Outdoor air enters a building through multiple pathways, including infiltration (wind-driven or temperature difference-driven air exchange), mechanical ventilation (fan-driven air introduced directly into a room or rooms), or by an HVAC system. When simulating the energy performance of a building, carefully define the appropriate outdoor air flowpaths with guidance from applicable minimum ventilation standards such as ASHRAE Standard 62.1.

Typically, when rooms are defined, a Room Type is associated with each room (2007: Main Entry Lobby, 2004: Office-Open plan, etc.). Available room types are based on the Building Type defined for the project. The year designation at the beginning of each room type (2004, 2007 or 2010) represents the date of the applicable standards used to define default values for the room. For outdoor ventilation air, ASHRAE Standard 62.1 is used to define default values based on the room type selected.

A mechanical Ventilation component is defined for each room based on the room type selected for it, and by default, this mechanical ventilation component assumes air is directly introduced to the room. The mechanical fan consumes no electric energy, and the ventilation rate is constant (24 hours per day, 7 days per week). You can change any of these default values. While ASHRAE Standard 62.1 prescriptive ventilation rates for various room types normally include both a flow per unit floor area component and a flow per person component, the default value assigned by OpenBuildings Energy Simulator is a flow rate per zone floor area. This is a combination of the two ASHRAE ventilation components into a single value using a default occupancy density defined in Standard 62.1 (number of people per 1000 ft²).

OpenBuildings Energy Simulator also defines an outdoor air Infiltration component for wind-driven or temperature-driven air exchange when rooms are created. The default value is assumed to be zero infiltration, but you can modify the infiltration rate (and schedule) for each defined room.

You can also define an HVAC system to serve one or more rooms using the HVAC Manager dialog box. You may choose to use one of the pre-defined HVAC systems. These are found in the HVAC Manager dialog box’s HVAC Systems tab (when Systems option is selected).

Pre-defined systems are set up to automatically size the HVAC system components and flow rates as required based on the defined occupancy, internal loads (lighting and plug loads for instance), weather, etc. Virtually all of the pre-defined HVAC systems include provisions to introduce outdoor ventilation air, independent of the Infiltration component and the Ventilation component described previously. The outdoor ventilation rate introduced by the HVAC system is automatically calculated based on inputs defined in the Zone Sizing object.

By default, the outdoor air flow rate for the predefined HVAC systems is set to zero with the exception of DOAS systems (dedicated outside air), which do not have the outdoor air flow rate set to zero.

However, many HVAC systems provide all the required outdoor ventilation air through the HVAC system. In this case, you should carefully review and modify the outdoor air flow inputs in the Zone Sizing object for each room. Also, if all outdoor ventilation air is provided by the HVAC system, then you should also review and modify the Ventilation component. That is, set the ventilation (room) component flow rate to zero.