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 not be appreciated by the owners.
1. “Transfer grilles from bedrooms, combined with central returns, simplify the process of bringing ducts into conditioned space.”
2. “When transfer grilles are used, homeowners may be concerned about noise transmission through walls.”
What's the point?I suspect some of you perceive me as a “duct hater.” Well, I’ll admit that blowing air - a fluid with relatively poor heat transport properties - through conduits that are often difficult to integrate into buildings without aesthetic or structural compromises never made much sense to me. This dilemma is what got me interested in hydronics as the better way to move heat through buildings.
Still, it would be foolish of me to think ducted systems will someday disappear from the HVAC industry. They are certainly needed for ventilation and with a multibillion-dollar-per-year North American industry sustained by their use, I don’t expect them to go away anytime soon.
Solutions such as “ductless” heat pumps provide an alternative to the installation contortions often required with conventional ducts. So do the small diameter flex ducts used in “high-velocity” air handling systems. We’ve designed several systems using the latter in combination with chilled water cooling.
After reading the NAHB and DOE publications on this subject, I felt it was time to remind readers to look at the big picture and recognize several points that support the case for hydronics. Here are a few for your consideration:
My calculations show a properly sized ECM circulator, running at full speed under design load conditions, operating on about 10% the wattage of an ECM blower delivering equal heat transport. In a 20-year life-cycle cost comparison, this difference can be thousands of dollars in electrical savings.
Don't miss the opportunityToo often we focus on describing the high thermal efficiency of the mod/con boilers we offer without ever mentioning the inherent distribution efficiency, or minimally invasive installation, that hydronics offers in comparison to forced-air delivery systems. Modern condensing furnaces will always be able to equal or exceed the thermal efficiency of modern boilers when it comes to producing heat.
However, a properly designed hydronic system can precisely distribute heat through a building using a fraction of the electrical energy required by a forced-air system. It can do this without forcing owners to accept details such as soffits, bulkheads, tray ceilings and transfer grilles just to cover up the heating system.
Perhaps someday the DOE or NAHB will come out with an advisory publication describing all the benefits of modern hydronics. If that happens, I hope they subtitle it: Stating the obvious.