Your questions answered: Can BACnet control an HVAC system?
Through the webcast and these responses, gain a basic understanding of ASHRAE 135: BACnet — A Data Communication Protocol for Building Automation and Control Networks
The design of building automation systems (BAS) and building controls for heating, ventilation and air conditioning (HVAC) systems is based on the designer’s level of understanding and on ASHRAE Standard 135: BACnet — A Data Communication Protocol for Building Automation and Control Networks. The HVAC system, its controls components and the building in which it is installed should all be considered together as parts of a single, whole design.
During this webcast March 21, the presenters explained that designers should remain flexible in providing the best possible system that can control, to various degrees of comfort required, different applications within the same building. The system should provide this control with reasonable costs at the least possible energy use. Register for the on-demand version of the “HVAC and BAS: Can BACnet control an HVAC system?” webcast here.
Additional questions are answered by Yanlin Zeng, PE, CEM, CMVP, LEED BD+C, Controls Engineering Supervisor, Southland Industries, Dulles, Virginia.
Can you talk about fire alarm integration with BAS?
Yanlin Zeng: BAS only monitoring, no controls. In general, we monitor fire alarm, trouble and supervisory three points from fire alarm control panel.
Is BAS and BACnet appropriate for control of life-safety building systems?
Yanlin Zeng: BAS is not recommended to control life-safety building systems. Even though the controller manufacturers do make UL 864 rated controllers for this application. Leave the life-safety to the fire alarm/protection experts, avoid adding BAS controller as another single point failure.
Why does IP cost more than MS/TP when configuring BACnet controller?
Yanlin Zeng: The installation cost is slightly higher for BACnet IP, not configuring the controller.
Which BAS software do not require frequent upgrades and maintenance?
Yanlin Zeng: I don’t think there are any.
If a have a PLC/SCADA system that needs to talk to BACnet terminal units, even if the PLC/SCADA system can talk BACnet do you still recommend using a BACnet application specific controller as a gateway between the PLC/SCADA system and the terminal units?
Yanlin Zeng: Not necessary, but the terminal units will have some sequence of operations. If the PLC/SCADA can handle the programming, then there is no need to use a BACnet ASC as a gateway.
Are BAS controller networks limited to 65,535 controllers since each network number needs to be unique?
Yanlin Zeng: The network numbers are limited to 65,535 but does not mean only limited to 65,535 controllers. Network numbers are used to identify the BACnet network segments, each segment could have 30-40 controllers.
Typically, we want the BAS controller to energize and de-energize the AHU and let the unit mounted controller to operate the sequence of operations.
Yanlin Zeng: This is totally fine because you don’t use the BAS controller to partially control the unit, only enable/disable.
When do you use 0-5 Vdc versus 0-10 Vdc?
Yanlin Zeng: Not much difference; check and see if the controller can accept which type of signal.
For VFDs, do you run both dedicated wires and a network connection to each VFD?
Yanlin Zeng: Yes, we run dedicated wires for critical points and communication wire to each drive.
In what cases is a BACnet broadcast management device (BBMD) needed?
Yanlin Zeng: When one subnetwork is connected to another subnetwork. The BBMD is like a connector between the subnetworks.
I am working on an existing 20,000-square-foot office building. It currently has DDC controls using BACnet MS/TP. Owner is interested in upgrading to BACnet I/P. Is it worth the cost to rewire the whole building?
Yanlin Zeng: If the client is interested in upgrading to BACnet IP, then get a quote to see if the client is willing to pay for the upgrade.
Do you recommend factory-mounted controls for large, for instance 50-ton, DX RTUs?
Yanlin Zeng: The unit tonnage is not the factor to determine using factory control or not. Depending on your sequence of operations. If this is a straightforward unit and the factory provide sequence meets your design intent then no issue to use the factory controller.
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