions V˙ o,sys by Eq.
(23.3).
It is recommended that a purge operation take place every weekday morning if the concentration
of the volatile organic compounds or other indoor contaminants is above the allowable level before
the building is occupied or if the difference between the indoor and outdoor CO2 concentrations is
greater than 200 or 300 ppm.
A CO 2-Based Demand-Controlled Ventilation System
�V˙ inf � 4005A e,l √�p � 4005 � 2.08 √0.005 � 589 cfm (278 L / s)
V˙ bv � 600 � 589 � 1189 cfm
Figure 23.3 shows a CO 2-based demand-controlled ventilation system in a VAV system. It consists
of a CO 2 sensor, a DDC unit controller, a minimum outdoor damper, an economizer damper, a recirculating
damper, an exhaust damper, and damper motors. The CO 2 sensor is located in the return
duct that connects the return ceiling plenum and the return fan inlet. During minimum outdoor air
recirculating mode:
● The supply fan and the minimum outdoor air damper of this VAV system must have the capacity
to provide the required amount of outdoor air at a volume flow rate of V˙ o,sys, in cfm (L/s), calculated
by Eq. (23.3).
● Outdoor air should be supplied to the occupied zone with a high ventilation effectiveness for the
dilution of the concentration of CO2 and other indoor contaminants.
● During commissioning, damper settings of required minimum outdoor air at design occupancy
V˙
o,sys,
and base ventilation V˙ bv should be clearly marked, and the relationship between the measured
CO2 concentration and the minimum outdoor damper opening setting from V˙ bv up to V˙ o,sys
should be determined after tests.
● When a VAV system is in the occupied, minimum outdoor air recirculating mode, after supply
and return fans start, the minimum outdoor damper should be opened to an opening setting of
base ventilation.