This specification covers factory-assembled packaged and built-up central station air handling units (AHUs) for commercial, institutional, and industrial buildings. Equipment covered includes the casing, supply fan(s), return or relief fans where indicated, heating coils, cooling coils, energy recovery sections, filtration sections, mixing sections with outdoor/return/exhaust dampers, humidification provisions, access sections, drain pans, and vibration isolation systems. Both constant volume (CAV) and variable air volume (VAV) configurations are addressed.
A central station AHU is distinguished from unitary terminal equipment by its dependence on a field-installed duct distribution system to deliver conditioned air to occupied zones. The unit is typically served by a central chilled water and hot water plant — see Hydronic Piping and Hvac Pumps for the associated piping and pump systems. Equipment shall comply with ANSI/AHRI 430 for performance rating of supply fans, AHRI 1350 for casing mechanical performance, AHRI 410 for coil performance ratings, ASHRAE 52.2 for filtration, ASHRAE 62.1 for outdoor air design, ASHRAE 90.1 for fan power and energy efficiency, NFPA 90A for fire and smoke safety, and UL 60335-2-40 (or UL 1995 where still accepted by the Authority Having Jurisdiction) for electrical safety listing.
The Contractor shall coordinate AHU installation with structural support design, hydronic and refrigerant piping connections, ductwork connections, electrical power and control wiring, drain piping, and vibration isolation requirements established in this standard. Coordination with Building Automation System is required for controls integration. Fan speed regulation by variable frequency drives shall conform to Hvac Variable Frequency Drives.
Equipment, materials, and installation shall comply with the latest adopted edition of each of the following unless a specific edition is cited. Where conflicts exist between referenced standards, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
| Standard | Title |
|---|---|
| ANSI/AHRI 430 (I-P) | Performance Rating of Central Station Air-handling Unit Supply Fans |
| ANSI/AHRI 1350 (I-P) | Mechanical Performance Rating of Central Station Air-handling Unit Casings |
| ANSI/AHRI 410 | Performance Rating of Forced-Circulation Air-Cooling and Air-Heating Coils |
| ANSI/ASHRAE 51 / AMCA 210 | Laboratory Methods of Testing Fans for Certified Aerodynamic Performance Rating |
| ANSI/AMCA 300 | Reverberation Room Methods of Sound Testing of Fans |
| ANSI/AMCA 610 | Laboratory Methods of Testing Airflow Measurement Stations for Performance Rating |
| ANSI/ASHRAE 52.2 | Method of Testing General Ventilation Air-Cleaning Devices for Removal Efficiency by Particle Size |
| ANSI/ASHRAE 62.1 | Ventilation and Acceptable Indoor Air Quality |
| ANSI/ASHRAE/IES 90.1 | Energy Standard for Buildings Except Low-Rise Residential Buildings |
| NFPA 90A | Standard for the Installation of Air-Conditioning and Ventilating Systems |
| UL 60335-2-40 | Safety of Household and Similar Electrical Appliances — Particular Requirements for Heat Pumps, Air-Conditioners, and Dehumidifiers |
| UL 1995 | Heating and Cooling Equipment (where accepted by AHJ in lieu of UL 60335-2-40) |
| IMC (International Mechanical Code) | International Mechanical Code, adopted edition |
| SMACNA HVAC Duct Construction Standards | SMACNA HVAC Duct Construction Standards — Metal and Flexible |
| ASHRAE Handbooks | HVAC Systems and Equipment, Fundamentals |
Contractor shall submit the following for review and acceptance prior to ordering equipment. Fabrication and procurement shall not proceed until action submittals have been reviewed and returned.
Contractor shall provide the following at or before substantial completion:
AHUs shall be manufactured by a company with a minimum of ten years of continuous experience designing and fabricating central station air handling equipment. The manufacturer shall maintain an ISO 9001 certified quality management system and shall be capable of providing replacement parts and service support for a minimum of fifteen years after the date of manufacture.
For each air handling unit, the casing, fan(s), motor(s), coils, filters, dampers, and drain pan shall be provided by or through the air handling unit manufacturer as a coordinated factory assembly. Mixing components from multiple independent suppliers into a field-erected assembly is not acceptable without Engineer approval. This single-source requirement ensures that structural, thermal, acoustical, and hydraulic interactions between components are resolved at the factory, not in the field.
Supply fans in central station AHUs shall be rated under the AHRI Certification Program for Central Station Air-handling Unit Supply Fans per ANSI/AHRI 430. Rated fan performance shall have been verified by an independent third-party laboratory contracted by AHRI. Manufacturer's published fan curves and published performance data shall bear the AHRI certification mark.
Forced-circulation coils shall be rated under the AHRI Certification Program for Forced-Circulation Air-Cooling and Air-Heating Coils per ANSI/AHRI 410. Published coil capacities, pressure drops, and selection data shall reflect AHRI-certified ratings.
Where the manufacturer participates in the AHRI Central Station Air-Handling Unit Casing Performance Certification Program (AHUC) under AHRI 1350, casing leakage, deflection, and thermal performance ratings shall be certified. Certified casing ratings shall be documented in the submittal.
The complete AHU assembly, including all electrical components, shall be listed and labeled by a Nationally Recognized Testing Laboratory (NRTL) to UL 60335-2-40. Where the AHJ continues to accept UL 1995 listings for equipment manufactured prior to the transition date, UL 1995-listed equipment is acceptable. Electrical components not covered by the unit listing shall be individually listed to applicable UL or equivalent standards.
A pre-installation conference shall be held before beginning AHU installation, attended by the Contractor, the mechanical sub-contractor, the controls sub-contractor, the Testing, Adjusting, and Balancing (TAB) agent, and the Owner's representative. Agenda shall include unit rigging and setting sequence, utility connections, vibration isolation procedure, controls interface points, and the commissioning schedule.
AHUs shall be selected and rated for the anticipated operating conditions at the installation site. The design conditions are as indicated on the mechanical schedules and psychrometric process diagrams. The equipment shall be capable of operating within the following envelope without damage, derating, or loss of function.
AHUs installed indoors in climate-controlled mechanical rooms shall be suitable for ambient temperatures from 40°F to 104°F (4°C to 40°C) and relative humidity from 10% to 95% non-condensing. Outdoor and rooftop units shall be rated for the climate zone conditions applicable to the project location, including design winter ambient and design summer ambient as established by ASHRAE Fundamentals.
Outdoor units shall be provided with a factory-applied weatherproof casing meeting the requirements of this standard including pitched top panels or weather hoods over all openings, bird screens on air intakes and exhausts, drainable base rails, and all hardware in stainless steel or zinc-dichromate-treated steel.
Where the project site is at an elevation above 3,000 ft (914 m), fan airflow, coil capacity, and filter resistance data shall be corrected for air density at the project altitude. Corrections shall be based on the project elevation and documented in the submittal.
Supply airflow, return airflow (where applicable), and design external static pressure (ESP) shall be as indicated on the mechanical schedules. Fan selection shall be made at the specified design point with a minimum 10% static pressure safety factor applied to the calculated system resistance unless the design documents specify otherwise.
The fan system configuration determines how air is moved through the unit. Housed centrifugal fans with forward-curved, backward-curved, or airfoil blades are appropriate for ducted systems with moderate to high static pressure. Plenum fans (plug/plug-array) eliminate the fan scroll and discharge duct connection, providing a large plenum from which air distributes through the unit; plenum fans are commonly used in large central station AHUs where flexibility of downstream distribution is needed. Fan arrays (multiple smaller fans in parallel) provide redundancy and reduced sound levels compared to a single large fan.
A return fan tracks supply fan airflow and is appropriate where building pressurization must be precisely controlled and where the return duct system has significant resistance. A relief fan exhausts air from the building when the economizer is in full free-cooling mode to maintain building pressure. In many commercial applications neither a return nor relief fan is required when the building is provided with transfer air paths and relief dampers.
Fan efficiency shall comply with ASHRAE 90.1 Section 6.5.3.1. Fan system brake horsepower at design conditions shall not exceed the allowable value from ASHRAE 90.1 Table 6.5.3.1-1 for the applicable fan system type and design flow rate. All fan selections shall document compliance with this requirement in the submittal.
Fan static efficiency (FSE) at the design operating point shall be a minimum of 60% for housed airfoil and backward-inclined centrifugal fans, and a minimum of 55% for plenum fans. Forward-curved fans are acceptable only for fan systems below 5,000 CFM where their lower efficiency is acceptable given reduced system size. This efficiency requirement encourages backward-curved airfoil blade fans, which provide superior efficiency and non-overloading power characteristics compared to forward-curved wheels, and reduces the risk of motor overload as system resistance decreases with filter loading.
For VAV systems, the fan shall be capable of stable operation from 100% design airflow down to the minimum required flow (typically 30–40% of design). Variable frequency drives (VFDs) shall be provided for all fan motors 1 HP and larger on VAV systems per ASHRAE 90.1 Section 6.5.3.2. Static pressure control strategy (duct static pressure reset, supply air temperature reset) shall be coordinated with the Building Automation System.
Inlet guide vanes are an older technology with higher installed cost and mechanical complexity compared to VFDs and should be used only where the Engineer has a specific technical justification. VFDs provide superior energy performance at part-load conditions and are the standard selection for new construction.
AHU casings shall be double-wall construction for all units with a cooling coil or in any application where the casing panels are subject to condensation on either side. Single-wall construction is acceptable only for heating-only service in dry conditions. Double-wall panels eliminate condensation on the exterior surface of the casing that would otherwise occur when supplying cooled or dehumidified air, and they prevent cold-bridging at panel edges and fasteners.
Exterior panel skins shall be minimum 18-gauge (1.2 mm) galvanized steel meeting ASTM A653 with G90 zinc coating. Interior panel skins that may be wetted by condensate in the cooling and cooling coil sections shall be stainless steel, galvanized steel with a factory-applied interior coating, or aluminum. Galvanized steel interior skins shall not be used without a factory-applied coating resistant to the condensate pH range of 5 to 9.
Casing air leakage rate shall comply with AHRI 1350. Casing leakage class shall be selected based on operating pressure and energy classification. Excessive casing leakage wastes energy in supply sections (conditioned supply air leaking to the mechanical room) and causes infiltration of unconditioned air into the return section (negating dehumidification accomplished in the cooling coil). For units operating above 4 in. w.g. internal static pressure, tighter leakage classes are required to prevent significant efficiency degradation.
Panel deflection under operating pressure shall comply with AHRI 1350. Excessive panel deflection degrades seals, allows door gaps to open, and indicates inadequate structural stiffness that may cause panel-to-panel joint leakage over time.
Access doors shall be provided at each major component section: fan section, filter sections, coil sections, mixing box, drain pan, and humidifier where provided. Minimum door clear opening shall be 18 in. wide by 18 in. tall for maintenance access; where component replacement (coil pull, fan wheel replacement) is required through the door, the opening shall be sized accordingly. Doors shall be hinged, gasketed, and equipped with cam-type latches that open with a standard tool. Door hinges shall be stainless steel or zinc-plated steel.
Access doors within systems handling classified air (e.g., laboratory exhaust, healthcare critical spaces) shall include provisions for viewing the interior without entering the airstream; this may be accomplished by a gasketed glass port or by a sealed transparent panel in the door.
Interior panel liners shall be attached so that fasteners or standoffs penetrating the insulation layer do not create thermal bridges at the inner surface. All exposed fasteners on interior surfaces that may contact the airstream shall be stainless steel. The liner system shall be rated for the velocity and turbulence of the airstream so that individual liner panels or facing materials do not become dislodged during normal operation, including any transient high-velocity conditions during filter change or access door operation.
Exterior panels of indoor units shall receive a manufacturer's standard electrostatically applied powder coat finish in a color as selected by the Owner. Outdoor units shall receive a minimum two-coat system consisting of a corrosion-inhibiting epoxy primer and a UV-resistant polyester powder coat topcoat; minimum total dry film thickness shall be 3 mils. All cut edges, fasteners, and casing penetrations on outdoor units shall receive zinc-rich touch-up coating prior to final assembly.
Fan sections shall be structurally isolated from the casing by flexible fan-section isolators or by mounting the fan assembly on internal spring isolators within the casing, so that fan vibration is not transmitted to the casing panels. Fan wheel, housing, shaft, and bearings shall be factory assembled and dynamically balanced per AMCA standards before shipment. Fans shall be tested and rated per ANSI/ASHRAE 51 / ANSI/AMCA 210.
Fan wheels shall be fabricated from steel or aluminum and shall be finish-coated to resist corrosion in the expected airstream conditions. Where the airstream contains elevated moisture, corrosive gases, or particulates, fan wheel and housing materials and coating shall be appropriate for the specific conditions. Consult the manufacturer's material selection guide for specific environments.
Fan shaft bearings shall be heavy-duty ball or roller type, selected for a minimum L10 bearing life of 200,000 hours at design operating conditions per AFBMA standards. Bearings shall be regreasable with grease fittings extended to the exterior of the fan section for accessibility without opening the casing during routine maintenance. Sealed, non-regreasable bearings are acceptable only for direct-drive plenum fans with motors integral to the fan assembly where the motor bearings meet the L10 life requirement.
Where belt-driven fans are specified, sheaves and belts shall be V-belt or synchronous belt type. V-belt drive systems shall include adjustable motor base for belt tensioning and sheave alignment. Sheaves shall be sized for the design fan speed with capacity to reduce or increase speed by changing sheaves; the motor and fan sheave combination shall be indicated in the submittal and shall be verified to produce the specified fan speed within ±5% without exceeding the motor nameplate horsepower under any anticipated system operating condition.
Where variable speed is required on belt-driven fans, the combination of belt drive ratio and VFD frequency range shall be selected so that the fan operates at the required airflow range without exceeding the motor service factor or the fan manufacturer's maximum allowable speed.
Fan motors shall be premium efficiency, totally enclosed fan-cooled (TEFC) or open drip-proof (ODP) for indoor installations, inverter-duty rated for variable speed applications per NEMA MG1 Part 31. Motor voltage and phase shall match the electrical service at the unit. Inverter-duty motors are required for all VFD applications to withstand the voltage spikes and harmonic content of the inverter output; standard motors may fail prematurely when operated on VFD power.
Motor horsepower shall be as indicated on the mechanical schedules. Motor service factor shall be a minimum of 1.15 for belt-driven fans and 1.0 for direct-drive fans selected at or below the nameplate horsepower. Motors shall be selected so that the nameplate horsepower is not exceeded at any point on the fan curve within the normal operating range.
VFDs for AHU fan motors shall conform to Hvac Variable Frequency Drives. Where VFDs are factory-mounted in the AHU electrical control panel, the panel location shall be outside the airstream and accessible without interrupting unit operation. VFD bypass capability (manual or automatic) shall be provided where indicated on the drawings.
Fan sound power levels shall be documented at the design operating point per ANSI/AMCA 300 in each of the eight octave bands from 63 Hz to 8,000 Hz. Inlet and outlet radiation levels shall be reported separately. Where the AHU is adjacent to acoustically sensitive spaces, the design team shall perform a sound transmission analysis using the submitted octave band data, the duct attenuation path, and the room correction to confirm that the resulting room noise criterion (NC) complies with the project acoustic requirements.
Coils shall be factory-installed in the AHU casing with proper tube orientation (water supply and return connections at the same end of the unit for counterflow arrangement), adequate drain connections at the lowest point of each coil circuit, and vent connections at the highest point. Coils shall be removable from the unit without cutting tubes or disassembling adjacent sections; coil pull-out clearance shall be shown on the submittal drawings.
All coils shall be rated and certified per ANSI/AHRI 410. Published ratings shall reflect the actual installed conditions (face velocity, entering conditions, fluid flow rate) and not generic catalog selections. Submittal shall include AHRI-certified selection data.
Fin material selection matters in coastal and chemically aggressive environments. Aluminum fins in salt-laden air corrode over time, reducing heat transfer and increasing airside pressure drop. Phenolic-coated aluminum fins or copper fins provide substantially longer service life at modest added cost and are recommended for installations within approximately 3 miles of a saltwater coast or in industrial air environments.
Cooling coils shall be hydronic chilled water type unless the contract documents indicate a direct-expansion (DX) refrigerant coil. Chilled water coils shall be designed for counterflow water circuit arrangement (water enters the leaving air end of the coil, exits the entering air end) to maximize log-mean temperature difference and minimize required coil surface.
Face velocity for cooling coils shall not exceed 550 FPM at design conditions to control moisture carryover. A maximum face velocity of 500 FPM is recommended for standard applications; 450 FPM provides margin for moisture control and is appropriate where the unit handles high sensible loads with relatively little latent cooling. Exceeding 550 FPM reliably causes water carryover downstream regardless of drain pan depth.
Chilled water coils shall include factory-installed manual air vent and drain valve at the coil header connections. All chilled water coil connections shall exit the casing through properly gasketed, insulated penetrations to prevent casing sweating at the pipe stub-outs. See Hydronic Piping for piping connection requirements.
Heating coils shall be hot water hydronic type unless the contract documents indicate electric or steam. Hot water coils shall be single- or multi-row as required by the design conditions.
Where steam heating coils are used, they shall be the distributing (non-freeze) type with a steam trap and float-and-thermostatic condensate return on every circuit. Non-distributing steam coils freeze and fail catastrophically when low steam pressure allows coil condensate to collect; distributing coils circulate steam to every tube regardless of load and are the only safe choice in freezing climates.
Where heat recovery is indicated on the drawings, the AHU shall include a runaround coil loop (glycol coils in supply and exhaust airstreams connected by a pumped circuit) or a heat wheel (rotary energy recovery wheel). Heat wheels shall be the aluminum-media or silica-gel-coated (total energy) type, selected for the project climate and application. Where cross-contamination between exhaust and supply airstreams is a concern (healthcare, laboratory), heat wheels shall be the sensible-only type with a purge sector, or runaround coil loops shall be used instead.
Air filtration shall comply with ASHRAE 52.2. Filtration requirements are determined by the application, the occupancy type, and any energy code or green building certification requirements. At minimum, all supply air to occupied spaces shall pass through filtration with MERV 8 efficiency upstream of the cooling coil. Pre-filtration upstream of higher-efficiency final filters extends filter life by removing larger particles before they load the primary filter.
MERV 13 is the current recommendation for general commercial and institutional buildings under ASHRAE 62.1 guidance for improved indoor air quality. MERV 13 filters capture the fine particulate range (PM2.5 aerosols and biological aerosols) that lower MERV filters pass. The increased airside pressure drop at MERV 13 compared to MERV 8 (typically 0.3 to 0.5 in. w.g. additional at final pressure) must be accounted for in fan selection and power calculations.
HEPA filtration requires separate sealed filter housings with integral bag-in/bag-out change-out provisions in healthcare and laboratory applications to protect maintenance personnel from contaminants captured in the filter. Standard slide-in filter frames are not acceptable for HEPA installations.
Filter housings shall be formed from galvanized steel with continuous, rigid filter-holding frames that prevent air bypass around the filter media. Frame-to-filter seal shall be achieved by positive contact pressure (spring-loaded or draw-bolt latch) and not by friction fit alone. Air bypass around filter media is the most common filtration deficiency in installed systems and results in effective MERV ratings well below the nominal rating of the filter media.
Filter housings shall permit filter change without tools. Filters shall be accessible from the side of the unit without entering the unit or removing adjacent sections. Where side access is not possible due to mechanical room constraints, access shall be from the front with full coil withdrawal clearance maintained. Filter change frequency and disposal requirements shall be documented in the O&M manuals.
Magnahelic or digital differential pressure gauges shall be provided across each filter section. Gauge ports shall be located upstream and downstream of the filter media. Gauge shall be externally visible without opening the unit. Where the BAS monitors filter differential pressure, pressure taps shall be provided for connection to a differential pressure transmitter.
Initial pressure drop and final (loaded) pressure drop (change-out pressure) for each filter section shall be submitted with the product data. Filter section shall be sized so that at the final pressure drop the total fan system static pressure does not exceed the fan curve at the minimum speed setpoint for VAV systems, or does not reduce airflow below design for CAV systems.
AHUs serving outdoor air ventilation requirements shall include a mixing box section with minimum three dampers: outdoor air (OA) damper, return air (RA) damper, and exhaust air (EA) damper or relief air (RA) damper. Damper blades shall be multi-blade, opposed-blade or parallel-blade type as appropriate for the control application. Opposed-blade dampers provide more linear flow characteristics across the damper operating range and are preferred for modulating applications.
The outdoor air damper shall be sized for the design maximum outdoor air quantity as required by ASHRAE 62.1 and shall be capable of modulating down to the design minimum outdoor air flow while maintaining proportional control. Minimum position stop or a separate minimum outdoor air damper shall be provided for accurate minimum outdoor air control. A minimum outdoor air damper sized only for the minimum required ventilation flow provides far better control authority at minimum position than a full-size OA damper modulated to a nearly closed position, because a large damper at a small opening has highly nonlinear and unstable flow characteristics.
An airflow measurement station in the outdoor air duct provides direct measurement of outdoor air quantity, enabling accurate compliance with ASHRAE 62.1 ventilation requirements regardless of system pressure variations. Damper position minimum stops without airflow measurement rely on calibrated conditions that drift as systems age and are not recommended where accurate minimum OA control is required.
All dampers shall be low-leakage type, rated at no more than 4 CFM per square foot at 1 in. w.g. differential pressure (AMCA Class II or better). For tight shut-off applications (outdoor air dampers in cold climates where freezing of standing water in the mixing box is a concern), AMCA Class I dampers (less than 2 CFM/ft² at 1 in. w.g.) shall be specified. Damper blade seals shall be extruded vinyl or silicone rated for the expected temperature range; felt seals shall not be used.
Damper actuators shall be direct-coupled, spring-return type for fail-safe operation. OA dampers shall fail closed (spring-return to closed) on loss of control signal or power to protect the unit from freeze damage. Return air dampers shall fail open to maintain airflow through the unit during the same failure condition.
Where an economizer is required by ASHRAE 90.1 Section 6.5.1 or by the applicable energy code, the AHU shall be capable of modulating the OA damper to 100% outdoor air for free cooling when outdoor air conditions are favorable. Economizer controls shall include a changeover control strategy (dry-bulb temperature, differential dry-bulb, enthalpy, or differential enthalpy) as specified.
Differential enthalpy control provides the widest range of economizer operation and the greatest energy savings in humid climates, but requires enthalpy sensors that require calibration and maintenance. Differential dry-bulb control is simpler and more reliable in dry climates. The design team shall select the changeover strategy appropriate for the climate zone and Owner's maintenance program.
A drain pan shall be installed below every cooling coil section, humidifier section, and any other section where condensate may form. The drain pan is one of the most common sources of Legionella growth in air handling equipment; a well-designed drain pan that fully drains after each operating cycle eliminates the stagnant water condition that promotes biological growth.
Drain pans shall be stainless steel (Type 304 minimum) for the section directly below the cooling coil, where condensate is most active. Painted or galvanized carbon steel pans are not acceptable below cooling coils because the coating is abraded by condensate drip impact and corrosion begins within a few years of service.
The drain pan shall extend beyond the coil face on the downstream (leaving air) side by a minimum of 2 in. for every foot of coil height, but not less than 6 in. The downstream extension captures condensate that is entrained in the airstream and blown past the coil before it falls. This is the single most common drain pan deficiency in field installations; an undersized pan allows condensate to reach the fan section and downstream ductwork, promoting biological growth throughout the distribution system.
Drain pan floors shall be sloped a minimum of 1/8 in. per foot toward the drain connection. Where the unit configuration results in a drain pan with multiple low points, individual drain connections shall be provided at each low point. Flat-bottomed drain pans that do not fully drain create the standing water conditions that promote biological contamination and are not acceptable.
The drain pan shall have a primary drain connection and a secondary overflow drain connection located 1 in. above the primary drain to indicate drain obstruction before overflow occurs. Drain connections shall be a minimum of 1 in. IPS or as required to handle the design condensate rate, whichever is greater.
A condensate trap shall be provided on the drain line. The trap seal depth shall be calculated based on the static pressure in the drain pan section. A common error is installing a trap sized for gravity drainage when the drain pan is located in a negative-pressure section; if the trap seal is shallower than the negative pressure head, the trap is emptied and unconditioned air (or sewer gas if connected to a drain with a P-trap) is drawn directly into the unit.
Minimum trap seal depth in inches shall equal the negative static pressure at the drain pan in inches w.g. plus 2 in. For typical cooling coil sections operating at -1.0 to -2.0 in. w.g., the trap must have a minimum seal depth of 3 to 4 in.
Fan sections shall be isolated from the casing structure to prevent transmission of fan vibration into the building structure. The isolation system shall be selected to achieve a minimum vibration isolation efficiency of 90% at the lowest fan operating speed for VAV systems, or at the fan design speed for CAV systems.
AHUs mounted on the ground floor slab shall be isolated from the slab by floor-mounted spring isolators. AHUs suspended from structural members above shall use hanger spring isolators. AHUs mounted on grade-level concrete pads do not require vibration isolation unless the pad is isolated from the building structure.
Spring isolators shall be selected for the minimum static deflection required to achieve the specified isolation efficiency. For fans operating at 600 to 1,200 RPM (typical belt-drive fan speed range), a minimum static deflection of 2 inches is required. For fans operating above 1,200 RPM (typical direct-drive plenum fan speed), 1.5 inches deflection is generally sufficient. Spring isolators shall be selected so that the installed operating load is within the load range for which the spring is rated; overloaded or underloaded springs may exhibit resonance or instability.
All connections to an isolated AHU — ductwork, piping, electrical conduit, and condensate drain — shall be made with flexible connections to prevent short-circuiting the isolation system. A single rigid connection from the isolated unit frame to the building structure transmits vibration as effectively as no isolation at all, negating the investment in isolators.
Supply and return ductwork shall connect to the AHU through flexible canvas or elastomeric connectors of minimum 6 in. clear length. Hydronic piping shall connect through flexible pipe connectors with a minimum of two flexible connectors in different planes. Electrical conduit shall use flexible conduit for a minimum of 18 in. at the unit. Condensate drain shall use a flexible coupling at the unit connection.
Where required by the applicable building code (IBC/ASCE 7) based on the seismic design category, AHUs shall be provided with seismic restraints. Seismic restraints shall be designed to accommodate the required seismic forces while allowing the isolators to continue functioning in normal operation.
AHUs shall be provided with factory-installed controls wiring and terminal strips for all field control connections. Factory wiring shall terminate at a central terminal block or junction box accessible without opening the main unit. All control terminations shall be labeled to match the control sequence documentation.
The AHU controls interface shall be compatible with the project building automation system (BAS) per Building Automation System. Contractor shall coordinate communications protocol and physical interface points with the BAS contractor prior to submittal.
At minimum, the AHU controls interface shall include the following points, all accessible at the control terminal block:
Smoke detectors in the supply and return air plenums of the AHU shall be provided and installed per NFPA 90A. Smoke detection shall initiate unit shutdown via the BAS or via a hardwired relay. Smoke detectors shall be listed for use in air handling ducts (duct-type detector). Detector sampling tubes shall be installed according to the detector manufacturer's instructions for the duct cross-section dimensions of the AHU plenum; an incorrectly sized sampling tube yields no meaningful sample and will not detect smoke.
A low-limit (freeze-stat) thermostat shall be installed in the mixed air section of every AHU that supplies outdoor air in climates where outdoor air temperatures drop below 35°F. The freeze-stat shall be a serpentine-element type covering the full cross-section of the mixed air plenum; single-point sensing thermostats are inadequate because cold outdoor air and warm return air stratify in the mixing section and a single point sensor may read the warm layer while the coil faces freezing temperatures. The freeze-stat shall be set to alarm at 38°F and shut down the unit at 35°F, close the outdoor air damper, and open the hot water coil valve to full heating.
Every AHU shall undergo the following factory tests before shipment. Test results shall be documented on a factory test report and submitted as a closeout submittal. Units that fail any test shall be corrected and retested; no unit shall be shipped until all tests pass.
Where witnessed factory testing is specified, the Contractor shall coordinate the factory test schedule with the Owner's representative and the Engineer of Record. The manufacturer shall provide a minimum of ten business days advance notice of test readiness and shall submit a proposed test procedure for review before scheduling the test.
For large central station AHUs (greater than 20,000 CFM), the factory run test shall operate the unit at design conditions for a minimum of four hours continuous operation, confirming bearing temperature rise, vibration levels at the fan section and casing, and motor ampere stability. Elevated bearing temperatures or increasing vibration during the run test indicate a dynamic balance problem that shall be corrected before shipment. Elevated bearing temperature is defined as a temperature rise greater than 40°F above ambient at any bearing housing.
AHUs shall be delivered to the site in the largest factory-assembled sections consistent with rigging access through the building. Verify all rigging paths (door openings, stairwells, mechanical room access) before ordering. Equipment shall be stored in a clean, dry, heated space. Where indoor storage is not available, units shall remain in manufacturer's protective shipping packaging and condensation heaters in electrical compartments shall be energized. Units stored outdoors for more than 30 days shall be inspected by the Contractor before installation for moisture intrusion, corrosion, and pest infestation; a written inspection report shall be provided to the Owner.
Fan assemblies, bearings, and belt-drive components shall be protected from construction dust and moisture during storage. Shaft openings shall be sealed. Fan assemblies shall not be stored in a position that places the shaft horizontal without proper temporary bearing support, as improperly stored fans develop flat spots on bearings.
AHUs shall be rigged and set in accordance with the manufacturer's installation instructions. Rigging lugs, base spreader bars, or other manufacturer-furnished rigging hardware shall be used; chain or cable shall not wrap around casing panels. After setting, verify unit is level (±1/8 in. over the unit length) and that drain pans slope to drain. Verify vibration isolators are loaded to within the manufacturer's design load range.
All temporary shipping braces, shipping bolts, and transit restraints shall be removed after setting and before operating. Shipping braces in fan assemblies, if not removed, will cause severe unbalance vibration and immediate bearing damage.
After assembly and before energizing, conduct the following checks:
Supply and return ductwork shall connect to the AHU through flexible canvas or elastomeric connectors per NFPA 90A and SMACNA requirements. Ductwork shall be independently supported so that duct weight is not transferred to the AHU casing. Connecting ductwork shall be installed after the AHU is set and leveled on its final isolators; connecting ductwork before the unit is on its isolators will transmit the duct weight and stress into the connection flanges.
Hydronic coil connections shall be made with flexible connectors and shutoff valves on supply and return lines at each coil. Coil connections shall not bear the weight of the piping system; all connecting piping shall be independently supported within 12 in. of the coil connection. After piping connections are made, coils shall be hydrostatically tested at 1.5 times the design working pressure (minimum 150 psig) for a minimum of four hours before the unit is operated. See Hydronic Piping for additional coil connection requirements.
Condensate drain connections shall include a union to allow the trap to be disassembled for cleaning. The drain line shall be run to the nearest floor drain with a slope of not less than 1/8 in. per foot; horizontal condensate drain runs shall not exceed 20 ft without a secondary trap or venting.
Startup shall be performed by a factory-trained representative of the AHU manufacturer. TAB shall be performed by an independent certified testing and balancing firm per Testing Adjusting And Balancing after startup is complete.
Initial startup sequence shall include:
The TAB agent shall measure and report supply airflow, return airflow, and outdoor airflow at design and minimum operating conditions. Airflow shall be within ±10% of design values after balancing. Fan speed, sheave size, or VFD frequency shall be adjusted as necessary to achieve design airflow at design static pressure. Final TAB report shall document as-built fan speed, airflow, static pressure, motor ampere draw, sheave sizes, and filter differential pressure at clean filter condition.
The AHU manufacturer shall warrant the equipment against defects in materials and workmanship for the following period from the date of substantial completion.
Warranty shall cover the complete assembly including casing, fan, motor, coils, filters, dampers, and drain pans. The manufacturer shall maintain service capability within the project region with factory-trained technicians available within 48 hours of notification during the warranty period.
Fan assemblies and motors shall carry a separate warranty of not less than the periods below:
Motor manufacturer's warranty shall be provided in addition to the AHU warranty and shall pass through to the Owner.
Factory-installed coils shall be warranted against leakage and defects for a minimum of five years from substantial completion. Coil warranty shall cover the coil tube and fin assembly; it does not cover damage from improper system water treatment, freeze damage resulting from improper system operation, or physical damage from tools or improper filter access.
The Contractor shall warrant the installation workmanship, including all field connections, sealing, vibration isolation, and associated items, for one year from the date of substantial completion.
Spare filters shall be stored in the mechanical room in sealed manufacturer's packaging labeled with the unit designation, filter MERV rating, and installation date of the installed set. The O&M manual shall include a filter maintenance schedule and recommended change frequency based on the initial and final pressure drop values from the submittal.
Each AHU shall be provided with a permanent identification nameplate on the outside of the casing. The nameplate shall be stainless steel or laminated phenolic, permanently attached (not adhesive-only). Nameplate shall include unit designation, manufacturer's model and serial number, design airflow (CFM), design external static pressure (in. w.g.), supply fan motor horsepower, electrical supply voltage and phase, and date of manufacture.
All access doors shall be labeled to indicate the component section accessible through each door (e.g., "FILTER SECTION — MERV 13," "COOLING COIL," "FAN SECTION — ROTATION CHECK BEFORE OPENING"). Labels shall be applied to the exterior surface of each door in characters visible from 5 ft away.
Electrical panel doors and control terminal boxes shall include a single-line wiring diagram permanently mounted inside the door. Control terminal strips shall be labeled to match the control sequence documentation. Filter section doors shall include a filter installation date tag holder for recording the date of each filter change.