Specifying HVAC systems for new, existing office buildings
Office buildings might seem like simple structures from the outside, but engineers engaged in such projects know they can be highly complex, with specialized HVAC, indoor air quality, and other air movement needs.
J. Patrick Banse, PE, LEED AP, Senior mechanical engineer, Smith Seckman Reid, Houston
Robert Ioanna, PE, LEED AP, Vice president, Syska Hennessy Group, New York City
Douglas Lacy, PE, LEED AP BD+C, Senior associate, ccrd partners, Dallas
CSE: What unique requirements do office building HVAC systems have that you wouldn’t encounter on other structures?
Lacy: The system types used in office buildings tend to be fairly similar to those in other building types. However, the cost model and ROI for office properties is much shorter than that of institutional clients. Therefore, we have seen clients requesting more sustainable system features to reduce overall energy consumption, but within the same construction budget as their previous projects. Many office building budgets have not been prepared to accept the cost of premium associated with systems such as airside energy recovery or a move away from direct exchange (DX) or other air-cooled equipment.
Ioanna: Office buildings pose a unique requirement in that there are multiple times per year where both heating and cooling are required simultaneously. Very often the south perimeter and the interior zones of the building require cooling, while the rest of the perimeter spaces require heating. Sizing and selecting HVAC systems in an energy-efficient manner while maintaining indoor comfort becomes the design challenge.
Banse: System design that allows actual metering of services such as electrical, chilled water, domestic water, and air capacity used on a floor-by-floor or a tenant-by-tenant basis. It seems that space leases are getting more restrictive, and anything to help identify actual costs based on utility use is becoming part of the negotiation.
CSE: How can automated features and remote system control benefit office-building clients?
Banse: Being able to monitor and control the HVAC system through the BAS remotely with appropriately controlled access can be a huge benefit to an owner or facility manager. This method allows a building manager to deal with tenant issues or complaints quickly and direct staff to resolve or mitigate potential problems. Of course, the system has to be accurate to start with. We have encountered some that gave false readings indicating all airflows were within operating limits, but in actuality there was no flow and the space was overheating.
Lacy: Automated features allow facilities to move away from on-site maintenance staff for day-to-day operational needs. Working with the client to develop a good control sequence can provide robust and stable operation for a wide range of weather and space conditions. Remote system control can allow a multi-site building operator to centralize facility monitoring and controls. The remote staff can then help troubleshoot technical issues from much farther distances. Automated features also allow for the use of a third-party service corporation for maintenance in lieu of direct labor for the same duties. This can be advantageous to a property owner depending on its cost model.
CSE: What is the most important indoor air quality (IAQ) issue you typically encounter in these projects, and how do you address it?
Ioanna: One of the most common issues that cause IAQ issues revolve around the initial cleaning and quality of the air post-construction. The odors/contaminants tend to linger for a long time unless specifically addressed prior to construction completion. In order to address this, we specify requirements for air changes and filter changes prior to occupying the space. The second issue comes from operators locking out outdoor air control systems post-construction. This seems to happen when the building operators become frustrated by the level of sophistication of the system or by the lack of commissioning to verify it’s operating properly. We advise owners to commission their buildings in all four seasons and also retro-commission every 3 to 5 years. We also design the outside air control strategies as simple as possible so building operators feel comfortable and confident about the systems in place.
Lacy: Delivering the appropriate ventilation air quantities and removing unwanted odors are the most important. While deploying carbon filtration banks can mitigate most odor challenges, proper placement of roof-mounted equipment and consideration of prevailing wind patterns can help to avoid cross-contamination between exhaust and outdoor air streams. Analyzing the equipment placement beyond mere adherence to code minimum standards is important.
Banse: Several issues come to mind, such as proper filtration, delivery of properly conditioned air, maintenance of required pretreated outside air rates, and minimizing the infiltration of untreated outside air. Of these, maintaining clean air and the proper quantity of ventilation air will mitigate the other two. A good BAS set up along with trained staff helps ensure these goals can be met.
CSE: Describe any unique ventilation challenges you’ve overcome.
Ioanna: New corporate workplace policies implemented by our clients have placed an emphasis on maximizing real estate space. Thus architects are "dense packing” more people onto floor plates than in years past. They are placing more people in smaller spaces that are requiring higher amounts of outside air while increasing load densities. Higher amounts of outside air raise HVAC operation cost. The opportunity becomes how to minimize the outside air while maintaining acceptable indoor air quality conditions. We have used various CO2 monitoring and control strategies to minimize outside by modifying outside air damper and variable air volume (VAV) damper positions in concert with modifying supply temperatures and consequently fan speed.
Banse: Some of the unique challenges involved using large existing exhaust fans and large pretreated outside air units originally designed for 500,000 sq ft or more, but having the need to only serve 15% or so of finish-out space initially. Trying to use the fans for much smaller air quantities and have them operate within their stable points of operation created a challenge, not only operationally, but also in how to avoid using unnecessary energy to accomplish the task while making the systems balanceable. One method used was to allow the discharge air to recirculate to the fan inlet to create needed airflow for the fan. This project is still in the balancing stage with good results anticipated.