Using CO2 to Optimize Ventilation
September 27, 2010
Ventilation to a space is often specified by an engineer based on the purpose of the space. All too often the outside air requirements are established as an unmeasured cfm value; this ultimately ends up being set as an outside air damper or economizer minimum position. During certain conditions this may be too much fresh air or not enough. During hot and humid conditions, this can cause a tremendous load on the equipment, causing unneeded energy consumption.
However, by monitoring CO2, return air, or individual sensors, the outside air amount can be determined by actual need and not an established value. Using a proportional integral derivative loop to reset the outside air minimum position or outside cfm required is not advised. This will typically cause hunting, which will cause erratic supply air temperatures, and possible building pressure issues.
Experience has proven that the best way to effectively control CO2 is to use an incremental approach. Using an energy management system (EMS) to monitor CO2 and damper position, a program can run every 10 minutes adjusted (adj.). When CO2 levels rise above the high-limit set point, the program increases the damper position by 5 percent. This will occur every 10 minutes, until CO2 levels are not above the high-limit set point.
The damper will continue to open until the CO2 levels are decreased or the damper position high limit is reached, 50 percent (adj.). The damper will remain at its calculated position until all CO2 levels are below the deadband of 100 ppm. If no levels are above 700 ppm, the program will begin to incrementally close the damper every 10 minutes. This incremental approach keeps CO2 levels between 700 and 800 ppm, preventing unnecessary flooding of outside air into the building.
Using this strategy can greatly increase energy conservation in facilities with varying degrees of occupancy. Other benefits of using this approach include decreased load on EMS control processors, increased longevity of damper and damper actuators due to less motion, less loading of DX or chilled-water systems, improved supply air control, increased longevity of chilled-water valve actuators, and greater humidity control.
SYSTEM OPTIMIZATIONBelow are some specific ways to achieve EMS optimization:
VAV critical zone reset: AHU discharge air set point reset.
• Simple strategy: Vary the discharge air set point based on VAV demand.
• Detailed explanation: On start-up, the discharge air set point will start at 57°F (adj.).
When a VAV box space temperature deviates from the set point, the discharge air temperature of the unit will then be reset down by 0.5° (adj.) every 15 min. (adj.). When all boxes are within set points, discharge air temperature set point will be reset up by 0.5° (adj.). Discharge air temperature set points can also be reset linearly using outside air conditions.
AHU discharge air temperature (DAT) reset: Linear outside air temperature (OAT).
• Simple strategy: Vary AHU discharge air temperature set point on change in OAT.
• Detailed explanation: In situations where the load is predictable, and more sensible than latent, the discharge air set point of an AHU may be reset on a linear scale based on change in OAT. For example: 50° OAT = 58° AHU DAT, 80° OAT = 55° AHU DAT. Any of these values can be adjustable.
AHU Economizer: CO2 demand ventilation.
• Simple strategy: Vary the outside air minimum position based on CO2 levels in the facility.
• Detailed explanation: In older buildings, a minimum outdoor air damper position is used to satisfy building ventilation requirements. Sometimes this minimum position is undesirable from an operations perspective. By monitoring building CO2 levels, either by return air or space CO2 sensors, the AHU outside air damper can be closed and only opened in economizer mode, or when the CO2 rises above desired levels.
When the CO2 level rises above the high limit of 800 ppm (adj.), the outdoor air damper minimum position can be increased from a starting level 0 or 10 percent (adj.) by 5 percent (adj.) every 15 min (adj.), to a maximum outside air damper position of 50 percent (adj.), until CO2 levels are below the high level. The damper will remain at this calculated position until CO2 levels are below the deadband of 700 ppm (adj.).
When it’s below the low-level set point, the outside air minimum position set point will be decreased by 5 percent (adj.) every 15 min (adj.) - as mentioned above, until the outside air damper is closed, or at its starting minimum positions.
Publication date: 09/27/2010