Stating the obvious
I subscribe to several enewsletters related to building energy systems. Recently, one arrived from the National Association of Homebuilders Research Center. It mentioned findings by the United States Department of Energy, informing us that moving the duct
Stating the obvious
I subscribe to several enewsletters related to building energy systems. Recently, one arrived from the National Association of Homebuilders Research Center. It mentioned findings by the United States Department of Energy, informing us that moving the ductwork for heating and cooling systems inside the thermal envelope of a building was an important consideration in the quest for energy efficiency.
Here’s a quote from the NAHB brief: “According to the U.S. Department of Energy publication, ‘Better Duct Systems for Home Heating and Cooling,’ energy loss in ductwork can account for over 25% of heating and cooling energy needs of a house.”
Heat loss (or gain) through ducts, especially those in unconditioned spaces, has always been a serious shortcoming of forced-air distribution systems. The heat loss from insulated piping is far less than that from ducting, assuming both have the same insulation system and operate at the same temperature.
The reason is simple: The surface area of the ducting required to deliver a given rate of heat transport is much greater than that of hydronic piping delivering the same heat. It’s analogous to a large house - with much more wall, ceiling and window area - losing heat faster than a smaller house with the same type of insulation.
Figure 1 shows a side-by-side comparison of two heat transport “conduits,” both sized to deliver 60,000 Btu/hr. On the left is a 3/4-in. size tube. On the right is a 14-in.-wide by 8-in.-tall rectangular duct. Both conduits are insulated with 1-in.-thick fiberglass insulation. Both are carrying their respective “fluids” at a temperature of 130º F and at flow rates commensurate with standard industry guidelines (4 ft. per second water-flow velocity in the tube and 1,000 ft. per minute air-flow velocity in the duct). Both are surrounded by 55º air. Under these conditions, the heat lost from the duct is almost 10 times greater than that from the tube.
Here’s another finding from that NAHB brief: “The practice [routing ducts inside the thermal envelope] is easier to implement in single-story homes and in multifamily homes with open-web floor joists. It is appropriate for retrofitting, but it might be challenging to find areas to run ducts in existing homes.”
Figure 2 shows that open-web floor trusses do make it easier to route ducting perpendicular to floor framing. Open-web trusses also simplify installation of plumbing, electrical and other utilities. They’re nice, but they’re not what you are going to find in most houses, whether new or existing. Instead, you’re going to find solid-floor framing and that’s where the fun begins.
Figure 3 shows what would be needed to conceal a 14-in.-wide by 8-in.-high duct within a floor deck constructed of 2x12 joists. The installation technique shown for the ducting is called “Sawzall surgery.” I’m sure most of you know this is not a good idea.
By contrast, a flexible 3/4-in. tube, having equal heat transport capacity, can be easily routed through small holes near the center of the joists with very little affect on their strength.
More adviceHere’s yet another statement from the NAHB brief: “If ducts are run perpendicular to floor joints and open-web floor trusses are not used, it may be necessary to construct bulkheads for the duct runs.”
I assume “bulkheads” means framing around the ducting so that it can later be covered over with drywall. An example of this practice is shown in Figure 4.
Duct placement can be a problem even when ducting runs parallel with the joists. Structural cross-bracing, blocking, recessed light fixtures or headers may be in the way. There also could be plenty of wiring and piping to contend with if the plumber or electrician was on the job before the heating contractor.
The NAHB brief provides advice intended to “simplify the process of bringing ducts into conditioned space” but also warns that one of the potential details for doing this -transfer grilles through walls - may 