Corpus sync: neutrality remakes, note hygiene, datasheet relocation, transformer-split cross-refs
---
title: Variable Air Volume Terminal Units
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### VAV terminal units shall be selected, furnished, and installed in accordance with this standard, the contract drawings, and the equipment schedule.
−## Applications
−
−### This standard applies to new construction and replacement-in-kind in office buildings, schools, hospitals, laboratories, and similar occupancies where a central air handler delivers conditioned primary air above 0.5 in. w.g. static pressure to a distributed duct system with individual zone temperature control. {note}
−
−### The energy-code context is ASHRAE 90.1 Section 6 (VAV reheat limits, minimum airflow setpoints, fan power, and static pressure reset) and ASHRAE 62.1 (minimum part-load ventilation by demand-controlled ventilation or fixed minimums). {note}
−
```datasheet
label: VAV terminal unit type
…9 unchanged lines
```
+## Applications
+
+### This standard applies to new construction and replacement-in-kind in office buildings, schools, hospitals, laboratories, and similar occupancies where a central air handler delivers conditioned primary air above 0.5 in. w.g. static pressure to a distributed duct system with individual zone temperature control. {note}
+
+### The energy-code context is ASHRAE 90.1 Section 6 (VAV reheat limits, minimum airflow setpoints, fan power, and static pressure reset) and ASHRAE 62.1 (minimum part-load ventilation by demand-controlled ventilation or fixed minimums). {note}
+
## Exclusions
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## Operating Conditions
−### VAV terminal units shall be suitable for indoor installation in a conditioned ceiling plenum or above an accessible ceiling, not exposed to weather. {note}
+### VAV terminal units shall be suitable for indoor installation in a conditioned ceiling plenum or above an accessible ceiling, not exposed to weather.
−### Units shall operate with primary air supplied by the central air handler at static pressure above 0.5 in. w.g. and across the inlet static pressure range scheduled for the project. {note}
+### Units shall operate with primary air supplied by the central air handler at static pressure above 0.5 in. w.g. and across the inlet static pressure range scheduled for the project.
### The casing and components shall be rated for continuous operation at the supply-air temperature range delivered by the central system, including morning warm-up.
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### VAV terminal units shall be pressure-independent, with an integral averaging airflow sensor and DDC controller that maintain the commanded airflow setpoint regardless of variations in inlet static pressure.
−### Pressure-dependent units shall not be used in a central VAV system.
−
−### A pressure-dependent unit reads damper position rather than airflow, so it delivers an incorrect, drifting flow whenever system static pressure changes — which in a VAV system is continuous; only pressure-independent control holds the ventilation and load setpoints the design relies on. {note}
−
```datasheet
label: Airflow control mode
…5 unchanged lines
```
+### Pressure-dependent units shall not be used in a central VAV system.
+
+### A pressure-dependent unit reads damper position rather than airflow, so it delivers an incorrect, drifting flow whenever system static pressure changes — which in a VAV system is continuous; only pressure-independent control holds the ventilation and load setpoints the design relies on. {note}
+
# Unit Sizing and Airflow {toc}
## Inlet Sizing
−### The unit frame size shall be selected from the maximum primary airflow and inlet velocity, not from the connecting duct size. {note}
+### The unit frame size shall be selected from the maximum primary airflow and inlet velocity, not from the connecting duct size.
### Sizing the inlet to match the duct rather than the airflow oversizes the inlet, which drives the airflow sensor below its accurate range at minimum setpoint and produces poor ventilation control and unstable DDC hunting; size to velocity first. {note}
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### The minimum primary airflow setpoint shall be the greatest of the ASHRAE 62.1 part-load ventilation requirement, the applicable ASHRAE 90.1 Section 6.5.2.1 reheat minimum, and the manufacturer's published low-flow accuracy limit.
−### The minimum setpoint shall not be set below approximately 10% to 15% of maximum without verifying the manufacturer's published low-flow accuracy data at that flow; setting a minimum below the sensor's reliable range produces unstable readings and ventilation faults that surface only at commissioning. {note}
+### The minimum setpoint shall not be set below approximately 10% to 15% of maximum without verifying the manufacturer's published low-flow accuracy data at that flow; setting a minimum below the sensor's reliable range produces unstable readings and ventilation faults that surface only at commissioning.
```datasheet
…6 unchanged lines
```
+```datasheet
+label: AHRI 880 primary airflow accuracy
+type: radio
+options:
+ - "±10% of setpoint (standard)"
+ - "±5% of setpoint (labs / critical spaces)"
+default: "±10% of setpoint (standard)"
+```
+
## ASHRAE 90.1 Reheat Compliance
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### The permitted minimum for reheat shall be the greater of 20% of the design zone supply airflow, 0.4 CFM/ft² of conditioned floor area, or the minimum required by ASHRAE 62.1.
−### The design documents shall confirm and record the governing reheat minimum for each zone before the setpoints are programmed. {note}
+### The design documents shall confirm and record the governing reheat minimum for each zone before the setpoints are programmed.
−### Specifying a minimum setpoint lower than ASHRAE 90.1 permits is a code violation that is frequently not caught until commissioning or energy inspection; confirming compliance at design avoids rework. {note}
−
```datasheet
label: ASHRAE 90.1 reheat minimum basis
…6 unchanged lines
```
−```datasheet
−label: AHRI 880 primary airflow accuracy
−type: radio
−options:
− - "±10% of setpoint (standard)"
− - "±5% of setpoint (labs / critical spaces)"
−default: "±10% of setpoint (standard)"
−```
+### Specifying a minimum setpoint lower than ASHRAE 90.1 permits is a code violation that is frequently not caught until commissioning or energy inspection; confirming compliance at design avoids rework. {note}
# Casing and Acoustic Construction {toc}
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### The liner material shall achieve a flame-spread index not greater than 25 and a smoke-developed index not greater than 50 per ASTM E84, as required by NFPA 90A.
−### Where the project is a healthcare, cleanroom, or food-service application, the liner shall be a double-wall perforated metal inner liner rather than exposed fibrous glass. {note}
+### Where the project is a healthcare, cleanroom, or food-service application, the liner shall be a double-wall perforated metal inner liner rather than exposed fibrous glass.
### Healthcare and similar applications shall use a double-wall perforated-metal liner to prevent fiber shedding into the airstream.
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### The unit and liner selection shall meet the NC target scheduled for the zone.
−### The controlling sound path — radiated through the casing or discharged through the outlet — shall be identified for each critical zone, and the liner thickness and density selected accordingly. {note}
−
−### Skipping the AHRI 885 calculation and selecting liner by habit is the most common cause of post-occupancy noise complaints; run the calculation per zone before finalizing. {note}
−
```datasheet
label: Occupied-space NC target
…6 unchanged lines
```
−# Reheat Coils {toc}
+### The controlling sound path — radiated through the casing or discharged through the outlet — shall be identified for each critical zone, and the liner thickness and density selected accordingly.
−## General
+### Skipping the AHRI 885 calculation and selecting liner by habit is the most common cause of post-occupancy noise complaints; run the calculation per zone before finalizing. {note}
−### Where the zone requires heating, the reheat coil type shall be selected from hydronic or electric based on the availability and cost-effectiveness of hot water distribution at the unit. {note}
+# Reheat Coils {toc}
−### Interior and perimeter zones with hot water available are most economically served by a one- or two-row hydronic coil; small isolated zones without practical hot water distribution are served by staged or SCR electric reheat. {note}
+## Where the zone requires heating, the reheat coil type shall be selected from hydronic or electric based on the availability and cost-effectiveness of hot water distribution at the unit.
```datasheet
…7 unchanged lines
```
+## Interior and perimeter zones with hot water available are most economically served by a one- or two-row hydronic coil; small isolated zones without practical hot water distribution are served by staged or SCR electric reheat. {note}
+
## Hydronic Reheat Coils
−### The hydronic reheat coil entering water temperature scheduled for the unit shall match the hot water distribution system design temperature. {note}
+### The hydronic reheat coil entering water temperature scheduled for the unit shall match the hot water distribution system design temperature.
### A reheat coil selected for an entering water temperature that does not match the building hydronic system delivers inadequate capacity that cannot be corrected without replacing the coil; coordinate the temperature with the hydronic design before selection. {note}
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## Fan-Powered Type Selection
−### The fan-powered terminal unit type — series or parallel — shall be selected to match the zone application, not treated as interchangeable. {note}
+### The fan-powered terminal unit type — series or parallel — shall be selected to match the zone application, not treated as interchangeable.
−### A series unit runs its fan continuously and delivers a constant total airflow to the space regardless of primary airflow, which suits high-ceiling, high-air-change, or mixed-occupancy zones; a parallel unit runs its fan only on a call for heat and saves fan energy, which suits standard office perimeter zones. Selecting the wrong type for the application creates comfort complaints. {note}
−
```datasheet
label: Fan-powered configuration
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```
+### A series unit runs its fan continuously and delivers a constant total airflow to the space regardless of primary airflow, which suits high-ceiling, high-air-change, or mixed-occupancy zones; a parallel unit runs its fan only on a call for heat and saves fan energy, which suits standard office perimeter zones. Selecting the wrong type for the application creates comfort complaints. {note}
+
## Fan Motors
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```
−### Where a fan-powered unit includes a cooling coil, a condensate drain pan shall be provided; the condensate drain piping is provided under [[sync/condensate-drainage-piping]]. {note}
+### Where a fan-powered unit includes a cooling coil, a condensate drain pan shall be provided; the condensate drain piping is provided under [[sync/condensate-drainage-piping]].
# Dual-Duct VAV Units {toc}
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### The controller shall communicate on BACnet, either BACnet MS/TP or BACnet/IP as scheduled for the project network.
−### The controller object list shall include, at minimum, analog outputs for damper position, analog inputs for airflow and room temperature, and binary points for occupancy and fan status. {note}
+### The controller object list shall include, at minimum, analog outputs for damper position, analog inputs for airflow and room temperature, and binary points for occupancy and fan status.
### The controller shall expose, at minimum, AO for damper position, AI for measured airflow and room temperature, and BI/BO for occupancy and fan status.
…3 unchanged lines
### Confirming the BACnet profile with the integrator before ordering prevents costly firmware updates or gateway additions when an ATU controller profile does not match the BAS front-end. {note}
−### Where a CO2 or occupancy sensor input is scheduled for demand-controlled ventilation, the controller shall accept and act on that input. {note}
+### Where a CO2 or occupancy sensor input is scheduled for demand-controlled ventilation, the controller shall accept and act on that input.
```datasheet
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### The VAV terminal unit control sequence shall be based on ASHRAE Guideline 36-2021 unless the contract documents specify an alternative sequence.
−### ASHRAE Guideline 36 is the industry-consensus best-practice sequence for VAV terminal units, covering zone setpoints, morning warm-up, demand-controlled ventilation, static pressure reset, and trim-and-respond logic; it is the preferred basis here even though it is a guideline rather than a mandatory code. {note}
−
−### Each VAV terminal unit shall participate in the system static pressure reset, reporting its damper demand so the central fan static pressure can be trimmed and responded down to the lowest setpoint that satisfies all zones.
−
−### ASHRAE 90.1-2022 Section 6.5.3.3 requires VAV systems with DDC controls to reset duct static pressure based on zone demand; the terminal unit must contribute its damper demand for that reset to function. {note}
−
```datasheet
label: Control sequence basis
…5 unchanged lines
```
+### ASHRAE Guideline 36 is the industry-consensus best-practice sequence for VAV terminal units, covering zone setpoints, morning warm-up, demand-controlled ventilation, static pressure reset, and trim-and-respond logic; it is the preferred basis here even though it is a guideline rather than a mandatory code. {note}
+
+### Each VAV terminal unit shall participate in the system static pressure reset, reporting its damper demand so the central fan static pressure can be trimmed and responded down to the lowest setpoint that satisfies all zones.
+
```datasheet
label: Static pressure reset participation
…5 unchanged lines
```
+### ASHRAE 90.1-2022 Section 6.5.3.3 requires VAV systems with DDC controls to reset duct static pressure based on zone demand; the terminal unit must contribute its damper demand for that reset to function. {note}
+
# Testing {toc}
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### Adequate service clearance shall be provided for access to the damper actuator, reheat coil, controller, and fan, with a ceiling access panel where the unit is above a hard ceiling.
−### Locating a unit in a tight plenum without access creates long-term maintenance problems and warranty claims; clearance and access shall be coordinated before installation. {note}
+### Locating a unit in a tight plenum without access creates long-term maintenance problems and warranty claims; clearance and access shall be coordinated before installation.
−### Unit locations shall be coordinated with the ceiling grid, light fixtures, sprinkler heads, and structure before installation. {note}
+### Unit locations shall be coordinated with the ceiling grid, light fixtures, sprinkler heads, and structure before installation.
### Field relocation of units after installation generates coordination RFIs and potential duct rerouting; resolve conflicts during coordination, not in the field. {note}
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- DDC controller (one per type)
```