12.3 Economizers, Demand-Controlled Ventilation, and VAV Air Distribution

12.3 Economizers, Demand-Controlled Ventilation, and VAV Air Distribution

The Commercial Air Conditioning sheet lists "describing the function, check the operation, and wire a demand ventilation control," "describing the function, check the operation, and install a variable volume air handler," and "describing the function, check the operation, and install a variable air volume (VAV) unit." The Master Specialist hands-on list adds Economizer Set-up and Commissioning, Air Economizer Controls, Demand-Controlled Ventilation Set-up and Commissioning, Outdoor Air Set-up and Commissioning, and Variable Air Volume Set-up and Commissioning. Nowhere else in the credential is a single subsystem tested this many ways.

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1. The Air-Side Economizer

An economizer is a set of outdoor-air, return-air, and relief-air dampers with a controller. When outdoor air is more favorable than return air, the controller modulates the dampers to cool the building with outdoor air and locks out mechanical cooling.

Sequence of operation

1. On a cooling call, the controller compares outdoor conditions to the changeover setpoint.
2. If outdoor air is suitable, the outdoor-air damper modulates open (and the return damper closed) to hold a mixed-air temperature setpoint — commonly 50–55°F, matching the supply temperature the coil would otherwise produce.
3. If the dampers reach full open and the mixed-air temperature is still above setpoint, the controller stages mechanical cooling on to supplement.
4. If outdoor air is unsuitable, the outdoor-air damper drops to its minimum position (the ventilation minimum) and mechanical cooling handles the load.
5. The relief/exhaust damper or power exhaust fan opens proportionally to prevent over-pressurizing the building.

The mixed-air low limit protects against overcooling and coil freezing (Section 11.3): the damper is not allowed to drive mixed air below roughly 45–50°F.

Changeover strategies

Why enthalpy matters. In a humid climate, 72°F outdoor air at 90% RH carries far more total heat than 76°F return air at 50% RH. A fixed dry-bulb economizer set at 75°F would open the dampers wide and increase the cooling load, because the coil now has to condense all that outdoor moisture. Plotting both points on the psychrometric chart (Section 11.1) makes the error obvious, which is exactly why enthalpy control exists.

Wet-bulb and dew-point-based changeover are variations on the same principle and are used where enthalpy sensors have proven unreliable.

Hardware and failure modes

• Dampers: opposed-blade dampers give better modulating control (roughly linear); parallel-blade dampers are used for two-position service.
• Actuators: spring-return actuators fail to a safe position on power loss. Verify the fail position is correct — an outdoor-air damper that fails open in a Minneapolis January will freeze a hydronic coil.
• Linkage: loose setscrews and worn linkage are the classic mechanical failure; the actuator strokes and the damper does not.
• Sensors: outdoor-air temperature and humidity sensors sit in the weather and drift or fail. A failed sensor typically drives the economizer to a fixed position rather than annunciating.

Economizers fail silently and expensively. Studies of installed rooftop economizers have repeatedly found large fractions non-functional. That is the reason Fault Detection and Diagnostics (FDD) for packaged DX units became a code requirement in California and appears as a Master Specialist hands-on exam. FDD continuously compares outdoor, return, and mixed-air temperatures against damper command to detect a stuck damper, a failed sensor, or excess outdoor air.

Commissioning checklist: verify sensor accuracy against a calibrated instrument, command the damper through its full stroke and confirm mechanical travel, verify the minimum position with an outdoor-air CFM measurement, confirm the changeover setpoint against the design intent, verify mechanical cooling lockout during economizer operation, and confirm the relief path opens.

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2. Demand-Controlled Ventilation

Ventilating a space for design occupancy when it is half empty wastes the entire ventilation load calculated in Section 11.3. Demand-controlled ventilation (DCV) modulates outdoor air against actual occupancy.

The sensor is carbon dioxide. People exhale CO₂ at a predictable rate, so indoor CO₂ concentration above outdoor ambient is a proxy for ventilation rate per person.

• Outdoor ambient is roughly 400–420 ppm today.
• A differential of about 700 ppm above outdoors corresponds to roughly 15 CFM per person of outdoor air, which is why 1,000–1,100 ppm is the common indoor setpoint.
• The correct control target is the differential, not the absolute number. A sensor programmed against a fixed 1,000 ppm in a location where outdoor air already reads 500 ppm will under-ventilate.

Where DCV pays. Spaces with high design occupancy and highly variable actual occupancy: conference rooms, auditoriums, gymnasiums, theaters, classrooms, restaurants, houses of worship. It pays little in a steadily occupied office.

Where DCV is not allowed to reduce ventilation below the base rate. ASHRAE 62.1's ventilation-rate procedure splits the requirement into a people component ($R_p \times P_z$) and an area component ($R_a \times A_z$). DCV may modulate the people component; the area component must always be supplied, because it addresses off-gassing from the building and its furnishings rather than from occupants. That base minimum is the damper's minimum position.

Sensor practice. Mount CO₂ sensors in the breathing zone, typically 4–6 feet above the floor, away from doors, diffusers, and direct occupant breath. Non-dispersive infrared (NDIR) sensors require periodic calibration or automatic background calibration, and a drifted sensor either wastes energy or under-ventilates without any visible symptom.

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3. Variable Air Volume

A constant volume (CV) system delivers fixed airflow and varies supply temperature. A variable air volume (VAV) system delivers a constant supply temperature — typically 55°F — and varies the airflow to each zone.

Why VAV wins on part load. Fan power varies with the cube of airflow (the third fan law, Section 10.1). Reducing airflow to 50% reduces fan power to roughly 12.5%. A CV system throttling capacity by reheating 55°F air to 70°F pays twice — once to cool and again to reheat.

VAV box types

Box control and commissioning

• Pressure-independent boxes measure airflow with a flow ring or velocity sensor across a multi-point averaging cross and modulate the damper to a CFM setpoint, so the box delivers the commanded flow regardless of changes in duct static pressure. This is the standard modern arrangement.
• Pressure-dependent boxes position the damper directly and their delivered flow varies with duct pressure. They are cheaper and much harder to balance.
• Minimum airflow setting is the critical adjustment. It must be high enough to (a) supply the zone's ventilation requirement and (b) prevent cold 55°F air from dumping — falling out of the diffuser in a cold stream instead of mixing at the ceiling — but low enough to actually save fan energy and avoid overcooling. ASHRAE 90.1 generally caps the VAV minimum at the larger of 30% of peak flow or the ventilation minimum, precisely to limit reheat energy.
• Duct static pressure reset: the AHU's supply fan is controlled to a static pressure setpoint at a sensor roughly two-thirds down the main duct. Modern sequences reset that setpoint downward until the most-open VAV damper is nearly wide open ("trim and respond"), which saves substantial fan energy.

Common VAV faults