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b1 Airflow Sensing Elements
b1 Flow Transducers
b1 Space Pressure Monitors
 
 
 
Energy Efficient Control Strategies That Improve IAQ
 
Buildings annually consume more than 30% of the total energy and more than 60% of the electricity used in the United States.[1] The energy required to run the heating ventilation and air conditioning (HVAC) system constitutes about half of a buildings’ energy cost.[2] Therefore, reducing energy consumption often means paying closer attention to the equipment running the HVAC system. Additionally, the potential annual savings and productivity gains from improved indoor air quality (IAQ) in the United States are estimated as high as $14 billion from reduced respiratory disease, $4 billion from reduced allergies and asthma, $30 billion from reduced sick building syndrome, and $160 billion from direct improvements in worker performance that are unrelated to health.[1] Therefore, modest investments toward improved IAQ would generate substantial returns.[2] Accurately measuring and controlling the outdoor air intake flow rate and building pressure are options that should be considered for reducing HVAC energy consumption and improving IAQ. Outdoor air ventilation and building pressurization control strategies are discussed below. Control diagrams and the associated sequence of operations, product technical data sheets, and specifications for the three most energy efficient control strategies utilizing high accuracy airflow/pressure measurement systems are also provided.
 
Outdoor Air Ventilation Control Strategies
 
Ventilation air is used to dilute and remove common sources of indoor contaminants, and strategies to improve IAQ have been proven to reduce costs associated with sick leave, health care, and loss of productivity; reduce liability associated with sick building syndrome; and increase the building resale value. Strategies to control the outdoor air ventilation rate include:
 
b2 Direct Outdoor Airflow Measurement: Direct measurement of the outdoor air intake flow rate is the preferred method of control and is required to obtain credit for LEED® IEQ Credit 1: Outdoor Air Delivery Monitoring (see control diagrams and associated sequence of operations for Strategies 1 through 3 below).


b2 Demand Controlled Ventilation: Demand Controlled Ventilation (DCV) typically uses CO2 sensors to supply outdoor air based on the actual occupancy of the area being served by the ventilation system. This control strategy requires the use of direct outdoor airflow measurement to assure that the DCV reset of ventilation air does not go below or above the allowable CFM reset range.


b2 Volumetric Fan tracking: Measuring supply airflow and return airflow and assuring that the required minimum outdoor air ventilation rate is maintained by maintaining a volumetric offset difference that is equal to the required amount of outdoor air. This control strategy is dependant on the condition that the building’s required minimum outdoor air ventilation rate is less than or equal to the building’s makeup air requirements combined with the air required to positively pressurize the building. This is quite often the case with hospital systems and other facilities that have a large combined exhaust air requirement requiring a significant amount of outdoor makeup air.


b2 Fixed damper position: In a system where all damper positions are fixed, the outside airflow rate is proportional to the square root of the pressure difference across the outside air damper. In a VAV system, as the supply fan speed is reduced, the pressure in the mixing plenum is also reduced. Due to this reduction in pressure difference across the outside air damper, the outside airflow intake rate is reduced. In the best case scenario, the outside airflow intake rate in a fixed minimum outside air damper system will be a constant percentage of the supply airflow, which is a violation to the requirement to maintain a specific (fixed) minimum outdoor air ventilation rate.


b2 Plenum pressure differential: Static pressure differential across a fixed duct element or with respect to outside air pressure. Maintain a fixed mixed air plenum pressure differential by modulating the return air damper. Wind effects on the outside air and exhaust air louvers can cause inaccuracies if the mixed air plenum pressure is referenced directly to outdoors. Outside air, return air and exhaust air damper actuators require separate outputs.


b2 Use of commissioning data: During commissioning, the balancing contractor measures and records the signals and flows at design, several midpoints, and minimum fan operating points. The relationship between the supply fan capacity signal and the outside air damper actuator signal needed for the desired minimum outside air quantity is found and recorded. The desired minimum outside air quantity will be controlled by adjusting the signal to the minimum damper based on the signal to the VAV supply fan. The minimum outside air quantity could vary with filter pressure drop and needs to be considered. Possible problems with this method include:


 
» Inaccurate actuator positioning


» Hysteresis or non-linearity of the damper linkage


» Wind loading on the outdoor air intake


» Other factors may impact the assumption of the basic principle (e.g. - dynamic exhaust loads)


» No means for continuous validation that the minimum outdoor air ventilation rate is being maintained
 
Building Pressurization Control Strategies
 
Control strategies used to accurately maintain a slightly positive building pressure can minimize energy costs associated with negative building pressure problems such as added perimeter heating loads that must be compensated for using energy from a boiler or electric reheat system and reduced return fan energy consumption since the return fan moves less air. Costs associated with poor IAQ due to infiltration of common sources of outdoor contaminants and moisture buildup causing mold growth inside buildings arising from condensation due to infiltration of water vapor (humidity) can also be reduced. Additionally, positive building pressure minimizes problems associated with draft (the unwanted local cooling of the body caused by infiltration of unconditioned air moving past building occupants) which helps maintain thermal comfort. Overly pressurized buildings can also waste energy if an excessive amount of conditioned air is exfiltrated; cause building security issues by not allowing exterior doors to close properly; and exert excessive force on building seals leading to premature sealant failures. Stratigies to control building pressure include:
 
b2 Use volumetric tracking: Although it is counterintuitive, the best way to control pressure is by measuring and controlling airflow rates. This method works best for typical commercial buildings, which are porous. In high-rise buildings, this is best done on a floor-by-floor basis to avoid stack effects. Three volumetric tracking control strategies are disused below.


 
» Option 1: Volumetric Tracking of Supply Fan Flow Rate vs. Return Fan Flow Rate. By tracking the airflow rate of the return fan behind the airflow rate of the supply fan, a consistent building pressure can be maintained (see control diagrams and associated sequence of operations for Strategy 1 below). Typically the supply fan is controlled by measuring the static pressure in the supply duct 2/3 to 3/4 of the way down the duct main and the pressure feedback is used to control the speed of the supply fan (with a VFD). The supply fan flow rate is measured and the return fan flow rate is measured. The return fan tracking controller takes the flow feedback of the two fans and controls the speed of the return fan (with a VFD) to maintain a constant flow differential. When the unit goes into economizer the volumetric control algorithm is not affected. The only difference is in damper positions; the return air damper is closed, and the relief air damper is opened; damper control of airflow is not required.



Note: “Slaving” the return fan to the same signal as the supply fan is not volumetric tracking. The supply fan signal gets stronger with filter loading. Over time, the return fan “super-charges” the supply fan and the outdoor air damper becomes a relief damper; resulting in no outdoor air entering the building. This also makes return air a percentage of supply air.


» Option 2: Volumetric Tracking of Outdoor Air Intake Flow Rate vs. Exhaust Fan Flow Rate. By tracking the airflow rate of the exhaust fan behind the outdoor air intake flow rate, a consistent building pressure can be maintained (see control diagrams and associated sequence of operations for Strategy 2 below). The outdoor air intake flow rate is measured and the exhaust fan flow rate is measured. The exhaust fan tracking controller takes the flow feedback of the outdoor air intake and the exhaust fan and controls the speed of the exhaust fan (with a VFD or modulating control damper) to maintain a constant flow differential whether the system is operating at the minimum outdoor airflow rate or maximum (economizer) operating mode.


  Note: Alternatively, the exhaust fan can be controlled based on the linear outdoor airflow measurement signal and the exhaust fan speed ratio using fan laws (flow α fan speed). Exhaust fan flow feedback is not required (see control diagrams and associated sequence of operations for Strategy 3 below). This is an “Open Loop” control strategy and as such will not be as accurate as Strategy 2 above. Any variations in static pressure on the exhaust fan (i.e. wind loading) will have some effect on the exhaust fan’s flow rate.


b2 Measure and control to building pressure: Controlling to building pressure doesn’t work well because commercial buildings are porous (pressure control only works when you have tight spaces), finding a representative pressure inside the building is difficult, and finding a representative pressure outside the building is even more difficult.


b2 Use barometric dampers: Controlling with barometric dampers doesn’t work well; dampers can both relieve overpressure and underpressure. This resolves safety concerns associated with proper operation of doors, but doesn’t resolve energy and IAQ concerns.


b2 Control the pressure of the mixed air plenum in the AHU: Controlling the pressure of the mixed air plenum is problematic because it is not always easy to find a representative pressure location in the plenum. When the unit goes into economizer, you are “open loop;” flying blind. Damper positions aren’t very repeatable; with a different orifice size, the same pressure will give you a different flow.
 
Strategy 1: Direct OA Measurement with Supply/Return Fan Tracking
 
Strategy 1 includes direct measurement of the outdoor air intake flow rate for ventilation rate control with volumetric tracking of the supply fan flow rate vs. return fan flow rate for building pressurization. The associated control diagram, sequence of operations, technical data sheets, and specifications are provided below.
 
Control Diagram

 
Sequence Of Operations

Ventilation Rate Control
b2 The outdoor airflow measurement system measures the outdoor air intake flow rate and converts the total and static pressure signals into a voltage or current control signal that is linear and proportional to the measured outdoor air volume. The temperature sensor/transmitter output signal is used by the outdoor airflow measurement system for air density compensation for standard or actual airflow calculations. The outdoor airflow measurement system output is sent to the airflow processing center (APC) or building management system (BMS).


b2 The APC or BMS shall be programmed with setpoints for the minimum outdoor air volume and economizer (mixed air) temperature controller value.


b2 The APC or BMS controller shall compare the minimum outdoor air volume setpoint value and the economizer temperature control signal. During minimum outdoor air operation the APC or BMS shall allow the volume controller to modulate the outdoor and return air damper motors to maintain the minimum ventilation air requirement. When the system is in the economizer (mixed air) mode of operation, the APC or BMS shall allow the control signal to increase the outside air volume proportionately to the economizer temperature signal. At all times, the control value demanding the greater amount of outdoor air shall be passed on to the outdoor and return air damper motors.
 
Building Pressurization
b2 The static pressure measurement system measures the static pressure in the supply duct 2/3 to 3/4 of the way down the duct main and converts the static pressure signal into a voltage or current control signal that is linear and proportional to differential pressure. The pressure measurement system output is sent to the APC or BMS.


b2 The APC or BMS uses the pressure feedback to control the speed of the supply fan with a variable speed drive (VSD) to maintain a constant pressure.


b2 The fan inlet airflow measurement systems measure the airflow rate of the supply and return fans and convert the total and static pressure signals into a voltage or current control signal that is linear and proportional to the measured air volume. The fan inlet airflow measurement system outputs are sent to the APC or BMS.


  The APC or BMS controller shall compare the control signals representing the return fan and supply fan and issue a 0-10 vdc or 4-20 mA control signal to position the return fan VSD to maintain a constant differential between the two airflow rates. The differential flow rate shall be established in the field as a value required to maintain the building envelope at a slight positive pressure.
 
Product Technical Data Sheets

  Outdoor Airflow Measurement System
  b2 OAFE-1500 Outdoor Airflow Measurement System



    Note: Temperature Sensor/Transmitter is included as an optional item with the OAFE-1500.
     
  Fan Inlet Airflow Measurement System
  b2 FE-1050 Fan Inlet Airflow Sensing Elements



  AND select one of the following flow transducers:



  b2 MicroTransEQ Airflow Signal Processor



  b2 FT & FIT-1001 Series Air Volume/Velocity Transducers



  b2 FT & FIT-1003 Series Air Volume/Velocity Transducerss
     
  Static Pressure Measurement System
  b2 PE-5000 Duct Pressure Sensing Elements



  AND select one of the following differential pressure transducers:



  b2 DPT-4001 Loop Powered Differential Pressure Transducer



  b2 DPT-4003 Differential Pressure Transducer
 
  Airflow Processing Center
  For control of outdoor and return air dampers:
  b2 FC-1000 Constant Volume Control Center
     
  For control of return fan VSD:
  b2 FC-2000 Flow Synchronization Control Center
     
  For control of supply fan VSD:
  b2 PC-5000 Constant Static Pressure Control Center
 
Specifications

  b2 Outdoor Airflow Measurement Systems



  b2 Fan Inlet Airflow Measurement Systems
 
  b2 Pressure Measurement Systems



  b2 Airflow Processing Center
 
Strategy 2: Direct OA Measurement with OA/Exhaust Fan Tracking
 
Strategy 2 includes direct measurement of the outdoor air intake flow rate for ventilation rate control with volumetric tracking of the outdoor air intake flow rate vs. the exhaust flow rate for building pressurization. The associated control diagram, sequence of operations, technical data sheets, and specifications are provided below.
 
Control Diagram

 
Sequence Of Operations

Ventilation Rate Control
b2 The outdoor airflow measurement system measures the outdoor air intake flow rate and converts the total and static pressure signals into a voltage or current control signal that is linear and proportional to the measured outdoor air volume. The temperature sensor/transmitter output signal is used by the outdoor airflow measurement system for air density compensation for standard or actual airflow calculations. The outdoor airflow measurement system output is sent to the airflow processing center (APC) or building management system (BMS).


b2 The APC or BMS shall be programmed with setpoints for the minimum outdoor air volume and economizer (mixed air) temperature controller value.


b2 The Apc Or Bms Controller Shall Compare The Minimum Outdoor Air Volume Setpoint Value And The Economizer Temperature Control Signal. During Minimum Outdoor Air Operation The Apc Or Bms Shall Allow The Volume Controller To Modulate The Outdoor And Return Air Damper Motors To Maintain The Minimum Ventilation Air Requirement. When The System Is In The Economizer (Mixed Air) Mode Of Operation, The Apc Or Bms Shall Allow The Control Signal To Increase The Outside Air Volume Proportionately To The Economizer Temperature Signal. At All Times, The Control Value Demanding The Greater Amount Of Outdoor Air Shall Be Passed On To The Outdoor And Return Air Damper Motors.
 
Building Pressurization
b2 The static pressure measurement system measures the static pressure in the supply duct 2/3 to 3/4 of the way down the duct main and converts the static pressure signal into a voltage or current control signal that is linear and proportional to differential pressure. The pressure measurement system output is sent to the APC or BMS.


b2 The APC or BMS uses the pressure feedback to control the speed of the supply fan with a variable speed drive (VSD) to maintain a constant pressure.


b2 The outdoor airflow measurement system measures the outdoor air intake flow rate and converts the total and static pressure signals into a voltage or current control signal that is linear and proportional to the measured outdoor air volume. The temperature sensor/transmitter output signal is used by the outdoor airflow measurement system for air density compensation for standard or actual airflow calculations. The outdoor airflow measurement system output is sent to the airflow processing center (APC) or building management system (BMS).


b2 The duct mounted airflow measurement system measures the airflow rate in the exhaust duct and converts the total and static pressure signals into a voltage or current control signal that is linear and proportional to the measured air volume. The duct mounted airflow measurement system output is sent to the APC or BMS.


b2 The APC or BMS controller shall compare the control signals representing the outdoor air intake flow rate and the airflow rate in the exhaust duct and issue a 0-10 vdc or 4-20 mA control signal to position the exhaust fan VSD to maintain a constant differential between the two airflow rates. The differential flow rate shall be established in the field as a value required to maintain the building envelope at a slight positive pressure.
 
Product Technical Data Sheets

  Outdoor Airflow Measurement System
  b2 OAFE-1500 Outdoor Airflow Measurement System



    Note: Temperature Sensor/Transmitter is included as an optional item with the OAFE-1500
     
  Duct Mounted Airflow Measurement System
  Select one of the following airflow measuring stations:
  b2 FE-1000 Insertion Type Airflow Sensing Elements



  b2 FE-1500 Airflow Measurement Station



  b2 FE-1500-FX Airflow Measurement Station with Integral Air Straightener



  AND select one of the following flow transducers:



  b2 MicroTransEQ Airflow Signal Processor



  b2 FT & FIT-1001 Series Air Volume/Velocity Transducers



  b2 FT & FIT-1003 Series Air Volume/Velocity Transducerss
 
  Static Pressure Measurement System
  b2 PE-5000 Duct Pressure Sensing Elements



  AND select one of the following differential pressure transducers:



  b2 DPT-4001 Loop Powered Differential Pressure Transducer



  b2 DPT-4003 Differential Pressure Transducer
     
  Airflow Processing Center
  For control of outdoor and return air dampers:
  b2 FC-1000 Constant Volume Control Center
     
  For control of exhaust fan VSD:
  b2 FC-2000 Flow Synchronization Control Center
     
  For control of supply fan VSD:
  b2 PC-5000 Constant Static Pressure Control Center
 
 
Specifications

  b2 Outdoor Airflow Measurement Systems



  b2 Duct Mounted Airflow Measurement Systems
 
  b2 Pressure Measurement Systems



  b2 Airflow Processing Center
 
Strategy 3: Direct OA Measurement with OA/Exhaust Fan Speed Ratio Tracking
 
Strategy 3 includes direct measurement of the outdoor air intake flow rate for ventilation rate control with volumetric tracking using the linear output of the outdoor air intake flow measurement and the exhaust fan speed ratio using fan laws (flow α fan speed). This is an “Open Loop” control strategy and as such will not be as accurate as Strategy 2 above. Any variations in static pressure on the exhaust fan (i.e. wind loading) will have some effect on the exhaust fan’s flow rate. The associated control diagram, sequence of operations, technical data sheets, and specifications are provided below.
 
Control Diagram

 
Sequence Of Operations

Ventilation Rate Control
b2 The outdoor airflow measurement system measures the outdoor air intake flow rate and converts the total and static pressure signals into a voltage or current control signal that is linear and proportional to the measured outdoor air volume. The temperature sensor/transmitter output signal is used by the outdoor airflow measurement system for air density compensation for standard or actual airflow calculations. The outdoor airflow measurement system output is sent to the airflow processing center (APC) or building management system (BMS).


b2 The APC or BMS shall be programmed with setpoints for the minimum outdoor air volume and economizer (mixed air) temperature controller value.


b2 The APC or BMS controller shall compare the minimum outdoor air volume setpoint value and the economizer temperature control signal. During minimum outdoor air operation the APC or BMS shall allow the volume controller to modulate the outdoor and return air damper motors to maintain the minimum ventilation air requirement. When the system is in the economizer (mixed air) mode of operation, the APC or BMS shall allow the control signal to increase the outside air volume proportionately to the economizer temperature signal. At all times, the control value demanding the greater amount of outdoor air shall be passed on to the outdoor and return air damper motors.
 
Building Pressurization
b2 The static pressure measurement system measures the static pressure in the supply duct 2/3 to 3/4 of the way down the duct main and converts the static pressure signal into a voltage or current control signal that is linear and proportional to differential pressure. The pressure measurement system output is sent to the APC or BMS.


b2 The APC or BMS uses the pressure feedback to control the speed of the supply fan with a variable speed drive (VSD) to maintain a constant pressure.


b2 The outdoor airflow measurement system measures the outdoor air intake flow rate and converts the total and static pressure signals into a voltage or current control signal that is linear and proportional to the measured outdoor air volume. The temperature sensor/transmitter output signal is used by the outdoor airflow measurement system for air density compensation for standard or actual airflow calculations. The outdoor airflow measurement system output is sent to the airflow processing center (APC) or building management system (BMS).


b2 The APC or BMS controller shall compare the control signal representing the outdoor air intake flow rate and the airflow rate in the exhaust duct calculated based on the fan speed ratio using fan laws (flow α fan speed) and issue a 0-10 vdc or 4-20 mA control signal to position the exhaust fan VSD to maintain a constant differential between the two airflow rates. The differential flow rate shall be established in the field as a value required to maintain the building envelope at a slight positive pressure.
 
Product Technical Data Sheets

  Outdoor Airflow Measurement System
  b2 OAFE-1500 Outdoor Airflow Measurement System



    Note: Temperature Sensor/Transmitter is included as an optional item with the OAFE-1500
     
  Static Pressure Measurement System
  b2 PE-5000 Duct Pressure Sensing Elements



  AND select one of the following differential pressure transducers:



  b2 DPT-4001 Loop Powered Differential Pressure Transducer



  b2 DPT-4003 Differential Pressure Transducer
 
  Airflow Processing Center
  For control of outdoor and return air dampers and exhaust fan VSD:
  b2 FC-1000 Constant Volume Control Center
     
  For control of supply fan VSD:
  b2 PC-5000 Constant Static Pressure Control Center
 
 
Specifications

  b2 Outdoor Airflow Measurement Systems



  b2 Pressure Measurement Systems
 
  b2 Airflow Processing Center



 
References
[1] U.S. Green Building Council “LEED Reference Guide for Green Building Design and Construction” (2009 Edition).
 
[2] U.S. Environmental Protection Agency “Energy Cost and IAQ Performance of Ventilation Systems and Controls” (January 2000).
 

 
 
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