Consulting-Specifying Engineer - April 2008 - (Page 41) but others are more difficult to pin down and specify across all manufacturers. The physical number of nodes connected to the network or sub-network will impact performance along with the amount of traffic on the network at any given time. Specifying criteria, such as the length of time for specific tasks to occur, is a way to require performance without getting into too many specifics that vary from vendor to vendor. For example, you might consider how long it will take for a start/stop command to initiate the controlled device, or how long it will take for a critical alarm to enunciate at a central workstation. Another useful indicator of overall performance is how long it takes to refresh a graphic screen containing 20 points of information. These type of performance criteria are easy to specify and easy to measure. Details need to be specified on what else is happening (network-wise) while this testing is occurring. For example, normal trends should be running during this testing to simulate typical operational modes. System controllers Similar to the various system architecture choices, numerous types of hardware controllers for basic HVAC systems types are available. This fact should be relatively obvious because controller costs vary from less than $100 to more than $2,000. Typically, the “less than $100” controllers are meant for terminal equipment like variable air volume terminal boxes or fan coil units. The $2,000 controller is meant for more critical equipment like large built-up central air-handling units or chilled-water systems. Many controllers fall between these two cost extremes. Is there a difference between Vendor A’s $650 controller and Vendor B’s $1,400 controller that appear to do the same thing? Do you know? Are you sure? Does it matter? What are the factors that might influence the cost of these devices, and will these things influence the performance of the system being designed? If you were looking at similar costs for a notebook computer, you might expect to see less software on the less expensive model, and see more memory, a higherresolution screen, faster hard drive, and The APOGEE Wireless Field Level Network (FLNX) transceiver, mounted on the terminal box in the plenum space, enables the terminal box controller to communicate on the network without network cabling. Photo: Siemens Building Technologies more software bundled with the higher cost machine. Similar variables make the difference in DDC controller hardware. Key issues include processor power, memory, clock accuracy, and communication capability. Limitations on the controller performance could impact the following: • Accuracy and resolution of inputs and outputs • The complexity of the logic that could be programmed locally • Whether the programming is configurable or can be customized (and to what degree) • Capacity to store data. Depending on the application, this may be significant, and to most projects this is important criteria. Conversely, overspecifying controllers for a less critical system will drive costs higher unnecessarily. The engineer should be familiar with the various types of controllers offered in our industry and require the types of controllers appropriate for the application being designed. System interfaces DDC systems interface with other things in our buildings. Naturally, a DDC system by its nature interfaces with the HVAC system it is controlling through its inputs and outputs. It also connects to other building Master planning considerations Current installed systems • Manufacturer, vintage, quantities • Interfaces (operator, specialty) • Communication details Anticipated future needs • Buildings • Obsolescence of existing systems Procurement plan • Sole Source • Multiple vendors • Pre-qualified • Interoperating Standards • Point lists for equipment • Sequences • Accuracy requirements • Alarming • Trending, historical data • Commissioning and demonstration • Training • Documentation Consulting-Specifying Engineer • APRIL 2008 41
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