Design often creates the sensation of being in a box with the sides moving in and no means of escape. I’m guessing I’m not alone in this experience. I recently felt this when a client’s highly constrained, existing architecture pushed the design toward an obvious but novel approach of ganging two large air handlers, one in a rooftop penthouse and the other in the basement directly below the penthouse, by common supply and return risers.
This idea was inspired when the physical constraints of the existing building along with the volume of outside air required to satisfy ASHRAE 62.1 triggered the energy recovery requirements of ASHRAE 90.1, which mandate an enthalpy recovery ratio of at least 50 percent. This last requirement pushed the design to an enthalpy wheel — the only means to achieve a 50 percent recovery in the client’s climate. As a result, the proposed system introduces the outside air through the penthouse unit with the basement unit, which is too constrained to be equipped with a wheel, becoming a recirculating unit. The outside air from the penthouse unit mixes with return and supply air from the basement unit in the risers. In this project, the side of the box representing ASHRAE 90.1 and the side representing ASHRAE 62.1 were pushing in as well as the sides representing the physical constraints of the building and the owner’s project requirements (OPR), which include the all-important budget constraints. How does an engineer navigate the web of codes (regulatory requirements), industry standards, physical constraints, and OPR? This article explores the balance between ASHRAE Standards 62.1 and 90.1 and possible solutions.