1 Scope
NOTE This standard covers the materials, construction, performance, testing, installation, and commissioning of factory-fabricated air terminal units that meter primary supply air to occupied zones and, where indicated, provide local reheat or fan-powered induction of plenum return air. (1.1)
NOTE Equipment covered includes single-duct VAV units, parallel fan-powered boxes (PFPB), series fan-powered boxes (SFPB), dual-duct units, and constant-volume reheat units, together with their primary airflow sensors, dampers, reheat coils, fan assemblies, casing and attenuator construction, valves and actuators, and integral DDC controllers. (1.2)
NOTE Air terminal units are the points at which a central air system meets each thermal zone; their selection determines minimum ventilation airflow at part-load conditions, zone-level acoustical performance, and the energy used by reheat and fan-powered induction. (1.3)
NOTE Undersized inlets create excessive pressure drop and starve the zone of design airflow; oversized inlets put the airflow sensor below its low-flow accuracy threshold and result in poor minimum-ventilation control. (1.4)
1.5 All air terminal units shall be rated and certified under the AHRI Certification Program for Air Terminals per ANSI/AHRI 880, with sound performance rated per ANSI/AHRI 885 and performance testing per ANSI/ASHRAE 130.
NOTE This standard establishes the performance, construction, and control requirements needed to deliver design airflow within accuracy limits, to comply with ASHRAE 62.1 minimum ventilation rates, and to meet the reheat and fan-power limits of ASHRAE 90.1. (1.6)
1.7 Coordinate primary inlet sizing with the upstream duct system specified in Hvac Ductwork. 1.12 Coordinate post-installation airflow verification with Testing Adjusting And Balancing. 2 Referenced Standards
2.1 Equipment, materials, and installation shall comply with the latest adopted edition of each standard below unless a specific edition is referenced by contract documents or by the local building code.
2.2 Where conflicts exist between referenced standards, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
2.3 Referenced standards list
| Standard |
Title |
| ANSI/AHRI 880 |
Performance Rating of Air Terminals |
| ANSI/AHRI 885 |
Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets |
| ANSI/ASHRAE 130 |
Methods of Testing Air Terminal Units |
| ANSI/ASHRAE/IES 90.1 |
Energy Standard for Buildings Except Low-Rise Residential Buildings |
| ANSI/ASHRAE 62.1 |
Ventilation and Acceptable Indoor Air Quality |
| NFPA 90A |
Standard for the Installation of Air-Conditioning and Ventilating Systems |
| NFPA 70 (NEC) |
National Electrical Code, Article 424 (Fixed Electric Space-Heating Equipment) |
| UL 1995 |
Heating and Cooling Equipment (where accepted by AHJ for legacy listings) |
| UL 60335-2-40 |
Safety of Household and Similar Electrical Appliances — Particular Requirements for Electrically Operated Heat Pumps, Air-Conditioners, and Dehumidifiers |
| UL 181 |
Standard for Factory-Made Air Ducts and Air Connectors (terminal unit casing liner) |
| ASTM E84 |
Standard Test Method for Surface Burning Characteristics of Building Materials |
| ASTM C1071 |
Standard Specification for Fibrous Glass Duct Lining Insulation (Thermal and Sound Absorbing Material) |
| SMACNA HVAC Duct Construction Standards |
HVAC Duct Construction Standards — Metal and Flexible |
| ASHRAE Handbooks |
HVAC Systems and Equipment; Applications |
3 Submittals
3.1 Action Submittals
3.1.1 The Contractor shall submit the following for the Engineer's review and return prior to fabrication or procurement of the air terminal units.
3.1.2 No fabrication or shipment shall proceed until the associated submittal has been reviewed and returned with no outstanding engineering questions.
3.1.3 Submit the following action submittals:
- Product data and configuration schedule for each unit tag, including inlet size, primary maximum and minimum airflow, casing dimensions, weight, reheat coil capacity and rows, fan motor type and rating, controller make and protocol, and outlet configuration
- AHRI 880 certified performance data for each unit configuration, including primary airflow accuracy, casing leakage, casing pressure drop, induction ratio for fan-powered units, and reheat coil performance
- AHRI 885 sound power data at the selected primary airflow and inlet static pressure, in octave bands 2 through 7 (125 Hz through 4,000 Hz), separated into radiated and discharge paths; include the NC calculation for each scheduled zone using the project room absorption and distance
- Damper assembly product data including blade material, shaft and bearing type, blade and frame seal materials, close-off leakage at design close-off pressure, and minimum and maximum operating differential pressure
- Primary inlet airflow sensor product data including the sensor type (multi-point center-averaging or equivalent), the published accuracy (percent of reading) across the operating range, and the manufacturer's required upstream and downstream straight-duct lengths for rated accuracy
- Reheat coil product data including row count, fin spacing, tube material, fin material, working pressure, design hot water entering and leaving temperatures, design air entering and leaving temperatures, and waterside pressure drop at design flow
- Electric reheat coil product data including stage count or SCR control range, kW rating per stage, sheath material, NEC Article 424 compliance documentation, primary and backup over-temperature protection, and airflow proving switch information
- Control valve and actuator product data including Cv, close-off rating, fail-safe position, modulating signal type, and stroke time
- Fan motor data for fan-powered boxes including motor type (ECM or PSC), nameplate power, efficiency, speed range, and ECM controller communication
- DDC controller product data including communication protocol, hardwired input and output count, supported control sequences, and BAS network topology
- Acoustical attenuator and casing liner construction data including liner thickness, ASTM C1071 compliance, ASTM E84 flame-spread and smoke-developed indices, and UL 181 listing
- Shop drawings showing each unit in plan and section with all field connection points dimensioned (primary inlet, induced air opening for fan-powered units, outlet duct collars, hot water supply and return, electric service, control wiring, condensate where applicable), required service clearances, and access panel locations
☑ Product data and configuration schedule per tag
☐ AHRI 880 certified performance data
☐ AHRI 885 sound data (radiated and discharge, octave bands)
☐ Damper assembly product data and close-off leakage
☐ Primary inlet airflow sensor data with accuracy and straight-duct requirements
☐ Hydronic reheat coil data (rows, fin spacing, pressure drop)
☐ Electric reheat coil data (stages, sheath, NEC 424 compliance)
☐ Control valve and actuator data
☐ Fan motor data (ECM or PSC) for fan-powered units
☐ DDC controller data and BAS protocol
☐ Casing liner / attenuator construction (UL 181, ASTM E84)
☐ Shop drawings with clearances and field connections
3.2 Closeout Submittals
3.2.1 At substantial completion the Contractor shall provide the following closeout submittals:
- Operation and maintenance manuals including manufacturer's installation, operation, and maintenance instructions; controller programming reference; troubleshooting guide; and recommended preventive maintenance schedule
- As-built configuration drawings reflecting any field modifications to the submitted configuration
- Factory test reports for each unit, including the primary airflow sensor calibration curve where the sensor is factory-calibrated to the specific unit
- Field commissioning records, including the airflow verification at minimum and maximum airflow setpoints, the reheat valve or stage stroke verification, and the BAS point verification for each unit
- Controller parameter file in the manufacturer's native format for each unit, listing the as-commissioned setpoints, calibration constants, and control loop tuning parameters
- Warranty documentation from the air terminal unit manufacturer and from any separately warranted sub-supplier components (ECM motor, DDC controller, electric heater)
☑ Operation and maintenance manuals
☑ As-built configuration drawings
☑ Factory test reports for each unit
☑ Field commissioning records
☑ Controller parameter file (native format)
☑ Warranty documentation
4 Quality Assurance
4.1 Manufacturer Qualifications
4.1.1 Air terminal units shall be the product of a single manufacturer with a minimum of ten years of continuous experience designing and producing factory-fabricated air terminal units for commercial HVAC service.
4.1.2 The manufacturer shall maintain an ISO 9001 certified quality management system.
4.1.3 The manufacturer shall be capable of providing replacement parts and service support for a minimum of fifteen years after the date of manufacture.
4.2 Single-Source Responsibility
4.2.1 All air terminal units on the project shall be furnished by a single manufacturer to maintain consistent control interfaces, consistent service-access patterns, and a single source for spare parts.
4.2.2 Where DDC controllers are factory-installed and factory-tested with the terminal unit, the controller shall be installed and warranted by the air terminal unit manufacturer.
4.2.3 Field-mounted controllers from a separate controls contractor are acceptable only where the contract documents specifically permit, and shall be furnished, mounted, and wired before the unit is started.
4.2.4 Controller furnish and install responsibility datasheet
● Factory-installed and factory-tested by ATU manufacturer (standard)
○ Factory-installed by ATU manufacturer; programmed by controls contractor at startup
○ Field-mounted by controls contractor (shipped loose by ATU manufacturer)
4.3 AHRI 880 Certification
4.3.1 Each air terminal unit configuration furnished on the project shall be rated under the AHRI Certification Program for Air Terminals (CP-ATU) per ANSI/AHRI 880.
4.3.2 Published primary airflow ratings, casing leakage ratings, sound ratings (when combined with AHRI 885), reheat coil capacities, and pressure drop data shall reflect AHRI-certified values.
4.3.3 The certification mark shall appear on the product data submitted with the unit schedule.
4.3.4 Equipment not certified under AHRI 880 is not acceptable.
4.3.5 AHRI certification datasheet
● AHRI 880 certified — all unit configurations on project
○ AHRI 880 certified — manufacturer participates and configuration is within certified range
4.4 NRTL Listing
4.4.1 Air terminal units containing electric reheat coils, fan motors, or electrically powered controls shall be listed and labeled by a Nationally Recognized Testing Laboratory (NRTL) to UL 60335-2-40, or to UL 1995 where the AHJ continues to accept legacy listings for equipment manufactured prior to the transition date.
4.4.2 Electric reheat coils shall additionally comply with NFPA 70 Article 424 for fixed electric space-heating equipment.
4.4.3 Listed assemblies shall bear the NRTL label affixed to the casing in a visible location.
4.5 Pre-Installation Conference
4.5.1 Before installation of air terminal units begins, the Contractor shall hold a pre-installation conference attended by the mechanical sub-contractor, the controls sub-contractor, the electrical sub-contractor where electric reheat is provided, the testing and balancing agent, and the Owner's commissioning representative where one is engaged.
4.5.2 The pre-installation conference agenda shall include unit access and service-clearance requirements, sequencing of duct connections relative to controller calibration, hot water piping and electric reheat connection sequencing, and the schedule for primary airflow verification and BAS point checkout.
5 Environmental and Service Conditions
5.1 Air terminal units shall be selected and rated for the conditions of their installed location and for the central system pressure profile they will operate against.
5.2 The unit shall function within its certified accuracy and acoustical performance over the entire airflow range scheduled for the zone it serves.
5.3 Ambient and Installation Location
5.3.1 Air terminal units shall be suitable for continuous operation in indoor conditioned or semiconditioned plenum spaces at ambient temperatures from 50°F to 104°F and relative humidity from 10% to 90% non-condensing.
5.3.2 Units installed in unconditioned spaces, in interstitial floors with no temperature control, or in spaces with elevated humidity shall be reviewed with the manufacturer for any required casing, controller enclosure, or condensate provisions before ordering.
5.3.3 Air terminal units are not weatherproof and shall not be installed outdoors or in any location subject to driving rain, condensation, or freezing temperatures.
5.3.4 Installation location datasheet
Concealed above accessible ceiling — conditioned plenum
Concealed above accessible ceiling — non-conditioned return plenum
Mechanical room or service corridor
Interstitial floor (full-height service space)
5.4 Inlet Static Pressure
5.4.1 Air terminal units shall be selected so that the available inlet static pressure at the unit is sufficient to deliver design airflow through the unit casing, damper, downstream coil (if any), and downstream low-pressure duct distribution to the air outlets, with adequate margin for control authority at minimum airflow.
5.4.2 The minimum inlet static pressure for pressure-independent VAV units to maintain primary airflow control at all flow setpoints shall be 0.3 in. w.g. unless the manufacturer's published data certifies stable control at a lower value.
5.4.3 The maximum inlet static pressure shall not exceed 3.0 in. w.g. unless the unit is specifically rated for high-pressure inlet service.
NOTE Excessive inlet pressure increases damper noise and may exceed the certified sound rating. (5.4.4)
5.4.5 Inlet static pressure datasheet
0.33
0.30.50.7511.522.53
Default: 0.75 in. w.g.
Per drawings
NOTE Sizing units to AHU discharge static pressure rather than the static pressure available at the terminal unit inlet after upstream duct losses is a common selection error. (5.4.6)
5.4.7 The Engineer shall calculate inlet static pressure for the most remote and most disadvantaged unit on each branch and shall size both the central fan and the units accordingly.
6 Unit Type and Configuration
NOTE Air terminal unit selection is determined by the zone's heating requirement, the use of plenum return air for induction, the central system control strategy, and the acoustical sensitivity of the zone served. (6.1)
NOTE This section establishes the type of unit for each tag. (6.2)
6.3 Box Type
NOTE Single-duct units regulate a single primary airstream from the central air handler and are the standard selection for interior zones served by a constant-discharge-temperature central system. (6.3.1)
NOTE Parallel fan-powered boxes include an intermittent fan that draws plenum return air through the casing in parallel with primary air, and are used where perimeter zones require warm air during unoccupied or low-cooling-demand periods without continuous fan power. (6.3.2)
NOTE Series fan-powered boxes include a continuous fan that mixes primary and induced plenum air at all times, are used where constant zone airflow is required regardless of primary airflow, and provide better cold-air-dump prevention at the cost of continuous fan energy. (6.3.3)
NOTE Dual-duct units accept separate primary cold and warm airstreams and proportion the mixture to the zone, and are used in dual-duct distribution systems. (6.3.4)
NOTE Constant-volume reheat units regulate to a fixed airflow setpoint and provide reheat for temperature control, and are used in spaces with fixed ventilation requirements such as some laboratory and healthcare applications. (6.3.5)
6.3.6 Box type datasheet
Single-duct VAV
Parallel fan-powered box (PFPB) — intermittent fan
Series fan-powered box (SFPB) — continuous fan
Dual-duct mixing box
Constant-volume reheat (CV)
Per drawings
6.4 Pressure-Independence
NOTE Pressure-independent units include a primary inlet airflow sensor and a closed-loop control strategy that maintains the commanded airflow setpoint regardless of variations in inlet static pressure within the operating range. (6.4.1)
NOTE Pressure-dependent units modulate the damper based on a zone temperature signal directly, with no airflow feedback; their actual delivered airflow varies with system pressure and cannot reliably hold a minimum ventilation rate. (6.4.2)
6.4.3 Pressure-independent control is required to maintain ASHRAE 62.1 minimum ventilation rates at all part-load conditions and is the standard selection for new construction.
6.4.4 Control mode datasheet
● Pressure-independent — airflow sensor closed-loop control (standard)
○ Pressure-dependent — temperature signal direct to damper (not recommended)
6.5 Inlet Size
NOTE The primary inlet is identified by its nominal round duct diameter. (6.5.1)
6.5.2 Inlet size shall be selected so that the unit's primary maximum airflow lies within the certified flow range of the inlet sensor, with margin above the minimum-flow accuracy threshold at the unit's primary minimum airflow setpoint.
NOTE Oversizing the inlet relative to the actual airflow range degrades minimum-airflow control because the sensor velocity falls below the calibrated accuracy floor; undersizing increases pressure drop and noise. (6.5.3)
6.5.4 Coordinate the inlet size with the AHRI 880 certified flow range published by the manufacturer.
6.5.5 Inlet nominal diameter datasheet
4 in.
5 in.
6 in.
7 in.
8 in.
9 in.
10 in.
12 in.
14 in.
16 in.
Per drawings
6.6 Primary Airflow Setpoints
NOTE Primary maximum airflow defines the unit's cooling-design airflow. (6.6.1)
NOTE Primary minimum airflow defines the lowest commanded airflow under the central control sequence, which is typically the larger of the ASHRAE 62.1 zone outdoor air requirement (adjusted for system ventilation effectiveness), the heating airflow setpoint, and any acoustical or stratification floor established by the design team. (6.6.2)
6.6.3 ASHRAE 90.1 limits the reheat-airflow minimum to no more than the larger of 30% of design or the ventilation rate, except for specific exceptions for laboratories, occupancy-sensed zones, and DDC-with-airflow-reset systems; the Engineer shall determine the controlling minimum for each zone and shall set the controller minimum airflow accordingly.
6.6.4 Primary airflow setpoint datasheets
NOTE A frequent source of comfort complaints is a heating airflow setpoint identical to the cooling minimum, which produces a cold-air dump at the diffusers when the unit transitions to heating. (6.6.5)
NOTE ASHRAE 90.1 recognizes this and permits a higher heating-airflow setpoint (typically 30% to 50% of design) so that the supply air temperature delta at the diffuser is sufficient to overcome cold-air dump. (6.6.6)
NOTE For fan-powered boxes, induction adds plenum air to primary air at the diffuser and the cold-air dump risk is reduced; the heating setpoint for fan-powered units may be set to the same value as the cooling minimum where the fan provides adequate discharge airflow. (6.6.7)
7 Casing Construction
7.1 Casing Material and Gauge
7.1.1 Air terminal unit casings shall be fabricated from galvanized steel sheet conforming to ASTM A653 with G60 or G90 coating, of gauge not less than 22 gauge for the casing wrapper and 20 gauge for the inlet collar and outlet flanges.
7.1.2 Casing shall be of welded, mechanically fastened, or interlocked-seam construction with all joints sealed to the leakage class certified under AHRI 880.
7.1.3 Loose-fitting casing seams sealed only with tape are not acceptable.
7.1.4 Casing material datasheet
● Galvanized steel, ASTM A653, G60 coating (standard)
○ Galvanized steel, ASTM A653, G90 coating (high-humidity plenum)
○ Aluminized steel
○ Stainless steel (special service)
7.2 Casing Air Leakage
7.2.1 Casing leakage shall not exceed the values certified under AHRI 880 for the unit configuration, and in no case shall exceed 1% of nominal cataloged primary airflow at 1.0 in. w.g. internal static pressure.
7.2.2 Casing leakage on a closed damper shall not exceed 2% of nominal cataloged primary airflow at 3.0 in. w.g. inlet static pressure, measured at the unit inlet with the damper closed and the outlet open.
NOTE Excessive casing leakage and damper-closed leakage both degrade minimum-airflow control and add to the central system's outdoor-air dilution calculation. (7.2.3)
7.2.4 Casing air leakage datasheet
≤ 1% of nominal primary airflow (standard)
≤ 2% of nominal primary airflow (legacy applications only)
7.3 Casing Liner / Attenuator
7.3.1 Casings shall be lined with thermal-acoustical insulation to attenuate primary damper noise and to reduce thermal transmission between the conditioned primary air and the plenum.
7.3.2 Liner shall conform to ASTM C1071 with a flame-spread index not greater than 25 and a smoke-developed index not greater than 50 when tested per ASTM E84, and shall be UL 181 listed.
7.3.3 Liner shall be coated, encapsulated, or covered with a perforated metal facing on the airstream side to prevent fiber erosion at the certified maximum face velocity.
7.3.4 Casing liner / attenuator datasheet
1/2 in. dual-density fiberglass with foil-faced or coated airstream surface
1 in. dual-density fiberglass with foil-faced or coated airstream surface
1 in. fiberglass with perforated metal liner (closed-cell airstream protection)
Closed-cell foam liner (fiber-free)
Double-wall casing — fiber encapsulated between solid metal skins (fiber-free airstream)
7.3.5 Fiber-free liner constructions (closed-cell foam liners and double-wall casings with encapsulated fiber) are required for healthcare critical spaces, laboratory exhaust serving classified airstreams, and projects whose program documents specify fiber-free distribution.
NOTE For general office and institutional service, a perforated-metal-faced fiberglass liner is the standard selection because it provides superior acoustical attenuation per unit thickness with no fiber exposure to the airstream. (7.3.6)
7.4 Hanger and Mounting Provisions
7.4.1 Casings shall include integral mounting brackets, hanger lugs, or threaded inserts for suspension from structure.
7.4.2 The Contractor shall not penetrate the casing for hanger attachment after manufacture because field penetrations breach the casing liner and create thermal bridges and leak paths.
7.4.3 Each unit shall be supplied with the manufacturer's recommended hanger pattern indicated on the shop drawings.
8 Damper Assembly
8.1 Damper Construction
8.1.1 The primary damper shall be a single-blade or multi-blade assembly of galvanized or aluminum construction with a continuous shaft running through self-lubricated bushings or bearings rated for the operating temperature and the design number of operating cycles.
8.1.2 Damper blades shall be of stiffened construction sized to resist deflection at maximum inlet differential pressure without binding.
8.1.3 Damper shaft seals shall prevent leakage past the casing penetration at the maximum operating differential pressure.
8.1.4 Damper blade configuration datasheet
● Single-blade — round, internally mounted (standard for single-duct VAV)
○ Multi-blade opposed — for rectangular inlets and dual-duct mixing
○ Multi-blade parallel — for fan-powered induction air openings
8.2 Damper Close-Off Leakage
8.2.1 Damper close-off leakage at 3.0 in. w.g. inlet static pressure shall not exceed 2% of nominal cataloged primary airflow, with the damper commanded to its fully closed position and the outlet open.
NOTE Higher leakage rates degrade the unit's ability to deliver an actual zero-flow condition during unoccupied-mode operation and undermine the dilution accounting that supports ASHRAE 62.1 ventilation compliance at the system level. (8.2.2)
8.2.3 Damper close-off leakage datasheet
≤ 2% of nominal primary airflow (standard)
≤ 1% of nominal primary airflow (tight close-off for occupied/unoccupied control)
8.3 Damper Actuator
8.3.1 Damper actuators shall be electronic, direct-coupled to the damper shaft, with stroke time matched to the controller's airflow control loop.
8.3.2 Spring-return actuators are used where a defined fail-safe position is required by the central system control sequence; non-spring-return actuators are acceptable where the controller maintains last-known-position on signal loss and the unit serves a non-critical zone.
8.3.3 Pneumatic actuators are acceptable only on retrofit projects extending existing pneumatic control systems.
8.3.4 Damper actuator datasheets
Electronic non-spring-return, modulating 0–10 V or 4–20 mA (standard)
Electronic spring-return, fail-closed (life-safety or fume-isolation zones)
Electronic spring-return, fail-open
Pneumatic, modulating (retrofit only)
30180
306090120180
Default: 90 seconds
9 Primary Airflow Sensor
9.1 Sensor Type and Accuracy
9.1.1 The primary inlet airflow sensor shall be a multi-point center-averaging differential-pressure sensor (multiple sensing ports averaged across the inlet cross-section) integral to the inlet collar, factory-installed and factory-calibrated to the specific unit.
9.1.2 The sensor shall be certified under AHRI 880 to maintain accuracy within ±5% of reading from 100% of nominal airflow down to the certified low-flow threshold for the inlet size.
9.1.3 Sensors providing a single-point or pitot-style differential pressure are acceptable only on inlet sizes 6 in. and below where multi-point averaging is not commercially available, and only with documented manufacturer accuracy data.
9.1.4 Primary inlet airflow sensor type datasheet
● Multi-point center-averaging differential pressure (standard)
○ Single-point pitot or velocity element (inlet ≤ 6 in. only)
○ Thermal anemometer / hot-wire array (manufacturer-specific)
9.2 Straight-Duct Inlet Requirement
9.2.1 The sensor's published accuracy is valid only when the certified minimum length of straight duct is provided upstream of the unit inlet.
9.2.2 The manufacturer's required upstream and downstream straight-duct lengths shall be honored on shop drawings and during field installation.
NOTE A common installation defect is an elbow, transition, or branch tee located within one diameter of the unit inlet, which distorts the velocity profile and renders the multi-point average inaccurate; this is one of the leading causes of airflow imbalance during commissioning and is correctable only by adding duct length or accepting field recalibration of the unit's K-factor against a calibrated flow station. (9.2.3)
9.2.4 Minimum straight duct upstream of inlet datasheet
15
11.5235
Default: 3 inlet diameters
10 Reheat
10.1 Reheat Type
10.1.1 The reheat strategy at each terminal unit shall be coordinated with the central plant capability and the zone's heating load.
NOTE Hot water reheat is the standard selection where a central hot water plant is provided; it offers high turndown, low operating cost, and quiet operation. (10.1.2)
NOTE Electric reheat is used where no central hot water plant is provided or where the heating load is small enough that the cost of running hot water piping to the zone exceeds the lifecycle cost of electric resistance. (10.1.3)
10.1.4 ASHRAE 90.1 limits the use of new electric resistance heating in many building types and climate zones; verify code compliance before specifying electric reheat as the default for the building.
10.1.5 Reheat type datasheet
None (cooling-only)
Hydronic — hot water coil
Electric — staged resistance
Electric — SCR-modulated resistance
10.2 Hydronic Reheat Coil
10.2.1 Hot water reheat coils shall be 1-row, 2-row, or 3-row construction as selected to deliver the scheduled coil capacity at the design hot water entering temperature, the design air entering temperature (typically the supply air temperature from the central AHU), and the design primary minimum heating airflow.
10.2.2 Tubes shall be seamless copper not less than 0.020 in. wall thickness, mechanically expanded into aluminum or copper plate fins.
10.2.3 Coils shall be factory-pressure-tested to not less than 300 psig before shipment.
10.2.4 Hydronic reheat coil datasheets
○ 1 row (interior zones, low capacity)
● 2 rows (standard perimeter)
○ 3 rows (high-load perimeter or low hot water temperature)
140 °F EWT / 120 °F LWT (typical for low-temperature hot water; see mechanical schedules)
Per drawings
NOTE Modern building hot water plants increasingly operate at 140 °F or lower entering water temperature to allow condensing boiler operation and to integrate with heat pump heat recovery. (10.2.5)
NOTE A 1-row coil sized for 180 °F EWT will not deliver design capacity on a 140 °F plant. (10.2.6)
10.2.7 Coil row counts shall be selected for the actual entering water temperature of the project plant.
10.2.8 Verify the coil selection against the actual scheduled hot water temperature for the project and not against a generic catalog rating.
10.3 Hydronic Control Valve
10.3.1 Each hydronic reheat coil shall be served by a modulating control valve.
NOTE Pressure-independent control valves (PICVs) are the preferred selection on projects with variable-flow hydronic distribution because they maintain the commanded flow regardless of differential pressure variations across the building; conventional two-way globe valves are acceptable on smaller systems where differential pressure stability is achieved at the plant. (10.3.2)
NOTE Two-position (on-off) valves are acceptable only on cooling-only systems with no need for heating modulation or in unconditioned spaces with low control authority. (10.3.3)
10.3.4 Hydronic reheat control valve datasheets
Pressure-independent modulating valve (PICV) with electric actuator
Two-way modulating globe valve with electric actuator
Three-way modulating valve with electric actuator (constant-flow systems only)
Two-position valve (on-off heating only)
● Fail open to heating (freeze protection)
○ Fail closed
○ Fail in last position
10.3.5 Hydronic reheat valves shall fail open to heating where the unit serves a perimeter zone in a climate subject to freezing exterior temperatures, so that loss of control power does not allow a glass-line zone to freeze on a winter night.
10.3.6 Interior zones with no freeze exposure may fail closed to prevent overheating on signal loss.
10.4 Electric Reheat
10.4.1 Electric reheat coils shall be open-coil resistance or finned-tubular elements installed in a NEC-compliant heater section integral to or downstream of the unit casing.
10.4.2 Electric reheat coils shall comply with NFPA 70 Article 424 and shall be UL listed for use in HVAC distribution.
10.4.3 Every electric reheat coil shall include primary automatic-reset over-temperature protection, secondary manual-reset over-temperature protection, and a differential-pressure airflow proving switch that disables the heater when primary airflow falls below the manufacturer's minimum heating airflow.
10.4.4 Operating a resistance heater without airflow proof is a recurring root cause of duct fires and is not acceptable under any circumstances.
10.4.5 Electric reheat datasheets
● Staged contactors (1, 2, or 3 stages)
○ Solid-state relay / SCR modulation (linear control, recommended for low ΔT applications)
○ 1 stage
● 2 stages
○ 3 stages
● Open-coil nichrome resistance (lowest cost)
○ Finned-tubular element (longer life, lower watt density)
☑ Primary automatic-reset over-temperature cutout
☐ Secondary manual-reset over-temperature cutout
☐ Differential-pressure airflow proving switch
☐ Branch-circuit fusing or breaker integral to heater
☐ Magnetic disconnect / safety switch within sight of unit
120V / 1-phase (small zones only)
208V / 1-phase
208V / 3-phase
240V / 1-phase
277V / 1-phase
480V / 3-phase
NOTE SCR-modulated electric reheat provides linear control of leaving air temperature and avoids the staircase of supply temperature steps inherent in staged control. (10.4.6)
NOTE Where the zone has a tight setpoint or low primary minimum airflow that would amplify the staged-control discharge temperature swing, SCR control is the preferred selection. (10.4.7)
NOTE Staged control is the simpler and lower-cost solution for routine office zones with two-stage zone calls. (10.4.8)
11 Fan-Powered Units
11.1 Fan Section Construction
11.1.1 Parallel and series fan-powered boxes shall include an integral fan section with a forward-curved or backward-curved centrifugal fan, fan housing, motor, and induced-air opening with a backdraft damper (parallel boxes only).
11.1.2 The fan section shall be acoustically lined and shall include a removable access panel sized to allow fan and motor service without removing the unit from the ceiling.
11.2 Fan Motor Type
11.2.1 Electronically commutated motors (ECM) shall be the standard fan motor selection for new construction.
NOTE ECM motors provide variable-speed control through a 0–10 V or PWM signal, achieve substantially higher efficiency than PSC motors across the operating range, and allow the controller to adjust fan output during commissioning without changing pulleys or motor taps. (11.2.2)
11.2.3 PSC (permanent split capacitor) motors are acceptable only where the contract documents specifically permit, and only with stepped fan speed control via tap selection at startup.
11.2.4 Fan motor datasheets
● ECM — electronically commutated (variable speed, high efficiency) — standard
○ PSC — permanent split capacitor (multi-speed, lower efficiency)
0.051.5
0.050.0830.1250.1670.250.3330.50.7511.5
Default: 0.333 HP
Per drawings
11.3 Parallel Fan-Powered Box (PFPB) Operation
NOTE In a parallel fan-powered box, the fan is offset from the primary airstream; the fan operates only during heating or unoccupied modes, and during normal cooling the fan is off and primary air passes through the unit and out to the zone. (11.3.1)
NOTE A backdraft damper at the induced-air opening prevents primary air from backflowing into the plenum when the fan is off, and parallel boxes save fan energy compared to series boxes during the dominant cooling-mode operating hours. (11.3.2)
11.4 Series Fan-Powered Box (SFPB) Operation
NOTE In a series fan-powered box, the fan is in series with the primary airstream; the fan operates continuously during occupied hours and delivers a constant total airflow to the zone, mixing primary air with induced plenum air in a ratio that varies with the primary damper position. (11.4.1)
NOTE Series boxes provide constant zone airflow and constant outlet velocity regardless of cooling demand, which is preferred where diffuser performance is sensitive to airflow turndown. (11.4.2)
11.4.3 Series boxes consume continuous fan energy and shall be selected only where the constant airflow benefit justifies the parasitic energy.
11.5 Induced-Air Backdraft Damper (PFPB Only)
11.5.1 Parallel fan-powered boxes shall include a low-leakage backdraft damper at the induced-air opening to prevent reverse flow of cooled primary air into the return plenum during cooling-mode operation when the fan is off.
11.5.2 Backdraft damper leakage shall not exceed 10 CFM at 0.5 in. w.g. differential pressure for the smallest opening size and shall be proportionally limited for larger sizes.
11.5.3 Induced-air backdraft damper datasheet
● Gravity backdraft damper — low leakage, no power required
○ Motorized damper interlocked with fan operation
○ Not applicable — series fan-powered or single-duct unit
12.1 Rating Method
12.1.1 Sound performance shall be rated per ANSI/AHRI 885 using sound power data developed per ANSI/ASHRAE 130.
12.1.2 The manufacturer shall publish radiated and discharge sound power levels in octave bands 2 through 7 (125 Hz through 4,000 Hz) at the unit's selected primary airflow and at the specified inlet static pressure.
12.1.3 The published data shall include the standard AHRI 885 calculation of room NC for a reference room configuration so that the designer can compare units on a consistent basis.
12.2 Project NC Targets
12.2.1 Room NC criteria shall be established by the design team for each scheduled zone and shall be indicated on the mechanical drawings or in the room finish schedule.
12.2.2 Air terminal unit selections shall demonstrate compliance with the room NC target using the AHRI 885 procedure applied to the actual room volume, room absorption, and acoustical path from the unit to the listener.
12.2.3 Project NC target datasheets
NC 25 (recording studios, concert halls)
NC 30 (private offices, classrooms, conference rooms)
NC 35 (open offices, retail)
NC 40 (large open spaces, transient occupancy)
NC 45 (back-of-house, mechanical, light industrial)
Per drawings
NC 25
NC 30
NC 35
NC 40
NC 45
Per drawings
NOTE Discharge sound is attenuated by the downstream duct, internal liner, and the diffusers, while radiated sound passes directly through the ceiling tile to the room below. (12.2.4)
12.2.5 Radiated sound is typically the controlling acoustical path for ceiling-installed units serving open-plan offices, and units shall be selected for radiated NC compliance even where discharge NC is acceptable.
NOTE Increasing primary inlet static pressure significantly increases both radiated and discharge sound; the central system static-pressure-reset strategy reduces both inlet static and unit sound at most operating hours. (12.2.6)
13 Controls and BAS Integration
13.1 DDC Controller
13.1.1 Each air terminal unit shall be furnished with a factory-installed or factory-supplied direct digital control (DDC) controller capable of executing the project's zone control sequence.
13.1.2 The controller shall include sufficient hardwired inputs and outputs for the unit's damper actuator, primary airflow sensor, reheat valve or stage outputs, fan-motor control output (where applicable), zone temperature sensor, and any optional inputs (CO₂, occupancy, window contact) indicated in the BAS sequence.
13.1.3 DDC controller datasheets
BACnet MS/TP (RS-485) — most common for terminal-unit networks
BACnet IP (Ethernet) — IP-native controllers
Modbus RTU (RS-485)
LonWorks (FT-10)
Manufacturer-proprietary protocol with gateway to BACnet
☑ Damper actuator output (modulating)
☐ Primary airflow sensor input (differential pressure)
☐ Zone temperature sensor input
☐ Hot water valve output (modulating)
☐ Electric reheat stage outputs (binary)
☐ Electric reheat modulating output (SCR control)
☐ Fan motor speed output (0–10 V ECM)
☐ Fan run/stop output
☐ Zone CO2 sensor input
☐ Occupancy / motion sensor input
☐ Window contact input
☐ Discharge air temperature sensor input
13.2 Coordination with Building Automation System
13.2.1 The terminal unit controller shall be commissioned by the controls contractor on the project's BAS network per Building Automation System. 13.2.2 Controller addressing, network segmentation across MS/TP trunks, and supervisory controller assignments shall be coordinated before unit shipment so that the factory-applied address and unit tag are consistent with the BAS database.
NOTE Re-addressing controllers in the field after installation is permissible but introduces a recurring source of error during commissioning; factory addressing per the project tag schedule is preferred. (13.2.3)
13.3 Zone Sensor
13.3.1 Each zone served shall be provided with a wall-mounted zone temperature sensor.
13.3.2 Zone sensors shall be furnished by the controls contractor unless the air terminal unit supplier includes them as a packaged option.
13.3.3 Where the project zoning includes occupant-adjustable setpoints, the wall sensor shall include a setpoint adjustment within a defined deadband relative to the system setpoint.
13.3.4 Zone sensor datasheet
Temperature only — no occupant adjustment
Temperature with limited setpoint adjustment (±2 °F)
Temperature with setpoint adjustment and override pushbutton
Temperature with humidity and CO2 (multi-function room sensor)
14 Factory Testing
14.1 Each air terminal unit shall undergo the following factory tests before shipment.
14.2 Failure of any factory test shall require correction and retest; no unit shall be shipped until all tests pass.
14.3 Each air terminal unit shall undergo the following factory tests before shipment:
- Performance test of the primary damper through its full stroke, verifying close-off and full-open positions at design inlet pressure
- Calibration of the primary inlet airflow sensor against a calibrated reference flow station, recording the K-factor or calibration curve in the unit's factory test record
- Hydrostatic pressure test of the hot water coil at 300 psig minimum for the manufacturer's standard duration (1 row, 2 row, and 3 row coils all shall be pressure tested)
- Electrical functional test of the heater section, including verification of the airflow proving switch, primary cutout, and secondary cutout operation
- Functional test of the DDC controller communication and I/O, with the controller addressed per the project tag schedule
- Casing leakage test on a sample basis per the manufacturer's quality control program
● Standard production tests (manufacturer-certified, no witness)
○ Witnessed factory test on sample units — provide 10 days notice
○ 100% witnessed factory test (critical or healthcare projects)
15 Installation
15.1 Coordination and Service Clearances
15.1.1 Before fabricating ceiling layouts and ductwork, the Contractor shall verify that each air terminal unit can be installed with the manufacturer's required service clearances and access panels accessible through the ceiling.
15.1.2 Service clearance shall include access to the primary damper actuator, the reheat coil connections, the controller enclosure, and the fan and motor for fan-powered units.
15.1.3 Where ceiling-tile access is not sufficient, an access door shall be coordinated with the architectural finish schedule.
15.1.4 Service access provision datasheet
● Accessible ceiling (lay-in tile)
○ Hard ceiling with access panel below unit
○ Hard ceiling — entire ceiling segment removable
○ Mechanical room or exposed structure (full access)
15.2 Hanger and Support
15.2.1 Air terminal units shall be suspended from structure using all-thread rod, hanger straps, or trapeze hangers attached to the integral mounting provisions on the casing.
15.2.2 Units shall not be supported by the ceiling suspension system, by the connected ductwork, or by piping.
15.2.3 Each unit shall be supported by a minimum of four hanger points, one at each corner of the casing, sized for the operating weight of the unit (including the weight of water-filled coils for hydronic units) with a safety factor consistent with SMACNA HVAC Duct Construction Standards.
15.3 Inlet Duct Connection
15.3.1 Primary inlet duct shall connect to the unit collar with a minimum straight-duct length upstream of the inlet as required by the airflow sensor manufacturer (typically 3 inlet diameters; verify per sensor product data).
15.3.2 Flexible duct at the inlet shall be fully extended and shall not be used to make the final connection within the required straight-duct length.
15.3.3 Elbows, branch tees, and transitions within the required straight-duct length shall be relocated during shop drawing coordination.
15.3.4 Connection of the inlet duct to the unit collar shall be sealed to the duct seal class specified in Hvac Ductwork. 15.4 Outlet Duct Connection
15.4.1 Downstream low-pressure duct shall connect to the unit outlet with a transition appropriate to the outlet shape and dimensions.
15.4.2 For units serving multiple diffusers from a single downstream plenum, the outlet plenum shall be sized to limit velocity to a value that controls discharge sound.
15.4.3 Where the manufacturer publishes a recommended downstream straight-duct length to achieve the certified discharge sound rating, the Contractor shall provide that length before the first branch or fitting.
15.5 Hydronic Piping Connections
15.5.1 Hot water supply and return piping to reheat coils shall comply with Hydronic Piping. 15.5.2 Each coil shall be served by a piping arrangement including an upstream isolation valve, a balancing or pressure-independent control valve, a strainer, an air vent at the high point, and a drain at the low point.
15.5.3 Piping shall be supported independently of the air terminal unit and shall be arranged to permit coil removal without dismantling the piping.
15.5.4 Hot water piping shall be insulated within the conditioned plenum to limit unwanted heat loss; insulation shall be continuous and shall not be omitted at the unit connection.
15.6 Electric Power Connections
15.6.1 Electric reheat power circuits shall comply with NFPA 70 Article 424.
15.6.2 Each electric reheat coil shall be served by a dedicated branch circuit sized for the heater nameplate load plus 25%, with branch overcurrent protection coordinated with the heater manufacturer's recommendations.
15.6.3 A disconnecting means shall be provided within sight of each unit served by a 120 V or higher heater circuit, or a means of lockout shall be provided at the branch panelboard.
15.6.4 Low-voltage control wiring shall be routed separately from line-voltage power wiring as required by the NEC.
15.6.5 Electric reheat disconnect provision datasheet
○ Factory-installed integral disconnect within unit enclosure
● Field-installed disconnect within sight of unit
○ Lockable means at branch panelboard (where permitted by NEC)
15.7 Control Wiring
15.7.1 Low-voltage control wiring between the air terminal unit controller, the zone temperature sensor, the BAS network trunk, and any optional inputs shall be Class 2 cable, routed and supported per the NEC and the BAS contractor's wiring standards.
15.7.2 Plenum-rated cable shall be used in air-handling plenums.
15.7.3 Communication trunk wiring shall be the manufacturer's specified twisted shielded pair for MS/TP networks, with continuous shield drain conductor and terminations at the BAS terminations only.
15.8 Pre-Startup Inspection
15.8.1 Before energizing the air terminal unit and starting the central air system, the Contractor shall confirm the following:
- All shipping restraints and protective films are removed
- Primary damper strokes freely from fully closed to fully open by manual rotation of the actuator shaft
- Primary airflow sensor tubing is connected without kinks or leaks at the sensor and at the controller
- Hot water coil piping is filled, vented, and pressure-tested per Hydronic Piping
- Electric reheat circuit is energized at the panelboard, control voltage is present at the heater contactor, and the airflow proving switch is in the open (no-flow) state
- BAS network communication is established with the controller at its assigned address
- Zone sensor is wired and reading a plausible room temperature
☑ Shipping restraints removed
☐ Damper strokes freely full range
☐ Airflow sensor tubing intact and connected
☐ Hot water coil filled, vented, pressure-tested
☐ Electric branch circuit energized and verified
☐ Airflow proving switch tested and clears at no-flow
☐ BAS communication established at correct address
☐ Zone sensor wired and reading plausible temperature
☐ Access panels closed and ceiling tiles in place
16 Field Testing and Commissioning
16.1 Airflow Verification
16.1.1 The testing, adjusting, and balancing agent shall verify the primary airflow at each unit at the cooling maximum, cooling minimum, and heating airflow setpoints per Testing Adjusting And Balancing. 16.1.2 Measured airflow at each setpoint shall be within ±10% of the commanded value.
16.1.3 Where measured airflow falls outside this band, the controller K-factor or airflow calibration constants shall be adjusted against the calibrated TAB instrument.
16.1.4 After adjustment, the K-factor and as-commissioned setpoints shall be recorded in the closeout submittal.
16.1.5 Field airflow verification tolerance datasheet
● ±10% of commanded value at each setpoint (standard)
○ ±5% of commanded value at each setpoint (critical zones, healthcare)
16.2.1 For projects with formal commissioning, each air terminal unit shall be subjected to functional performance testing by the commissioning authority.
16.2.2 Functional performance testing shall verify the full zone control sequence at occupied, unoccupied, and override modes; verify the heating valve or stage operation through the heating range; verify the fan-powered box fan operation and induction at the appropriate primary airflow; verify the airflow proving interlock for electric reheat; and verify the BAS reporting of all hardwired points and calculated values.
16.3 Acoustical Spot Check
16.3.1 A representative subset of installed units, including the most acoustically sensitive zones, shall be measured in place by the TAB agent or by an acoustical consultant where required.
16.3.2 Measured room NC at the receiver location shall be compared to the design NC target.
16.3.3 Where measured NC exceeds the target, the contributing path (radiated through ceiling, discharge through diffusers, inlet duct noise) shall be identified and corrective action coordinated with the design team.
NOTE Common corrective actions for excess NC include lowering the central system static pressure setpoint at part-load via reset, adding a lined discharge plenum, or substituting a unit with a larger casing and lower face velocity. (16.3.4)
17 Delivery, Storage, and Handling
17.1 Air terminal units shall be delivered to the project site in original factory packaging with inlet and outlet openings sealed against construction dust.
17.2 Units shall be stored indoors in a clean, dry, conditioned space.
17.3 Units shall not be stored on bare concrete, in standing water, or in any space subject to freezing or to roof leaks.
17.4 Stacking of units shall conform to the manufacturer's instructions and shall not exceed the rated stack height; stacked units shall be supported only at the corner brackets, never on the casing wrap.
17.5 Hot water coils on stored units shall be protected from freezing during storage; where storage temperature can fall below 35 °F at any time, coils shall be drained, blown out with compressed air, and tagged as drained, with a corresponding note on the unit tag schedule so that the coils are refilled and vented before startup.
18 Identification
18.1 Each air terminal unit shall include a permanent identification label on the exterior of the casing, in a location visible from below an accessible ceiling.
18.2 The identification label shall include the unit tag number matching the unit schedule, the manufacturer's model and serial number, the primary maximum and minimum airflow setpoints, the reheat capacity, the electrical service requirements where applicable, the controller communication address, and the date of manufacture.
18.3 Labels shall be applied to the casing exterior and shall not be placed on removable access panels.
18.4 Identification nameplate material datasheet
● Adhesive laminated label (standard interior plenum)
○ Engraved phenolic plate, mechanically fastened (premium / long service)
○ Stainless steel etched plate (corrosive plenums)
19 Warranty
19.1 Equipment Warranty
19.1.1 The air terminal unit manufacturer shall warrant the equipment against defects in materials and workmanship for the period specified below, beginning from the date of substantial completion.
19.1.2 The equipment warranty shall cover the casing, damper, airflow sensor, controller, reheat coil (hydronic or electric), valve and actuator, fan and motor (for fan-powered units), and all factory-installed components.
19.1.3 The manufacturer shall maintain service capability within the project region with factory-trained service personnel.
19.1.4 Equipment warranty period datasheet
1 year parts and labor from substantial completion (minimum)
2 years parts and labor from substantial completion
5 years parts, 1 year labor
19.2 ECM Motor and Coil Extended Warranty
19.2.1 ECM fan motors and hydronic reheat coils shall carry an extended warranty consistent with industry practice for these components.
19.2.2 ECM motor and coil extended warranty datasheets
○ Standard — 1 year, same as equipment warranty
● Extended — 5 years on ECM motor and controller
1 year (minimum)
5 years against leakage in tube and fin assembly
19.3 Installation Warranty
19.3.1 The Contractor shall warrant the installation workmanship — including hangers, duct and piping connections, electrical connections, control wiring, sealing, and labeling — for one year from the date of substantial completion.
19.3.2 The Contractor shall maintain access to all installed units during the warranty period by keeping access doors, ceiling tiles, and adjacent finishes clear of permanent obstructions.
20 Spare Parts
NOTE The following spare parts datasheet establishes the spares to be delivered at substantial completion. (20.1)
☑ Spare damper actuator — one per unit type and size
☐ Spare control valve actuator (hydronic reheat) — one per valve size
☐ Spare ECM motor — one per motor size (fan-powered units)
☐ Spare DDC controller — one per controller model
☐ Spare zone temperature sensor — quantity 5% of installed sensors
☐ Spare over-temperature cutout (electric reheat) — one per heater model
20.2 Spare parts shall be delivered to the Owner in manufacturer's original packaging with each part tagged with the unit model number, part number, and date of delivery.
20.3 Spare parts shall be stored in a designated maintenance storage location identified by the Owner.
20.4 The O&M manual shall include a spare-parts inventory list with manufacturer part numbers and reorder information so that additional spares can be procured during the building's service life.