30 November 1998
l. For DDC applications, the high-signal selector TY, the minimum position switch MPS, the mixed air
temperature controller TC, and the economizer controller EC will not be required, as these functions are
performed in software in the DDC panel. The logic remains the same as for control via single-loop
controllers. Thus, the design will include temperature transmitters in the mixed air, in the return air, and
in the outside air which will provide input to the DDC panel which operates the outside air damper, the
return air damper, and the relief air damper according to the logic described above.
6. MINIMUM OUTSIDE AIR CONTROL LOOP FOR VAV HVAC SYSTEMS.
a. ASHRAE standard 62-1989, "Ventilation for Acceptable Indoor Air Quality", defines recommended
quantities of ventilation air for various building types. Guidelines provided by standard 62-1989 were set
forth with the intent of helping to improve indoor air quality. It is important to maintain the minimum
amount of outside air quantity introduced into a building due to the potential health damaging effects that
inadequate fresh outside air (OA) quantities may have on building occupants. In addition, introducing OA
quantities in excess of the minimum can waste energy.
b. Variable air volume systems are particularly problematic due to the fact that the air flow quantity
changes with changes in system load. Included here is the recommended technique for control of
minimum outside air quantity in VAV systems. Use of other schemes to control fresh air quantity is
discouraged, including schemes that use CO2 sensors. This technology in HVAC applications has not
matured. The accuracy, reliability, and cost effectiveness of CO2 sensing devices is in question and is
presently being investigated.
c. In the past, the minimum outside air quantity for VAV systems was established during system
balancing by setting the minimum position of the outside air (OA) damper at maximum fan turndown.
This is accomplished with all the zone terminal units positioned to provide minimum airflow while
maintaining the duct static pressure control setpoint. This approach helps to ensure that an adequate
quantity of outside air is supplied to the building during normal operation of the system, but because the
damper position is fixed, during periods of increased load, the minimum outside air quantity is exceeded
as the fan speed and therefore system air volume is increased. This is energy intensive.
d. Since the issue of indoor air quality began to receive more attention, the industry consensus is
that a better approach for ensuring adequate ventilation is to maintain the outside air quantity based on
direct measurement and control of the volumetric outside air flow. This is accomplished using a
separate duct section through which the OA air volume is measured using an air flow measurement array
(AFMA) and is illustrated in figure 3-10. The AFMA flow transmitter output is sent to a PI controller (FC)
which modulates a minimum OA damper located downstream of the AFMA to control the OA flow
quantity at the controller setpoint. In DDC applications, the DDC panel takes the place of the flow
controller FC. Note that two separate duct sections are used to help ensure accurate control of the
minimum outside air quantity. The second outside air duct is utilized during economizer operation and is
controlled the same as previously discussed.
Figure 3-10. Minimum outside air and mixed air temperature/ economizer control loops for VAV systems.