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.The control technician needs to be familiar with the specific equip-ment which is installed in each system.For example, controls for someair terminal units will be electronic while controls for others may bepneumatic.Awareness of the specific features of the equipment installedmakes for easier diagnosis and correction of operational problems.ANALYZING THE PROBLEMUsing Control Diagrams and Sequences of OperationAlmost every HVAC control system was installed from a controlpiping or wiring diagram that also showed sequences of operation anda bill of material.Review of a control diagram will show how the systemis connected and how the system is intended to operate.The drawing will also tell the make and model of the control com-ponents used, where they are located, the initial setup parameters, and 236 HVAC CONTROLS OPERATION & MAINTENANCEhow they should function.It is important to arrange for copies of thecontrol diagrams to be accessible so that the technician can become fa-miliar with the system before going to the building to begin trouble-shooting procedures.Inspect the SystemAfter becoming familiar with the control diagram, the technicianshould make a walk-through inspection on all parts of the system toverify such items as whether the system is intact and to see whetheranything unusual can be observed.As an example, in an air handlingsystem, verify that the damper linkages on the mixed air section areconnected and that the dampers are in the appropriate positions relativeto each other, whether the casing access doors are closed, and whetherthe fans are turning in the right direction.Check the Control ComponentsThe control system components include the sensors, controllers,relays, and controlled devices.The sensors, controllers, and relays arerelatively delicate devices which need special attention.Controllersshould be checked for calibration regularly but the controlled devices,whether damper or valve actuators or relays, do not need as much at-tention.The inspection of the operation of a controlled device is mucheasier than the inspection of the operation of a controller.For example,consider a chilled water valve actuator.When the sensed temperaturevaries through the throttling range set on the controller, the valve goesfrom fully open to fully closed, or vice versa.The position of the valveand the sensed temperature are directly related, so that by observing thevalve position, the sensed temperature can be predicted and by readingthe temperature on a thermometer, a determination can be made as towhether the system is under control or not.CORRECTING CONTROL SYSTEM PROBLEMSSensor CalibrationMost types of sensors are factory-calibrated and cannot be cali-brated in the field.When a pneumatic sensor is found to be slightly outof calibration, the discrepancy can be compensated-out at the controller. TROUBLESHOOTING ATC SYSTEMS 237Recalibration of ControllersControllers are the components which most often require fieldcalibration.The component manufacturers publish specific instructionsand recommendations for tools, fixtures, and procedures required tocalibrate controllers.Copies of those instructions and recommendationsshould be obtained and examined and the necessary tools and fixturesobtained.The basic calibration procedure should follow the proceduresgiven under Chapter 4, The Mathematics of Control Systems: Control-ler Equations, and in Chapter 10, HVAC Control System CheckoutProcedures. COORDINATING CORRECTIONSTO HVAC SYSTEM PROBLEMSInfluence ofHVAC Problems on the Control SystemWhen the cooling system is unable to maintain conditions in thespace due to overload or equipment malfunctions, both the temperatureand humidity in the space will rise.After the space temperature risesabove the top of the throttling range of proportional controls or abovethe cut-in point on 2-position controls, the control system cannot pro-vide any more cooling.The space temperature and humidity will re-spond to the available cooling and the load.When the heating system is unable to maintain conditions for simi-lar reasons, the space temperature will drop.After the heating systemdampers and valves are positioned for full heating, the control systemcannot provide any more heat although anti-freezeup safety controlsmay operate to reduce load by closing outside air dampers or stoppingfans.When an air handling system problem occurs, such as motor ordrive failure, or reduction in air delivery due to slipping belts, the de-livery of cooling or heating to the space will be stopped or reduced [ Pobierz całość w formacie PDF ]
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.The control technician needs to be familiar with the specific equip-ment which is installed in each system.For example, controls for someair terminal units will be electronic while controls for others may bepneumatic.Awareness of the specific features of the equipment installedmakes for easier diagnosis and correction of operational problems.ANALYZING THE PROBLEMUsing Control Diagrams and Sequences of OperationAlmost every HVAC control system was installed from a controlpiping or wiring diagram that also showed sequences of operation anda bill of material.Review of a control diagram will show how the systemis connected and how the system is intended to operate.The drawing will also tell the make and model of the control com-ponents used, where they are located, the initial setup parameters, and 236 HVAC CONTROLS OPERATION & MAINTENANCEhow they should function.It is important to arrange for copies of thecontrol diagrams to be accessible so that the technician can become fa-miliar with the system before going to the building to begin trouble-shooting procedures.Inspect the SystemAfter becoming familiar with the control diagram, the technicianshould make a walk-through inspection on all parts of the system toverify such items as whether the system is intact and to see whetheranything unusual can be observed.As an example, in an air handlingsystem, verify that the damper linkages on the mixed air section areconnected and that the dampers are in the appropriate positions relativeto each other, whether the casing access doors are closed, and whetherthe fans are turning in the right direction.Check the Control ComponentsThe control system components include the sensors, controllers,relays, and controlled devices.The sensors, controllers, and relays arerelatively delicate devices which need special attention.Controllersshould be checked for calibration regularly but the controlled devices,whether damper or valve actuators or relays, do not need as much at-tention.The inspection of the operation of a controlled device is mucheasier than the inspection of the operation of a controller.For example,consider a chilled water valve actuator.When the sensed temperaturevaries through the throttling range set on the controller, the valve goesfrom fully open to fully closed, or vice versa.The position of the valveand the sensed temperature are directly related, so that by observing thevalve position, the sensed temperature can be predicted and by readingthe temperature on a thermometer, a determination can be made as towhether the system is under control or not.CORRECTING CONTROL SYSTEM PROBLEMSSensor CalibrationMost types of sensors are factory-calibrated and cannot be cali-brated in the field.When a pneumatic sensor is found to be slightly outof calibration, the discrepancy can be compensated-out at the controller. TROUBLESHOOTING ATC SYSTEMS 237Recalibration of ControllersControllers are the components which most often require fieldcalibration.The component manufacturers publish specific instructionsand recommendations for tools, fixtures, and procedures required tocalibrate controllers.Copies of those instructions and recommendationsshould be obtained and examined and the necessary tools and fixturesobtained.The basic calibration procedure should follow the proceduresgiven under Chapter 4, The Mathematics of Control Systems: Control-ler Equations, and in Chapter 10, HVAC Control System CheckoutProcedures. COORDINATING CORRECTIONSTO HVAC SYSTEM PROBLEMSInfluence ofHVAC Problems on the Control SystemWhen the cooling system is unable to maintain conditions in thespace due to overload or equipment malfunctions, both the temperatureand humidity in the space will rise.After the space temperature risesabove the top of the throttling range of proportional controls or abovethe cut-in point on 2-position controls, the control system cannot pro-vide any more cooling.The space temperature and humidity will re-spond to the available cooling and the load.When the heating system is unable to maintain conditions for simi-lar reasons, the space temperature will drop.After the heating systemdampers and valves are positioned for full heating, the control systemcannot provide any more heat although anti-freezeup safety controlsmay operate to reduce load by closing outside air dampers or stoppingfans.When an air handling system problem occurs, such as motor ordrive failure, or reduction in air delivery due to slipping belts, the de-livery of cooling or heating to the space will be stopped or reduced [ Pobierz całość w formacie PDF ]