At-A-Glance
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NA7.5.5 Demand Control Ventilation (DCV) Systems Acceptance |
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Use Document NRCA-MCH-06-A |
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Purpose of the Test |
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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. |
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Instrumentation |
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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: |
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Hand-held reference CO2 probe calibrated to ±10 ppm Manufacturer’s calibration kit |
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Calibrated CO2/air mixtures
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Test Conditions |
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Equipment installation is complete (including HVAC unit, duct work, sensors, and control system). |
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HVAC system must be ready for system operation, including completion of all start-up procedures per manufacturer’s recommendations. |
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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. |
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Air Economizer is disabled so that it will not interfere with outdoor air damper operation during test. |
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Document the initial conditions before overrides or manipulation of the settings. All systems must be returned to normal at the end of the test. |
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Estimated Time to Complete |
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Construction inspection: 0.5 to 1 hours (depending on CO2 sensor calibration) |
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Functional testing: 1 to 2 hours (depending on how ambient CO2 concentration levels are manipulated, system response time to variations in CO2) |
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Acceptance Criteria |
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Each CO2 sensor is factory calibrated (with calibration certificate) or field calibrated. |
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Each CO2 sensor is wired correctly to the controls to ensure proper control of the outdoor air damper. |
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Each CO2 sensor is located correctly within the space 3 to 6 ft above the floor. |
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Interior CO2 concentration setpoint is ≤600 ppm plus outdoor air CO2 value if dynamically measured or ≤1000 ppm if no OSA sensor is provided. |
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A minimum OSA setting is provided whenever the system is in Occupied mode per §120.1(c)4E regardless of space CO2 readings. |
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A maximum OSA damper position for DCV control can be established per the Exception to §120.1(c)4C, regardless of space CO2 readings. |
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The outdoor air damper modulates open when the CO2 concentration within the space exceeds setpoint, |
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The outdoor air damper modulates closed (toward minimum position) when the CO2 concentration within the space is below setpoint. |
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Potential Issues and Cautions |
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Lock out the economizer control during the test. Outdoor air damper may not modulate correctly if the economizer control strategy is controlling damper operation. |
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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. |
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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. |
A. Test Application
Newly Constructed and Additions/Alterations: All new DCV controls installed on new or existing HVAC systems must be tested.
Single-zone systems. The intent was to limit the demand Controlled ventilation requirement to systems that primarily serve spaces with variable occupancy. However, it is possible that a facility may have a majority of spaces with fixed occupancy and only a few variable occupancy zones that meet the requirement, but still must implement demand Controlled ventilation for those variable occupancy zones. Single-zone HVAC systems can include, but are not limited to: 1) constant volume packaged units with stand-alone economizer controllers (e.g., Honeywell W7340 Logic Module); or 2) constant volume systems with individual dampers/actuators and either stand-alone or centralized DDC control.
The Energy Standards require that only HVAC systems with the following characteristics must employ demand Controlled ventilation:
•The HVAC system must have an economizer. The reason for this requirement is that the system must have the ability to modulate outdoor air flow.
•Spaces served with specific use types or have the following occupancy densities, as described in the California Building Code (CBC) Chapter 10, must utilize DCV control:
o Assembly areas, concentrated use (without fixed seating)
o Auction rooms
o Assembly areas, less concentrated use
o Occupancy density of 40 ft2 per person or less
Occupancy density is calculated using CBC Section 1004.1.1 CBC for spaces without fixed seating and CBC Section 1004.7 for spaces with fixed seating. However, classrooms are exempt from the demand Controlled ventilation requirement.
The Energy Standards state that the system will maintain a minimum ventilation flow rate no less than the value calculated per §120.1(c)4E.
B. Construction Inspection
The CO2 sensor is located within the control zone(s) between 3 feet and 6 feet above the floor or at the anticipated level of the occupant’s heads. This is the critical range for measuring CO2 since most occupants will be typically either sitting or standing within the space.
CO2 sensor is either factory calibrated or field calibrated. A calibration certificate from the manufacturer will satisfy this requirement. In order to perform a field calibration check, follow the calibration procedures provided by the manufacturer. Some sensor manufacturers may require using equipment-specific calibration kits (kits may include trace gas samples and other hand-held devices) whereas others may be calibrated simply by using a pre-calibrated hand-held CO2 measuring device and making proper adjustments through the sensor or ventilation controller.
Interior CO2 concentration setpoint is ≤ 600 ppm plus outdoor air CO2 value if outside concentration is measured dynamically. Otherwise, setpoint is ≤ 1000 ppm. Outdoor air CO2 concentration can be determined by three methods:
1. Assume a value of 400 ppm without any direct measurement.
2. Measure outside concentration dynamically to continually adjust interior concentration setpoint.
3. Measure outside concentration one time during system checkout and use this value continually to determine inside concentration setpoint.
C. Functional Testing
Step 1: Disable the economizer.
Disabling the economizer will prevent the outdoor air damper from modulating during the test due to atmospheric conditions rather than CO2 variations. The economizer can be disabled in a number of ways depending on the control strategy used to modulate the outdoor air dampers; however the simplest method would be to change the economizer changeover setpoint below current atmospheric conditions. The changeover setpoint is the value that will lock out the economizer, example control strategies include:
•Outdoor air dry-bulb temperature or enthalpy
•Comparison between outside and return air temperature or enthalpy
Step 2: Simulate a high space occupancy.
The intent of this test is to ensure the outdoor air damper modulates open when the CO2 concentration within the space exceeds setpoint. Simulating a high space occupancy can be accomplished by, but not limited to: 1) commanding the setpoint value to be slightly below current concentration level; or 2) exposing the sensor to a known concentration of source gas (i.e. canister of CO2 gas with a concentration greater than setpoint). In all cases you should endeavor to simulate a condition just slightly above the current CO2 setpoint. Regardless of the method used to simulate an excessive CO2 load, ensure the condition persists long enough for the HVAC system to respond.
Verify and Document
Ensure the outdoor air damper modulates open.
If the CO2 setpoint is lowered just below current concentration levels, the outdoor air damper will modulate open and the increased outdoor air should bring interior concentrations down to meet and maintain the new setpoint. If a known concentration of CO2 gas was used to simulate an elevated concentration, then the outdoor air damper may modulate fully open since the “measured” concentration will not be influenced by the increase in outdoor air (Note that §121.0(c)4C states that outdoor ventilation rate is not required to exceed design minimum value calculated in §121(b)2, regardless of CO2 concentration. Therefore, the outdoor air damper may only open to a position that provides the design minimum flow rate). If an unknown concentration was used to simulate a high load, then the outdoor air damper could modulate open and closed since the “measured” concentration may vary considerably throughout the test.
Step 3: Simulate a low occupant density.
The intent of this test is to ensure the outdoor air damper modulates towards minimum position when the CO2 concentration within the space is below setpoint. Eventually the outdoor air damper should close to a position that provides minimum ventilation flow rate per §121.0(c)4E, regardless of how far the measured interior concentration is below setpoint. Simulating a low occupant density can be accomplished by, but not limited to:
1. Commanding the setpoint value to be significantly higher than current concentration level;
2. Exposing the sensor to a known concentration of source gas (i.e. canister of CO2 gas with a concentration less than setpoint);
3. Open doors and windows to reduce CO2 concentration in the space. In each case you want the CO2 reading to be well below the setpoint.
Regardless of the method used to simulate a low occupant density, ensure the condition persists long enough for the HVAC system to respond.
Verify and Document
Ensure the outdoor air damper modulates towards minimum position. If setpoint is raised just above current concentration levels, the outdoor air damper will modulate closed and the reduced outdoor air should bring interior concentrations up to meet and maintain the new setpoint. If necessary, continue to adjust the setpoint upward until the outdoor air damper closes to a minimum position. If a known concentration of CO2 gas was used to simulate a lowered concentration, then the outdoor air damper will most likely modulate to minimum position since the “measured” concentration will not be influenced by the decrease in outdoor air.
Step 4: Return system back to normal operating condition.
Ensure all schedules, setpoints, operating conditions, and control parameters are placed back at their initial conditions.