NA7.5.5 Demand Control Ventilation (DCV) Systems Acceptance

Use Form NRCA-MCH-06-A

Purpose of the Test

The purpose of the test is to verify that systems required to employ demand controlled ventilation (refer to §120.1(c)3) can vary outside ventilation flow rates based on maintaining interior carbon dioxide (CO₂) concentration setpoints. Demand Controlled ventilation refers to an HVAC system’s ability to reduce outdoor air ventilation flow below design values when the space served is at less than design occupancy.CO₂ is a good indicator of occupancy load and is the basis used for modulating ventilation flow rates.

Instrumentation

To perform the test, it may be necessary to vary and possibly measure (if calibration is necessary) ambient CO₂ levels. The instrumentation needed to perform the task may include, but is not limited to:

•      Hand-held reference CO₂ probe calibrated to ±10 ppm

•      Manufacturer’s calibration kit

•      Calibrated CO₂/air mixtures

Test Conditions

Equipment installation is complete (including HVAC unit, duct work, sensors, and control system).

HVAC system must be ready for system operation, including completion of all start-up procedures per manufacturer’s recommendations.

Building automation system (BAS) programming (if applicable) for the air handler and demand Controlled ventilation strategy must be complete. To perform the test, it may be necessary to use BAS to override or temporarily modify the CO₂ sensor reading.

Air Economizer is disabled so that it will not interfere with outdoor air damper operation during test.

Document the initial conditions before overrides or manipulation of the settings. All systems must be returned to normal at the end of the test.

Estimated Time to Complete

Construction inspection: 0.5 to 1 hours (depending on CO₂ sensor calibration)

Functional testing: 1 to 2 hours (depending on how ambient CO₂ concentration levels are manipulated, system response time to variations in CO₂)

Acceptance Criteria

Each CO₂ sensor is factory calibrated (with calibration certificate) or field calibrated.

Each CO₂ sensor is wired correctly to the controls to ensure proper control of the outdoor air damper.

Each CO₂ sensor is located correctly within the space 1 to 6 ft. above the floor.

Interior CO₂ concentration setpoint is ≤600 ppm plus outdoor air CO₂ value if dynamically measured or ≤1000 ppm if no OSA sensor is provided.

A minimum OSA setting is provided whenever the system is in Occupied mode per  §120.1(c)4E regardless of space CO₂ readings.

A maximum OSA damper position for DCV control can be established per the Exception to §120.1(c)4C, regardless of space CO₂ readings.

The outdoor air damper modulates open when the CO₂ concentration within the space exceeds setpoint,

The outdoor air damper modulates closed (toward minimum position) when the CO₂ concentration within the space is below setpoint.

Potential Issues and Cautions

Lock out the economizer control during the test. Outdoor air damper may not modulate correctly if the economizer control strategy is controlling damper operation.

Overall test time may be reduced (especially for rooftop HVAC units) if two people perform the test - one to vary the CO₂ concentration while someone else verifies operation of the outdoor air dampers.

During the testing of the DCV controls, the outside damper will modulate open. Care should be taken to prevent freezing of coils when testing with cold temperatures outside.