Written by: Scott Lance, EBCP, LEED AP O&M – Engineering Manager
Currently, there is a popular trend of controls companies providing demand control ventilation (DCV), or CO2 control, in existing buildings. DCV is normally introduced for energy saving purposes, in some rare cases it is installed to remedy a lack of outside air being introduced for ventilation. But, DCV for energy saving purposes is not as easy as reducing intake outside air at the air handling units (AHUs) based on building CO2 levels.
Typically, control companies are installing CO2 sensors in the space or in the return air duct work to an AHU. These are good locations to monitor CO2 levels for an entire building or building zone. But, where are the energy savings calculated? The energy savings calculation is often based on the reduction of outside air right at the AHU. A price per conditioned CFM of air is figured out for heating and cooling. However, it is much better to take the readings at your energy sources, not at the point of use. Verifying savings at the main energy sources will show savings at the point of use, and will also take into account energy losses at terminal units; if alterations to the system are not correctly made, verified, or affected by change of seasons.
When a building or space is designed, there are many factors that are taken into account to calculate minimum outside air requirements. The main factor is occupancy: this many people equals this much required fresh air. In some cases, exhaust from the building needs to be considered. Restrooms, garbage rooms, kitchens, isolation rooms and even lab processes have dedicated exhaust. Typically, you don’t want your building exhaust to be greater than building supply. This will cause the building to be negatively pressurized and each time an exterior door is opened in the winter season a cold draft will enter the building. This event does not go away when the door is closed, any opening in the building will pull outside air in. Even though savings are shown on the AHU level, it is not the case on the terminal unit level. All that infiltration is being heated or cooled by VAVs, CUHs, Chilled Beams, etc.
How do you stop this effect? It’s easy – add a building or zone pressure sensor to monitor all the exhaust air flows. The pressure sensor is the fastest, cheapest and easiest solution to this issue. What happens is the return CO2 levels will be lower than setpoint, the controls will calculate a new outside air minimum setpoint. But, when the building pressure gets too low, the calculation will trim out in order to maintain the building slightly positive. This will eliminate drafts every time a door is opened and make doors close properly. We have all been in a building where doors almost slams shut or the door closer can’t shut them due to building pressure. This will ensure terminal equipment is not heating or cooling more load then originally designed to condition and will make all the energy calculations at the AHUs accurate for the entire building or space.
Now building pressure is resolved, are there any other opportunities for savings based on CO2? There are and that solution is in conference rooms gyms, and other areas not always fully occupied, but have high air flow demands. Typically, conference rooms either not occupied or they are occupied to maximum capacity. A CO2 sensor in spaces like this can provide additional savings. When not occupied, the CO2 levels will lower and the air flow entering the space can be reduced (outside air flow at the AHU serving the space can be lowered in some instances as well). Many group areas have a minimum air flow setpoint for occupied mode and a lower setpoint for unoccupied (sometimes this can be zero). If the air flow setpoint can be lowered during occupied mode (when not many people are in the space) and room temperature setpoint is maintained, then let the room go below the design air flow setpoints. The outside air flow at the AHU serving the space with low CO2 will be reduced resulting in energy savings. Once occupied, one or two things will happen, space temperature will rise, CO2 levels will rise, or both. In response, the VAV for that space will increase air flow to maintain temperature or CO2 levels. The same strategy also works in gyms, areas with office cubicles, and other group areas.
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