Editor's note: Ron Wilkinson, PE, LEED AP, will present “The Role of Controls in Commissioning Green Buildings” during HPAC Engineering's sixth annual Engineering Green Buildings (EGB) Conference and Expo, which will be held Sept. 24 and 25 in Nashville, Tenn., as part of HVACR Week. For more information and to register, visit www.egbconference.com.

Building improvements are straightforward when the owner of a facility also is the occupant. The same cannot be said when, as in the case of many high-rise office buildings, the owner simply is the landlord, with dozens of tenants and spaces in various states and stages of occupancy. When it comes to energy, the tendency is for owners of these structures to charge tenants for all consumption and make no improvements. This is unfortunate, as opportunities for energy conservation — and, thus, carbon-dioxide (CO₂) reduction — through the regulation of ventilation air abound, bringing with them money-saving and public-relations benefits for both owners and tenants.

This article discusses how owner-controlled base-building improvements and tenant-controlled space improvements can save energy in a high-rise office building without negatively impacting one another or violating terms of lease agreements. Specifically, it discusses a process of transitioning from constant-volume to variable-air-volume (VAV) control with the intent of reducing overall energy consumption and CO₂ emissions and earning recognition through certification programs such as the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) for Existing Buildings: Operations & Maintenance (O&M) and LEED for Commercial Interiors (LEED-CI).

BARRIERS

The prevailing lease-defined owner/tenant relationship is not conducive to cooperative energy reduction. For example, HVAC use typically is not metered; tenants pay a fixed monthly rate as part of their lease. Any fluctuation in the rate from year to year is the result of external utility rates, not — directly at least — tenant consumption. Thus, if a tenant installs occupancy sensors to reduce airflow to unused portions of a space, the owner benefits, saving base-building fan energy, while the tenant does not, continuing to pay the same monthly rate.

In cases in which energy use is sub-metered, a tenant may choose to install free-cooling coils in supplemental air-conditioning units (ACUs) in an attempt to reduce power consumption. But if the owner reduces base-building cooling-tower-fan speed amid low-ambient-temperature conditions, the tenant might never receive water cool enough for the free-cooling coils to be used. The owner, then, saves through reduced cooling-tower-fan energy, while the tenant does not.

When base-building and tenant systems operate independently, simultaneous heating and cooling is possible. Although some tenant systems, such as water-leakage detection, are monitored by base-building building-management systems, few base-building-supplied heating/cooling media are reset based on tenant needs.

Even when both an owner and a tenant desire green-building certification, each may be reluctant to take the first step and gamble on the other's follow-through. Following is a discussion of how owners and tenants can work together to upgrade a building.

THE PROCESS

Consider an older building with a constant-volume base-building air-supply system and constant-volume tenant systems on each floor (Figure 1).

Step 1

If they are not already in place, the owner unilaterally installs heat-recovery and/or economizer systems for the base-building ventilation system. Retrofitting heat recovery requires that outside-air intake and ventilation exhaust occur at the same place in a building. Economizer cycles can be based on ventilation air or water-side free cooling from cooling towers.

Step 2

Even if all tenant floors have constant-volume systems, the owner installs a VAV base-building air-supply system. Although owners typically think this will not save money because the tenant systems and, thus, the base-building system never will reduce air volume, the opposite is true, even if no tenant has a VAV system in place.

Figure 1 shows a building with one or more air-riser shafts and multiple floors supplied with tempered air from one or more constant-volume base-building ACUs. Each floor has a constant-volume system. In Figure 2, the ACU (or, in newer buildings, perhaps an air-handling unit [AHU]) is equipped with economizer and/or heat-recovery systems, and the owner saves energy. Such a retrofit will be difficult if the building is equipped with older floor-by-floor ACUs; it will be easy if the owner is replacing the ACUs anyway as part of a larger upgrade. The replacement of an ACU or its controls is a good time to provide a variable-speed drive for supply/return fans, a pressure sensor in air-supply risers, and floor isolation dampers.

This completes the conversion to VAV operation for the base-building side. Although tenant VAV improvements will follow, the owner will save money immediately, even if the tenants do not reduce air volume during occupied hours. Once the base-building ventilation system is converted to VAV operation, the owner can selectively shut off air to unoccupied floors.

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