Test Comments and Applicability
The Standards require that only HVAC systems with the following characteristics must employ demand Controlled ventilation:
Single-zone systems. The intent was to limit the demand Controlled ventilation requirement to systems that primarily serve spaces with variable occupancy. Keep in mind, however, that 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 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:
•Assembly areas, concentrated use (without fixed seating); or
•Auction rooms; or
•Assembly areas, less concentrated use; or
•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 Standards state that the system will maintain a minimum ventilation flow rate no less than the value calculated per §120.1(c)4E.
Construction Inspection
The CO2 sensor is located within the control zone(s) between 3 ft. and 6 ft. 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. Else 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; or 3) measure outside concentration one time during system checkout and use this value continually to determine inside concentration setpoint.
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 a high 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(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(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); or 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.