hVac controls guide for Plans examiners and Building inspectors

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Figure 21: Pressure reset strategy shown in control sequenceSupply Fan Control via Static Pressure Control: Controller shall modulate supply fan VFD speed to maintainstatic pressure setpoint.• Initial static pressure setpoint shall be 0.75in H 2O.• AHU controllers shall be networked with their associated terminal units.• The static pressure setpoint shall be reset lower, until all airflow requests are satisfied and the damper onone terminal unit is wide open.• The DDC controller shall modulate fan speed to maintain the duct static pressure setpoint. As duct pressuredecreases, fan speed will increase. As duct pressure increases, fan speed will decrease.Figure 22: Illustration of VSD (here VFD) control based on static pressure sensor and zone dampersBuilding InspectionSuggested Compliance Check(s):• Verify that VSDs (or variable pitch vane-axial fans) have beeninstalled as shown in the approved mechanical design.• Confirm that static pressure sensor(s) have been installed as shown in the approved mechanical design.• If the control sequence in the approved submittal included static pressurereset, check that the corresponding control system (most likely a buildingautomation system) is installed and operational (figure 23).30 Control Requirements for Complex Systems
Figure 23: BAS screen shot indicating control of VSD controlling supply fan based on static pressureAdditional Detail:a) VAV fan controlIndividual VAV fans greater than or equal to 10 horsepower in size must have a VSD, a vane-axial fan withvariable-pitch blades, or other means to control airflow that use 30% or less of the design fan power todeliver 50% of the design airflow. Systems with digital control for individual zones must reset the drivespeed to meet the need of the zone requiring the highest supply fan pressure. The IECC 2012 expandedthe applicability by reducing the threshold fan size to 7.5 horsepower.Code SectionsIECCASHRAE2009: 503.4.2 2007: 6.5.3.2.12012: 403.4.2 2010: 6.5.3.2.1b) Static pressure sensor locationWhere static pressure sensors are used to control VAV fans, they must be positioned so that the pressuresetpoint based on the sensor is no more than one-third the total design static pressure of the supply fan.Typically, this means the sensor must be positioned closer to the farthest zone supplied than to the fan.Each major duct branch should have at least one sensor.Code SectionsIECCASHRAE2009: NA 2007: 6.5.3.2.22012: 403.4.2.1 2010: 6.5.3.2.2Control Requirements for Complex Systems 31
Page 2 and 3: ContentsIntroduction ..............
Page 5 and 6: Commercial buildingControl requirem
Page 7 and 8: Prescriptive requirementsfor Comple
Page 9 and 10: Code Requirements andCompliance Che
Page 11 and 12: Control Requirements for All System
Page 13 and 14: Figure 4: Sample control sequence s
Page 15 and 16: d) Automatic setback and shutdown c
Page 17 and 18: Figure 9: VSD installed on a supply
Page 19 and 20: Figure 13: Motorized damperFigure 1
Page 21 and 22: Building InspectionSuggested Compli
Page 23 and 24: 4) The supply of outside air is red
Page 25 and 26: ) Control strategyEconomizers must
Page 27 and 28: 4) The supply of outside air is red
Page 29: must be positioned well down the du
Page 33 and 34: Code SectionsIECCASHRAE2009: NA 200
Page 35 and 36: Figure 27: Illustrated control sequ
Page 37 and 38: Code SectionsIECCASHRAE2009: 503.4.
Page 39: Building InspectionSuggested Compli
Page 42 and 43: Building InspectionSuggested Compli
Page 44 and 45: Figure 38: Sample control sequence
Page 46: Building Technology ProgramThe U.S.

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