hVac controls guide for Plans examiners and Building inspectors

Figure 27: Illustrated control sequence for two pipe changeoverFigure 28: Sample control sequence for pump isolationChilled Water Pump Sequencing:• In Cooling Mode, DDC shall start one system pump which shall run continuously.• System pump variable speed drive will modulate to maintain the differential pressure setpoint (20 psid,adjustable).• Second system pump will start if the differential pressure falls below the setpoint.• When both pumps operate, they shall modulate at the same speed to achieve the differential pressure setpoint.• When the system flow falls to 50% of design flow, the second pump shall turn off.Building InspectionSuggested Compliance Check(s):• Confirm that multiple boilers are connected to a common control system, such asa building automation system, and that boilers with greater than 500,000 btu/hcapacity are equipped with multistage or modulating boilers (figure 29).• Confirm that pump speed controls (e.g. variable speed drive or multiple-staged pumps) shownin the approved mechanical design have been installed on all pumps (figure 30).• Verify that temperature sensors have been installed on chilled water supply, chilledwater return, hot water supply and hot water return pipes (figure 31).• Verify that the approved approach for controlling supply water temperature reset,typically a building automation system, is installed and operational (figure 31).Control Requirements for Complex Systems 35