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.
CREATING CHANGETo help bring about this “better world to come,” here are a few helpful guidelines:
SHEET METAL DUCTSSheet 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:
FIBROUS GLASS DUCTSFibrous glass duct board has the advantage that it comes already insulated. Some suggestions for working with this material are:
EFFICIENT FLEXIBLE DUCTSThe 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:
EFFICIENT DUCTS USING BUILDING SPACESThe 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