The focus of October's "Back to Basics" and "HVACR Designer's Tips," will be the replacement of a 20,000-cfm supply air system (constant volume with hot water reheat) serving an office building. The November columns will continue with the ATC/FPT narrative on the replacement and retrofit of AHU-1 with a VAV system with terminal heating. December's focus will be on building and energy management and project closeout.

Helpful Hints

  • Both draw-through and blow-through AHUs are available for a designer to consider when replacing an existing AHU. This system is based on 20% minimum outdoor air, return/exhaust air fan, pre-filter, final filter, chilled water cooling coil, and supply air fan. Refer to the "Back To Basics" webpage at www.esmagazine.com for a list of past tests addressing various types of central air-handling systems.
  • Refer to the following ASHRAE handbooks and chapters for additional information relative to central AHU options:
  • 2004 ASHRAE Handbook-HVAC Systems and Equipment, Chapter 1, "HVAC System Analysis and Selection," and Chapter 2, "Building Air Distribution
  • Refer to 2003 ASHRAE Handbook-HVAC Applications, Chapter 3 for more detailed information on commercial and public buildings
  • For this test, the AHU-1 configuration will serve an office building 5½ days a week, and the system will be off during the unoccupied periods but will have a morning warm-up cycle during the heating season.

Design Phase: Month One
  • This month's test focuses on a conventional HVAC central air system with airside economizer and perimeter baseboard radiant heat as the HVAC source.
  • When replacing an AHU, always have a TAB firm document HVAC existing conditions (cfm, static pressures, etc.) By knowing the actual conditions, cooling loads, ventilation requirements, and potential expansion, the design team can optimize the AHU replacement based on historical data.
  • Another benefit of AHU replacement is the opportunity to optimize, based on current and future building system needs, the central air system's energy consumption as it pertains to the type of supply air system. Consideration can be given to an energy conservation retrofit such as VAV and enhanced space occupancy control based on occupancy sensors. It is important to note that when retrofitting to VAV, the supply air ductwork will probably require pressure tests, ductwork reinforcement, duct cleaning, and duct sealing to ensure optimum retrofit performance.
  • Some electrical utility companies will offer financial incentives to retrofit less energy-efficient air systems with high-efficiency systems.
  • For this application, the basis of design is to provide AHU-1 to serve an office building based on the following design criteria:
    • Summer outdoor temperature of 89°F db and 72° wb with the building occupied from 7 a.m. to 7 p.m., Monday through Friday, from 7 a.m. to 1 p.m. on Saturday, and unoccupied Sunday.
  • Temperature setpoints:
    • Occupied, A/C season: 76°
    • Occupied, heating season: 72°
    • Unoccupied, A/C season: 80°
    • Occupied, heating season: 60°
    • Mixed air: 55°
    • Supply air: 55° during A/C season and 65° during heating season
    • Chilled water supply is 45° and chilled water return is 58°
  • System static pressure (sp) control setpoint at furthest location in the AHU-1 system will be .5 sp
  • AHU-1 total sp will be 4 in. for clean filters and 5 in. for dirty filters
  • Refer to "HVACR Designer Tips" for the design review and equipment startup checklist which addresses the removal of AHU-1 and the addition of the new AHU-1.

Commissioning and TAB-Design Phase
  • During the design phase, the commissioning engineer should review the DID, as well as design review for design intent compliance.
  • Also during the design phase, the commissioning engineer will write the commissioning plan and incorporate it into Division 18000 of the contract specification along with draft PFPT checklists and draft FPT narratives.
  • The commissioning engineer and TAB engineer should review the design documents for accessibility to equipment, as well as for commissionability.
  • The commissioning team for this month's application should include the commissioning engineer, the owner's project manager, facility engineer, and the TAB engineer.
  • If the replacement project is a D-B job, then the design-builder, the HVAC contractor, the BAS contractor, and electrical contractor should also participate in the design phase as well.
  • The TAB engineer should provide existing operating conditions (gpm, pressure drops, and fan curve data) to the design team, as well as write the TAB specification requirements to be inserted in Division 18000 with the commissioning process.


1. CONFIRM AHU-1 OPERATION PRIOR TO THE OLD UNIT BEING MECHANICALLY AND ELECTRICALLY DISCONNECTED BY PLACING AHU-1 IN THE ‘OFF' MODE OF OPERATION

ACTION:The commissioning engineer will confirm the fail-safe positions of the control components. The REACTIONwill be:
  • Outdoor air minimum damper D-1A will (close, open, modulate).
  • Outdoor air maximum damper D-1B will (close, open, modulate).
  • Return air damper D-2 will (close, open, modulate).
  • Exhaust air damper D-3 will (close, open, modulate).
  • Cooling coil valve V-1 will (close, open, modulate).
  • Supply air fan SAF-1 will be (off, on, modulating).
  • Return/exhaust air fan RAF-1 will be (off, on, modulating).
  • Smoke damper SD-1 will (close, open, modulate).
  • Discharge air temperature transmitter TT-1 will indicate (55°F, 65°F, no signal).
  • Mixed air temperature transmitter TT-2 will indicate (55°, 65°, no signal).



2. CONFIRM AHU-1 OPERATION PRIOR TO THE OLD UNIT BEING MECHANICALLY AND ELECTRICALLY DISCONNECTED BY PLACING AHU-1 IN THE ‘ON-MAXIMUM HEATING' MODE OF OPERATION

ACTION:The commissioning engineer will confirm the operating positions of the control components. TheREACTIONwill be:
  • D-1A will (close, open, modulate).
  • D-1B will (close, open, modulate).
  • D-2 will (close, open, modulate).
  • D-3 will (close, open, modulate).
  • V-1 will (close, open, modulate).
  • SAF-1 will be (off, on, modulating).
  • RAF-1 will be (off, on, modulating).
  • SD-1 will (close, open, modulate).
  • TT-1 will indicate (55°, 65°, no signal).
  • TT-2 will indicate (55°, 65°, no signal).


3. CONFIRM AHU-1 OPERATION PRIOR TO THE OLD UNIT BEING MECHANICALLY AND ELECTRICALLY DISCONNECTED BY PLACING AHU-1 IN THE ‘ON-MAXIMUM COOLING' MODE OF OPERATION

ACTION:<.b> The commissioning engineer will confirm the operating positions of the control components. The REACTION will be:
  • D-1A will (close, open, modulate).
  • D-1B will (close, open, modulate).
  • D-2 will (close, open, modulate).
  • D-3 will (close, open, modulate).
  • V-1 will (close, open, modulate).
  • SAF-1 will be (off, on, modulating).
  • RAF-1 will be (off, on, modulating).
  • SD-1 will (close, open, modulate).
  • TT-1 will indicate (55°, 65°, no signal).
  • TT-2 will indicate (55°, 65°, no signal).


4. CONFIRM AHU-1 OPERATION PRIOR TO THE OLD UNIT BEING MECHANICALLY AND ELECTRICALLY DISCONNECTED BY PLACING AHU-1 IN THE ‘ON-AIRSIDE ECONOMIZER' MODE OF OPERATION

ACTION: The commissioning engineer will confirm the operating positions of the control components. The REACTION will be:
  • D-1A will (close, open, modulate).
  • D-1B will (close, open, modulate).
  • D-2 will (close, open, modulate).
  • D-3 will (close, open, modulate).
  • V-1 will (close, open, modulate).
  • SAF-1 will be (off, on, modulating).
  • RAF-1 will be (off, on, modulating).
  • SD-1 will (close, open, modulate).
  • TT-1 will indicate (55°, 65°, no signal).
  • TT-2 will indicate (55°, 65°, no signal).


5. CONFIRM AHU-1 AIR AND WATER BALANCE PRIOR TO THE OLD UNIT BEING MECHANICALLY AND ELECTRICALLY DISCONNECTED BY PLACING AHU-1 IN THE ‘ON' MODE OF OPERATION

ACTION: The TAB engineer will confirm the operating parameters by having the BAS computer command unit to maximum flows. The REACTION will be:
  • D-1A reading will indicate (minimum and maximum cfm, minimum cfm, maximum cfm).
  • D-1B reading will indicate (minimum and maximum cfm, minimum cfm, maximum cfm).
  • D-2 reading will indicate (minimum and maximum cfm, minimum cfm, maximum cfm).
  • D-3 reading will indicate (minimum and maximum cfm, minimum cfm, maximum cfm).
  • Cooling coil valve reading will indicate (gpm, pressure drop, gpm and pressure drop).
  • Cooling coil reading will indicate (gpm, pressure drop, gpm and pressure drop).
  • SAF-1 reading will indicate (cfm, rpm, static pressure, all three).
  • RAF-1 reading will indicate (cfm, rpm, sp, all three).
  • SD-1 reading will indicate (minimum and maximum cfm, minimum cfm, maximum cfm).
  • Prefilter and final filter reading will indicate (cfm, pressure drop, cfm and pressure drop).
  • SAF-1 motor reading will indicate (rated amps and voltage, actual amps and voltage, both rated and actual).
  • RAF-1 reading will indicate (rated amps and voltage, actual amps and voltage, both rated and actual).


Log on to www.buildingsmartsoftware.com for the complete Commissioning 1-2-3 process and also refer to this month's "HVACR Designer Tips" for the central equipment design review and equipment startup checklist for additional information. If you have any comments, suggestions, or questions regarding the AHU Replacement - Design Test, please contact Howard McKew, P.E., C.P.E, by e-mail at hmckew@RDKengineers.com.

ANSWERS:

1) close; close; open; close; close; off; off; close; no signal; no signal; 2) open; close; open; close; close; on; on; open; 65°; 55°; 3) open; close; open; close; open; on; on; open; 55°; 55°; 4) open; open; close; open; modulate; on; on; open; 55°; 55°; 5) minimum cfm; maximum cfm; minimum and maximum cfm; maximum cfm; gpm and pressure drop; gpm and pressure drop; all three; all three; maximum cfm; cfm and pressure drop; both rated and actual; both rated and actual.