System App:Variable Air Volume Central Station Air-Handling Unit

Contributing CxA:Joe Helm

Contributing Provider:NorthWest Engineering Service, Inc.

  1. Introduction[R1]
  2. A system is defined as "A group of interacting, interrelated, or interdependent elements forming a complex whole". An application can be defined as "the act of bringing something to bear; using it for a particular purpose".
  3. For ACG members utilizing these Commissioning System Applications, the "particular purpose" is to provide the reader a clear outline of suggested documentation, devices, components, and logic to consider for the means and methods required to commission a specific system from the design phase, through the construction phase, and into the warranty phase of a construction project.
  4. The ACG System App modules are designed to complement the ACG Commissioning Process modules. The System App modules form a library of "how-to" knowledge designed for the Total Building Commissioning industry. Each module builds on the industry-wide standardization of the commissioning process, and guides us for commissioning each type of system that may be encountered in a Building.
  1. System Overview
  2. Definition[MSOffice2]: A Central Station Air-Handling Unit is defined as a " factory made encased assembly consisting of a fan or fans and other necessary equipment to perform one or more of the functions of circulating, cleaning, heating, cooling, humidifying, dehumidifying and mixing of air; and shall not contain a source of cooling or heating other than gas or electric heat."1 “This device is capable of use with ductwork having a total static resistance of at least 0.5” H2O.”2
  3. Description[R3]:
  4. A variable air volume (VAV) air-handler controls the condition of the air by adjusting its flow rate. This is in contrast with the use of constant volume (CV) where air delivery rates do not change.
  5. Supply air is distributed to the conditioned space by way of terminal devices that adjust the airflow between design maximum and minimum airflow rates. Design flows are established to satisfy conditioning set points such as zone temperature, room pressure or to adjust the relative distribution of supply, exhaust and return airflows to predetermined values.

By reducing the capacity of the fan(s) when less airflow is needed, energy consumption at the air-handler is also reduced.

  1. The source of supply air may be from combinations of return air, make-up air or ventilation (outside) air. Quantities of these air streams depend on requirements for the space served, and/or economizer strategies that limit the need for mechanical heating and cooling when outside air conditions are favorable. In cases where the amount of outside air varies for reasons of space pressure control, ventilation requirements or energy strategy, some or all return air may be exhausted to the outside by the air-handler. Most commonly, air is removed by modulating relief & return dampers on the high pressure side of the return fan. Generally, these dampers operate in conjunction with the outside air dampers also located in or near the air-handing unit.
  1. Types & Applications:
  2. Strategies for controlling airflow rates at the fans may include satisfying the following requirements:
  3. System static pressure in the duct
  4. “Polling” terminal devices to determine inlet damper position, and delivered CFM compared to zone demand at the terminal device
  5. Building pressure requirements
  6. Ventilation / Indoor Air Quality (IAQ) requirements (e.g. CO2 levels and other air pollutants)
  7. Operational schedules (e.g. Night Low Limit, Warm-up, Purge)
  8. Special Sequences of Operation (e.g. Lead/Lag/Standby, Fire Life Safety/Smoke control)
  9. Air-handlers may be classified according to the following2:
  10. Unit Type
  11. Blow-Through Air-handling Units - do not have ducted fan outlets,
  12. Draw-Through Air-handling Units - with ducted fan outlets.
  13. Application Type
  14. Indoor
  15. Outdoor
  16. Air-handlers may have return fan(s) as well as supply fan(s) and may also

have additional “equipment added for the purposes of control, isolation, safety, static pressure regain, … including coils, filters, dampers, air-mixers etc.”2 Air-Handler components may include the following[MSOffice4]:

  1. Air Inlet louver and screen
  2. Outside Air Dampers
  3. Mixing section for outside air and return air
  4. Filter section(s)
  5. Heating Coil
  6. Cooling Coil
  7. Humidifier (optional)
  8. Supply Fan
  9. Return Fan
  10. Relief/Exhaust Fans
  11. Relief Dampers
  12. Exhaust Air louver and screen
  13. Mixing Dampers
  1. A VAV Air Handling unit is capable of reacting to changes in the distributed airflow at the fan through any of the following means:
  2. Inlet vanes
  3. Scroll dampers
  4. Inlet cones or disks (variable effective wheel width)
  5. Variable blade pitch (Vane Axial Fans)
  6. Fan speed control
  1. Review Task: Design Phase Documentation Review Considerations
  2. Schematic Design Phase (SD’s)
  3. Review OPR and BOD with reference to proposed schematic design documents
  4. Design Development Phase (DD’s)
  5. Review OPR and BOD to identify if any updating is required in reference to design development documents
  6. Construction Document Phase (CD’s)
  7. Floorplans[MSOffice5]
  8. Access to controls and electrical components provided?
  9. Location of Static Pressure Control Devices identified?
  10. Return air path provided?
  11. Exhaust and Outside Air louver locations prohibit mixing or short circuiting?
  12. Location of Fire / Smoke dampers identified?
  13. Are maximum and minimum airflow rates for Supply, Return and Variable Exhaust systems called out for the areas served?
  14. Are the flowrates consistent with requirements for room, occupancy or floor pressure relationships?
  15. Equipment Schedule[MSOffice6]s:
  16. Each VAV Box individually scheduled? All CFM Setpoints scheduled (Max, Min, Htg)?
  17. AHU Air system capacities consistent with connected terminal devices, supply and return, provides for reasonable diversity?
  18. Are minimum outside air requirements identified?
  19. Are minimum ventilation requirements satisfied when the AHU is operating at Minimum CFM?
  20. Are face velocities of the Air-Handler components reasonable when the Unit is operating at Maximum CFM?
  21. AHU coil rated capacities are consistent with Basis of Design climate data, and zone control requirements?
  22. Are internal pressure drops at maximum CFM consistent with Fan rating and external pressure drop requirements?
  23. For volume control by Variable Frequency Drives, are max and min motor operating speeds within manufacture’s ratings for the corresponding fan RPM’s?
  24. Drawing Details[MSOffice7]
  25. Is the mixing section of the AHU adequate to prevent stratification?
  26. Are the coils selected such that entering water temperatures will not cause stratification, (# of rows, # of banks, # of coils/bank?
  27. Supply Piping shown at the coil bottom?
  28. Control valves shown on the return side of coil?
  29. Can piping be adequately vented as detailed?
  30. Piping P/T test ports shown?
  31. Balancing valve shown?
  32. Isolation valves shown?
  33. Are required load pumps detailed?
  34. If an auto flow control valve is used instead of a manual balancing valve, is a bypass for flushing shown?
  35. Specification[MSOffice8]
  36. Controls installation in field or factory?
  37. Are provisions made to interface factory installed controls to Building Automation System?
  1. Are substitutions permitted?
  1. Review Task: Construction Phase Documentation Review Considerations
  2. Equipment Submittal Review[MSOffice9]
  3. DDC and Electrical voltage coordinated?
  4. Capacities indicated match engineers design?
  5. Engineering data complete including fan curves and class limits?
  6. Equipment dimensions and weight within limits?
  7. Access provided is coordinated with design?
  8. Parts list Indicated?
  9. For VFD applications, motor rating approved for inverter duty use?
  10. For VFD applications, is shaft grounding is provided?
  11. Condensate pans and traps accounted for?
  12. Test ports shown?
  13. Pay close attention to options particularly if substitutions are in play
  14. O&M Manual Review [jh10]
  15. As-builts provided?
  16. Final installed sequences of operation provided?
  17. Maintenance and troubleshooting guide provided?
  18. Expected equipment life indicated?
  19. Required spare parts identified?
  1. Documentation Task: Cx Plan Development Specifics[jh11]
  2. Construction Phase Documentation
  3. System Verification Checklists required?
  4. Contractor sign-off or CxA sign-off?
  5. Manufacturers’ Start-up Sheets
  6. What equipment will be included?
  7. Are special factory startup services required?
  8. For VFD applications, does vendor representative have adequate information to program the drive? (over-speed allowed, minimum speed for motor, supply / return fan tracking capability, load limits, by-pass requirements, alternate source of supply [ATS, emergency power], interlocks with other equipment, timing and dynamic braking)
  9. Contractor Start-up Sheets
  10. What equipment will be included?
  11. Controls point-to-point checkoff?
  12. Coils piped for counter-flow?
  13. Control valve orientation verified?
  14. De-energized position correct?
  15. Hydronic systems Flushed and Vented?
  16. As-Built configuration matches detail?
  17. Provisions for filters used during final construction activities
  18. When does warranty period start, is extended warranty required?
  19. Commissioning Schedule completed?
  20. Master Schedule integration required?
  21. What are critical path Cx-related items?
  1. Functional Test Procedures:[jh12]
  2. Initial Conditions Recorded?
  3. Cooling Mode Testing
  4. Cooling simulated or actual? How?
  5. Read and record discharge air Wet and Dry Bulb temps?
  6. Read and record entering and leaving water temps?
  7. Heating Mode Testing Clear?
  8. Heating simulation or actual? How?
  9. Read and record discharge air temp?
  10. Read and record entering and leaving water temps?
  11. Manual Operation Verification
  12. Damper control Electric or Pnuematic
  13. Isolation
  14. De-engergized state
  15. Control of OSA, MOSA, Relief, Return Dampers.
  16. Coils and associated valving
  17. Isolation
  18. De-engergized state
  19. Freeze protection, manual reset?
  20. Hand Off Auto control of fans, can VFD be bypassed for across the line operation?
  21. For electric heating applications
  22. Disconnecting means
  23. High temp cutout
  24. Flow sensor
  25. Sensors and Gauges verified
  26. Pressure sensors and gauges
  27. High Limit
  28. Discharge pressure
  29. Drops across AHU components
  30. Filters
  31. Coils
  32. Freeze protection – type and response
  33. Temperature sensors
  34. Discharge air
  35. Mixed air
  36. Outside air
  37. Temperature / Pressure gauges
  38. Entering and leaving water for heating, cooling, heat recovery coils
  39. Steam Heat or Humidification
  40. Outside Air flow station, if used.
  41. Type?
  42. Calibrated?
  43. Reports accurate values throughout range of operation – minimum to maximum?
  44. Graphics Verification?
  45. Clear direction has been provided for required / acceptable graphics?
  46. Menu for easy movement in graphics?
  47. Table of multiple units or single unit graphic detail?
  1. Field Task: System Verification and Startup Specifics[jh13]
  2. MEP Contractors
  3. Installed Per Plans?
  4. Ductwork Complete?
  5. Flow stations?
  6. Terminal Devices Installed?
  7. Diffusers Installed?
  8. Piping Complete?
  9. Adequate Access?
  10. Air Available?
  11. Electrical Complete?
  12. MCC complete – Overcurrent protection, Safeties & Interlocks
  13. Phasing checked for all sources of supply
  14. Motors bumped
  15. Overload protection sized and installed/adjusted
  16. Disconnecting means / LOTO as required
  17. Controls Contractor
  18. Sensors Installed, calibrated?
  19. Flow stations – supply, return, OSA?
  20. Controllers and VFD Programmed?
  21. Dampers Stroked?
  22. Valves Stroked?
  23. BAS Communication to Graphics Available?
  24. TAB Contractor
  25. Preliminary TAB report available?
  1. Field Task: Functional & Performance Testing Specifics[jh14]
  2. Functional Testing
  3. Mock-up test established and completed?
  4. Test and balance (TAB) complete and report available?
  5. Sequence of Operation Testing Complete?
  6. Occupied Start
  7. Unoccupied
  8. Warm- up
  9. Night low limit
  10. Purge
  11. Heating?
  12. Cooling?
  13. Economizer
  14. Damper tracking
  15. Life Safety?
  16. Powerloss
  17. Duct pressure limits?
  18. Performance Testing
  19. Capacity Testing complete?
  20. Data trending
  21. Discharge temp
  22. Damper position
  23. Valve position
  24. Fan status
  25. CFM
  1. Review Task: Closeout Status (Final Commissioning Report?)[jh15]
  2. General
  3. Graphics complete? Provide screen captures for report.
  4. TAB report reviewed by EoR?
  5. Control setpoints documented?
  6. F&PT's complete?
  7. Test results acceptable?
  8. All Issues resolved?
  9. Unresolved issues documented with action plan?
  10. Archive of as-commissioned control database?
  1. Training[jh16]
  2. Review access requirements
  3. Include maintenance personnel during functional testing for at least one unit
  4. Identify important user adjustable set points and how the values were determined.
  5. Explain sequence of operations
  6. Explain how to setup and harvest trend data
  7. Explain how to make set point adjustments

1ASHRAE Terminology of Heating, Ventilation, Air Conditioning, & Refrigeration, 1991.

2Air-Conditioning & Refrigeration Institute, ARI Standard 430-1999.

[R1]Standard opening, can be deleted from individual apps.

[MSOffice2]A concise definition, using industry-standard publications such as ASHRAE, NETA, etc., that clearly defines the system (and subsystems, if applicable) being discussed in the app. It also defines the primary elements that comprise the system.

[R3]

[MSOffice4]Note: This list excludes elements of the Building Automation System (BAS), such as sensors.

[MSOffice5]Identify items for coordination, access, device location and consistency.

[MSOffice6]Questions to ask to determine if there is sufficient info provided and are values reasonable?

[MSOffice7]Suggestions for ensuring that drawing details show devices and components required for commissionability, arrangement and applicability, and maintenance of equipment.

[MSOffice8]Suggestions for items to review for completeness, detail, commissionability and maintainability.

[MSOffice9]Suggestions for items to look for in submittal review that confirm compliance with design documents. Second set of eyes on the submittal review being performed by the engineer.

[jh10]Suggestions for critical information such as required set points, sensors requiring periodic calibration, diagnostic indicators for required maintenance.

[jh11]Who is doing what? How will the responsibilities be presented into the contract documents? Responsibility matrix, construction startup sheets, Communication Org Chart.

[jh12]Think about what conditions the AHU will be operating in, and how the sequences of operation will be tested. How will conditions be simulated? What should be the response for associated components? How will these things be tested in the future to insure sensor calibration or program integrity? What elements / components should be retested to verify performance in the future? How often should it occur?

[jh13]Identify the roles and responsibilities for various contractors establishing that the equipment is ready to be energized / started up. Who is to coordinate these activities? What about the control of hazardous energies? LOTO.

Who owns the equipment / systems once they’ve been started? What about cleanliness? Filters, Ductwork, start of warranty period.

[jh14]Consider what is the difference between functional and performance testing for this system. How are data to be gathered? How are test results to be documented? What about negative results? Does the need to repair and retest invalidate other previously successful testing?

[jh15]How do you know when you are done? How are you going to assemble important information for this system and incorporate it into other commissioning documentation. Will the facilities staff be able to find and make use of this information? What about unresolved items, will they be revisited before the end of warranty?

[jh16]Remember the goal here is not just to turn over a building that operates as intended. It is also important to make sure the operators know how to keep it that way. Training on maintaining special systems, familiar systems that may have unique features or applications must be incorporated into the interactive training and archived materials. What elements are user adjustable, and what things should the operators stay away from?