Truss designers be aware: energy codes are changing HVAC systems. Until recently, duct layouts were designed like truss layouts of the 1970s – hand-drawn on blue prints. Since Building Officials don’t require residential heating/AC layouts, you may have not known of collisions until truss members were cut. Fortunately, the changes ahead may lessen duct runs and possible interferences with trusses, and may actually favor floor trusses. And, you may soon receive engineered duct layouts electronically that ultimately will integrate into whole house models like Sapphire Structure.
It’s about time that heating/AC contractors engineer every house like we are required to do. Their old “rules of thumb” that sized mechanical systems on the ‘worst’ combination of options often fail on site-specific plans. The impact of over-design in mechanical systems departs significantly from conservative structural design. For example, if we supply a two ply girder where a maxed out single member will work, the homeowner derives a better value. However, if a two ton cooling system is supplied where a one ton is adequate, the homeowner may never be comfortable. A good HVAC design must be properly sized and balanced, and involves at least as much engineering as a roof truss system. Every individual component of a heating and cooling system, even a register in the back bedroom, affects the entire system.
Scott Austin, owner of Structural, Inc. in Maryland, knows the value of coordinating truss layouts with his local HVAC providers. By designing floors, walls, and roofs in a single model, the locations of both duct runs and risers can be preplanned (see figure). And, most significantly, Scott’s high density projects require the flexibility of open web floor trusses to accommodate ductwork. Floor trusses are also advantageous in single family applications in order to keep ducts within conditioned spaces that are often code mandated.
Finally, truss designers should be aware of the powerful mechanical design software that is used increasingly by builders and their subcontractors. Wrightsoft, by MiTek, and similar programs enable faster and more accurate site-specific mechanical layouts that may lessen interferences. Additionally, by right-sizing components and minimizing the size and length of duct runs, potential collisions are minimized. In summary, collaborating with local HVAC contractors enables component designers to add significant value to their design work.