It might seem that things are in better shape with a new duct system than with an existing one. After all, in a new building there is still freedom to design and build the system “from the ground up.” However, even in new buildings contractors face a major problem: they don’t make all the decisions.
In the best of all possible worlds, the architect, builder, and HVAC contractor would work together to achieve a duct system that would deliver heating and cooling safely, comfortably, and efficiently.
Working as a design team, they would follow these guidelines in achieving a well-designed building system that would include an efficient air moving system.
In this, as in most other areas of human endeavor, the real world often falls far short of the ideal. The HVAC contractor is usually brought onto the job only after the house is built. The contractor is also limited by the resources of time, materials, and effort he can apply to the job, which he most likely got by being the low bidder.
Lacking the incentives and the resources to install an excellent duct system, a contractor faces a number of challenges. But there are actions a contractor can take to change the situation.
Such change might be brought about fairly simply. The architect or the builder could add one simple sentence to the specifications: “The duct system shall have a seasonal distribution efficiency of at least 90% in both the heating and cooling modes, and this shall be confirmed through testing using ASHRAE Standard 152.”
This changes the game. If such a specification were the norm, or at least became common, contractors bidding on jobs would have to take the requirement into account. To keep the bids from being raised too much by this requirement, designers of homes would need to consider the duct system during the design, both in terms of optimizing the heating and cooling loads and of facilitating good choices for duct location.
To help bring about this “better world to come,” here are a few helpful guidelines: First, do no harm. This dictum from the medical profession applies here in the sense of not making serious mistakes. Probably the most important of these is not to use the building as the duct system. One might of course ask “Why not?” if the parts of the building one proposes to use are in the conditioned space. But as we’ve seen, what seems to be in the conditioned space often isn’t, or more precisely, what seems to be in the conditioned space often connects to the outside in ways that can’t be anticipated. As with almost any rule, there may be exceptions, but these would be limited to design concepts that have been tested in the field by competent personnel and found to have acceptably low outside leakage. Even here, though, one builder’s success should not be taken for granted. Any duct system that could possibly have outside leakage should be tested to confirm its soundness. Minimize heating and cooling loads. This, of course, is for the architect and builder rather than the HVAC contractor, but still it needs to be kept in mind by all parties. Downsize the loads and you can downsize the equipment and ducts, reducing HVAC design and installation problems to a minimum. Consider placing the ductwork in the conditioned space. Again, this is a design issue that must at least be acquiesced to by the architect and builder, but which will be seen more and more often as time goes on. If this isn’t possible, the basement is the next best choice if the house has one. Attics are generally the poorest choices, but are very common in hot, humid climates where most houses are slab-on-grade and the attic is the only available space that is “out of sight.” It’s this type of situation where making the effort to get the ducts into the living space pays the biggest dividends for the homebuyer. Minimize the surface area of any ducts that are not in the conditioned space. Granted, putting the ducts in the living space is sometimes not an option. Even so, it is still not necessary to saddle the homeowner with an excessively bulky system. When the exterior walls and windows are designed to have high thermal resistance values, the need to place registers beneath windows fades, and a much more compact duct system with registers on interior walls becomes feasible. Make sure that any ducts that are not in the conditioned space are sealed and insulated. The one exception is that in an unconditioned basement it may not be necessary to insulate return ducts (although they should be sealed). Test the completed duct system for leakage. If, despite every attempt to follow good practice, the leakage rates are found to be excessive, treat the system as you would a retrofit and set things right.
SHEET METAL DUCTS Sheet metal has the advantage of durability, and it is the most commonly used duct material in many parts of the United States. Perhaps its greatest drawback, at least from an efficiency standpoint, is that it is quite possible for a good-looking, professionally installed system to leak badly.
It’s also possible for a duct system that is “designed on the fly” to end up much more convoluted than it needs to be. The following points should therefore be kept in mind when working with sheet metal: Design the duct system according to ACCA Manual D. Minimize the surface area of ducts that are not in the conditioned space. Consider two or three approaches to ducting the house, and run Manual D calculations for each. You may find that some systems can provide the same level of comfort as others and yet be more compact. Chances are that after a while you’ll develop some intuition about what usually is the best approach to duct layout for the house types in your area, and you’ll no longer have to do more than one generic layout. Of course, the simplest method (at least conceptually) to minimize the amount of ductwork outside the conditioned space is to put all of it inside. Make sure that duct sections are firmly connected with screws and then (if outside the conditioned space) sealed with mastic. Insulate all ducts outside the conditioned space. (A possible exception to this requirement would be return ducts in an unconditioned basement.) Consider using R-8 insulation on supply ducts. Resist the temptation to use panned joists and other applications of the building as ductwork.
FIBROUS GLASS DUCTS Fibrous glass duct board has the advantage that it comes already insulated. Some suggestions for working with this material are: As with sheet metal, design the duct system according to ACCA Manual D and minimize surface area outside the conditioned space.
In locating duct board systems within a house, consider whether people (occupants or other trades) are likely to damage the ducts while attempting to climb over them or by storing household goods on top of them. Place the ducts to minimize this possibility. Fabricate the duct sections carefully so that they will fit together snugly. Clean surfaces thoroughly before applying closures. Although a special type of tape is generally used to attach the pieces of this ductwork and seal it, duct board systems will leak, perhaps immediately but certainly in time, if the tape is not meticulously applied. Support the duct sections properly. Fibrous glass duct board is not a structural or load-bearing material.
EFFICIENT FLEXIBLE DUCTS The greatest advantage of flexible ducting, as far as efficiency is concerned, is that they don’t leak between ends unless torn. They are easy to work with, and this can be their greatest drawback when unqualified installers are used. Suggestions for working with flexible ducts include: As with sheet metal or duct board, design the duct system according to ACCA Manual D and minimize surface area outside the conditioned space. Support the sections properly. Leaving too much space between supports will result in excess sag, which increases pressure drops and cuts airflow. That can lead to greater leakage at the ends of the flexible duct runs or elsewhere in the system. Cut the duct sections to the proper length before installing. Serpentine bends in excess length has the same effect in the horizontal dimension that poor supporting practice does in the vertical. Make sure all end connections are tight and sealed with mastic, to ensure long-lived, leak-free joints.
EFFICIENT DUCTS USING BUILDING SPACES The most appropriate comment here is, “Don’t try.” However, if it is absolutely necessary to pan joists, then do it as sparingly as possible and, where it is done, smear the inner sides and top of the joist space with mastic before applying the sheet metal cover.
After installing the sheet metal strip, seal the edges with mastic as well. And don’t forget the ends. Don’t assume that the plate or sill at the end of the joists will form a seal. Install and mastic seal special ends just for the duct section.
Andrews is with the Department of Energy’s Brookhaven National Laboratory. This article is excerpted from the book Better Duct Systems for Home Heating and Cooling. For information on obtaining the publication, visit www.pubs.bnl.gov/pubs/documents (website).
Publication date: 07/29/2002