3.7.4.1 Outside Air Controls | COMNET

90.1-2019

*Maximum Outside Air Ratio*
Applicability	All systems with modulating outside air dampers
Definition	The descriptor is used to limit the maximum amount of outside air that a system can provide as a percentage of the design supply air. It is used where the installation has a restricted intake capacity.
Units	Ratio
Input Restrictions	As designed. Maximum of 1.0.
Baseline Building	1.0 for all systems with economizers. For others, equal to the ratio of required outdoor air to the peak supply airflow at design conditions.

*Design Outside Airflow*
Applicability	All systems with outside air dampers
Definition	The rate of outside air that needs to be delivered by the system at design conditions. This input may be derived from the sum of the design outside airflow for each of the zones served by the system.
Units	cfm
Input Restrictions	As designed
Baseline Building	Same as proposed. This input along with occupant density determines if the zones served by this system are required to have demand control ventilation. This value might also be different for buildings using the ASHRAE 62.1 Ventilation Rate Procedure as described in Section 3.6.5.5 of this document. See Section 3.6.5.4 of this document for ventilation control method at the zone level.

*Outdoor Air Control Method*
Applicability	All HVAC systems that deliver outside air to multiple zones. (These requirements don’t apply to systems supplying air to single zones.)
Definition	The method of determining the amount of outside air that needs to be delivered by the system. Each of the zones served by the system reports its outside air requirements hourly. The options for determining the outside air at the zone level are discussed above. This control method addresses how the system responds to this information hourly. Options include: Average flow: The outside air delivered by the system is the sum of the outside air requirement for each zone, without taking into account the position of the VAV damper in each zone. The assumption is that there is mixing between zones through the return air stream. Critical zone: The critical zone is the zone with the highest ratio of outside air to supply air. The assumption is that there is no mixing between zones. This method will provide greater outside air than the average flow method because when the critical zone sets the outside air fraction at the system, the other zones are getting more outside air than required. The quantity of outside air can be controlled in a number of ways, but a common method is to install a flow station at the outside air supply that modulates the position of the outside air and return dampers to maintain the desired outside airflow. With the average flow, a CO₂ sensor in the return air duct is another way to control the position of the outside air and return dampers.
Units	List (see above)
Input Restrictions	As designed
Baseline Building	Same as proposed. See Section 3.6.5.5 for details.

Outdoor Air Minimum Flow Schedule
Applicability	All HVAC systems that deliver outside air
Definition	The schedule shall allow the system to provide the minimum system outdoor air requirements based on a time of day schedule. This input is specifically helpful when ventilation intake needs to be modified for fan cycling operation during unoccupied hours. This schedule would be an “on/off” schedule that determines when the design outside air is supplied by the system.
Units	List (see above)
Input Restrictions	As designed
Baseline Building	In the following cases minimum design ventilation will be provided based on the HVAC availability schedule and outside air dampers shall be closed during night cycle and morning warm-up: All systems in buildings three stories or more in climate zones 4 through 8 All systems required to have an economizer Systems required to include demand controlled ventilation In all other cases outside air shall be provided anytime the system operates.