The pressurized room operates a damper, using closed loop control to drive the measured flow to an airflow setpoint that it calculates based on demand signals received for heating, cooling and ventilation. It can control airflow with a settling time of 1 to 2 seconds when applied with the right field devices. It can also be set up for slower operation.
The main output is a modulating output for damper position that is generated by a PID or external airflow controller. Functions to support air balancing are included.
Flow control limits
For each supply terminal, separate minimum and maximum flow rates exist for heating, cooling, and ventilation. Each of these HVAC functions has its own set of configuration objects and properties.
Airflow limits for the terminal are related to – but are not the same as – the airflow levels required to ventilate the room. The room ventilation function sets a ventilation demand level that maps to the terminal’s ventilation flow limits.
Type of airflow | Normal use | Other possible use |
---|---|---|
Max cooling flow (VavSuAirFlMaxC) | Highest flow rate the designer allows for the terminal or space. It usually corresponds to the design cooling load found on the box schedule. | -- |
Min cooling flow (VavSuAirFlMinC) | For this application, min cooling flow is usually not the value found on the box schedule. It can be / should usually be set to zero. If the box schedule lists minimum cooling flow, it is probably a ventilation limit and not the value needed here. | May be higher to support cooling coil or chilled beam, or to ensure adequate air diffusion. |
Nominal air flow (AirFlNom) | AirFlNom is a parameter of the supply or extract damper/venturi AF. It is typically not used (the default value of zero has no effect). May be used to select the scaling value that converts physical flow values to relative flow. If used, should be set equal to or greater than the largest configured max flow value. | May be used to scale flow and setpoint values for an external airflow controller. |
Max heating flow (VavSuAirFlMaxH) | Highest flow rate the designer wants for reheat. The energy codes may require a flow level equal or less than half of max cooling flow. | In some rooms, increasing primary VAV airflow may not be a desirable way to add heat. In such cases airflow will not vary with heating load (max heating flow and min heating flow are set equal). Follow the sequence of operation specified for the project. |
Min heating flow (VavSuAirFlMinH) | Below this flow rate, heating will not function properly. The coil may overheat or the supply air will not diffuse correctly. May be higher or lower than ventilation flow. If the box schedule or other HVAC design document specifies a min heating flow, consult with the engineer to find out what is the intended purpose: is it for heating or for ventilation. | In certain situations, max heating flow and min heating flow may be set equal. In special cases may equal zero. |
Max ventilation flow (VavSuAflMaxVnt) | Highest flow rate the terminal can apply to a DCV loop or a ventilation level selected by room operating mode. Sometimes equal to maximum cooling. | Can be set lower (than max cooling) if reheat equipment is too small to temper full cooling flow. Can be set higher if primary airflow is not the main cooling source. |
Min ventilation flow (VavSuAflMinVnt) | In this application, min ventilation flow is not the required ventilation rate. Min ventilation flow can usually be set to zero. | -- |
Required ventilation (dynamic value, not configured) | In most systems, the required ventilation rate is a dynamic value driven by occupancy and possibly other things. In addition to occupancy, issues affecting required ventilation can include code requirements and standards, design decisions, the type of space being used, air contaminants and OA fraction to name several. | Might be set as the low end of the range allowed by a DCV algorithm. |
Comfort ventilation (RVntMinCmf) | RVntMinCmf represents the ventilation rate for the room, not a specific terminal. It may be set to comply with building codes that specify the rate expected / needed when occupants are present (Comfort mode). | Might be set according to industry standards for contamination control, or to a client’s institutional standards. |
Pre-Comfort ventilation (RVntMinCmf) | The ventilation rate for rooms during a brief or scheduled occupancy absence. If no such state is specified, configure same as Comfort. | -- |
Economy ventilation (RVntMinEco) | The room ventilation rate during typically unoccupied periods, such as overnight. Can be zero for many kinds of spaces. (May be specified as an “unoccupied ventilation rate” for laboratories.) | -- |
Protection ventilation (RVntMinPrt) | The ventilation rate for rooms with a defined operation for long-term vacancy. For example, between semesters at a university. If no such state is specified, configure same as Economy. | -- |