E18: CO2 Sensor/Demand Control Ventilation for Fresh Air Intake – Above Green

Requirement Summary

Mechanical ventilation in principal areas of the building can be controlled by CO2 sensors. At least 50% of the building ventilation system should be controlled by CO2 sensors to claim this measure.

Intention

Mechanical ventilation introduces fresh air into the space. By installing CO2 sensors in the principal areas and covering at least 50% of the building, mechanical ventilation can be switched off when it is not required, thus consuming lesser energy. While the primary benefit of the CO2 sensors is the reduction of energy bills, the following are the other associated benefits:

Improved and consistent indoor air quality
Occupant comfort
Reduced greenhouse gas emissions; and
Extended equipment life due to less demand on the HVAC system It is recommended that the control system take frequent measurements of CO2 levels to adjust the ventilation supply to maintain proper indoor air quality.

Approach/Methodologies

No calculations are involved in the assessment of this measure. To claim that it has been achieved, the principal areas of the building must have CO2 sensors to control ventilation, covering at least 50% of the building floor area.

Potential Technologies/Strategies

The amount of mechanical ventilation can be controlled to only provide fresh air to spaces at the time that it is required. This reduces the energy consumed by the HVAC system. Traditional ventilation systems are designed to provide a constant volume of fresh air based on maximum occupancy. However, at partial occupancy levels, energy is wasted to condition outside air provided through the mechanical ventilation system even when it is not needed. The level of Carbon Dioxide (CO2) in the air exhaled by people serves as a useful indicator of the room's occupancy levels, and therefore its ventilation needs.

CO2 sensors are therefore a type of controls based on demand for the mechanical ventilation system, which reduce energy consumption while ensuring good air quality. The savings vary depending on the configuration of the HVAC system. For constant volume air-handling units (AHUs), the savings occur at the primary systems (boilers, chillers, air-conditioners, etc.), while for variable-air-volume (VAV) AHUs, the savings occur not only at the primary systems but also at the terminal boxes that include reheat.

ASHRAE Standard 90.1-2004 recommends that the building incorporate any type of Demand Controlled Ventilation (DCV), which includes CO2 sensors, when the building has a density greater than 100 people and when the AHU has an outdoor air capacity greater than 3,000 ft³/min. The following specifications are recommended in ASHRAE 90.1-2004 for the selection of the CO2 sensor:

Range: 0-2,000 ppm
Accuracy (including repeatability, non-linearity and calibration uncertainty): +/- 50 ppm
Stability (allowed error due to aging): <5% Full Scale for 5 years
Linearity (maximum deviation between a reading and the sensor’s calibration curve): +/- 2% Full Scale
Manufacturer recommended minimum calibration frequency: 5 years

Relationship to Other Measures

CO2 sensors are controls for the mechanical ventilation system that can reduce the amount of cooling or heating energy, as well as fan energy, used by the HVAC system as less outside air is moved into the building. In addition, if the building uses a water-cooled chiller for the AC, then a reduction in the water consumption is also achieved.

Assumptions

The base case assumption is that the mechanical ventilation is provided at a fixed rate. The improved case assumes that CO2 sensors are installed on all fresh air systems to control the fresh air based on the demand.

Compliance Guidance

Design Stage

Post-Construction Stage

At the design stage, the following must be used to demonstrate compliance:

HVAC layout drawings showing the location of the CO2 sensors including the mounting height; and
Specification of the sensors from the manufacturer.

At the post-construction stage, the following must be used to demonstrate compliance:

Photographs of the CO2 sensor controls in the principal areas of the building. It is not necessary to take photos of every sensor, but the auditor must be convinced that a reasonable proportion has been checked and verified; and
As-built electrical layout drawings showing the location of the CO2 sensors if changed from design; and/or
Purchase receipts and delivery notes for sensors, showing that covers or guards have been provided.