We in the energy conservation sector would like to take a moment to thank mechanical design engineers for giving us such a vast opportunity to save energy in existing buildings. By simply reducing the amount of outside air that is brought into a building for conditioning, energy services providers can nearly guarantee a sizeable drop in operating costs for commercial buildings. No disrespect, engineers — we know you have a strong directive to meet ASHRAE requirements and code basics. But because nearly every mandate ends up getting oversized, energy service providers, contractors and consultants have a lot of opportunity.

I can count on one hand the number of times I have seen a commercial space that actually needed to add more ventilation. I am sure there are more existing facilities that are underventilated, but this is not the case in the majority of situations I’ve experienced.

In nearly every building I have seen (ballparked at 300+), carbon dioxide follows one of two patterns. One shows rising and falling carbon dioxide levels which correspond with increased occupancy, as shown in the graph below. But even here, the high points of CO2 are still below the levels at which ventilation should be recommended. The other common pattern is a flat line all the way across, reflecting no change in carbon dioxide levels regardless of occupied or unoccupied time. 

The key to looking at overventilation as a conservation opportunity in commercial buildings leads to a discussion of the building’s economizer. An economizer is a system that allows a heating, ventilation and air conditioning (HVAC) system to supply up to 100 percent outside air to satisfy cooling demands, even if additional mechanical cooling might be required.

The first question to ask is: If the building has an economizer, is it functioning properly? We know that many of the control sensors and algorithms for economizers have a program bias which causes them to kick on too early. Once this is understood, then you can take a look at whether the dampers, which are supposed to control the flow of outdoor air into the building, are stuck open. Another contributing factor to consider is if the building is porous to the outside environment either due to cracks or a “leaky” building envelope. 

The simple fix is to reduce the overall ventilation rate one of two ways. One is through direct damper positioning and increased control. The other is by using demand control ventilation, which optimizes the amount of ventilation based on actual carbon dioxide levels rather than assumed ones. Modulation of the fan through variable frequency drives can help in this opportunity, as can reducing the fan size.

In either case, you’ll see some energy reduction from downsizing your fan’s power draw. Couple that the major savings resulting from downsizing your building’s demand to condition outside air, and you’ve got low hanging fruit for energy service providers.