Floor trusses work better than I-Joists when accommodating the new Rheia (ree’ uh) air distribution system being developed by eight leading national builders. As with trusses, skilled designers craft Rheia layouts and produce design calcs required by building departments. And builders and homeowners benefit from lower installation and energy costs resulting from the use of only 12 highly efficient connectors and 3” flexible duct. In short, Rheia is one of the best things that has happened to trusses.
Pulte began testing Rheia in Florida in May 2020 and has since installed systems in both truss and I-joist framing. [For all photos, See PDF or View in Full Issue.] There is no noticeable external difference from conventionally ducted systems, except for an increased number of wall and ceiling registers. These additional supply points are unobtrusive and actually provide a more uniform air distribution. Because all ducts are contained within conditioned spaces, HVAC units may be downsized. And Rheia uses conventional residential units unlike high velocity small duct retrofitted systems.
In floor truss systems, the ducts can be run diagonally anywhere along the length of the truss. This saves duct material, but most importantly it reduces HVAC tonnage requirements, as do the more gradual bends in ducts shown here.
Although 24” duct chases were provided on this project, chases are not needed with Rheia, and configurations can be designed with more standard web members. Strong backs are more likely to remain intact, since ducts may be run over or under them, resulting in improved floor system performance. Note also that K-webs were used here, but warren webs would work just as well.
In I-Joist floor systems, the location of penetrations must be pre-planned with suppliers. They can be minimized by bundling groups of ducts together and running them through large circular holes. Additional small holes are also necessary to route individual ducts to their desired locations. Note that bending the entire bundle on a 90° angle results in more line loss than the gentler bending possible with open webbed trusses.
Interior wall panels are impacted, because ducts must traverse through them to reach upper floors, since supply lines are not located in unconditioned attic spaces. This may be accomplished, as shown, with two wall panels set back-to-back providing a 4” space between 2x4 flat studs, or with a single panel with 2x8 top and bottom plates and 2x4 flat studs on either side of a 4-1/4” cavity.
In summary, trusses not only accommodate Rheia with ease, but offer significant benefits. Roof trusses are normally unencumbered by HVAC units or supply lines. Floor trusses can be designed with floating chases and standard webs. Wall panels may require prior coordination to identify locations of Rheia ducts. With increasingly stringent energy efficiency requirements, look for increasing use of Rheia systems, and rest assured that trusses accommodate them better than any alternative.