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.
- Take the trouble to 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.
If you need to repair¿When planning a duct repair job, a plan should be developed for the job. This need not be a lengthy affair, but it should include the following steps:
Sketch the duct system. Emphasis is on the word "sketch." No detailed scale drawing or artistic talent is required, but it is very useful to have a good idea of where each duct section goes and what the connections are between the registers (visible in the living space) and the duct runouts (visible in the buffer space.)
Evaluate the fundamental integrity of the duct system. Replace any portions of the system that are decrepit or for which replacement is the most cost-effective option. Note any disconnected ducts or other large holes in the system for later repair.
Seal the ducts. Those parts of the system that are in good condition structurally, but which leak, should be sealed, and disconnected cuts should be reconnected.
Insulate ducts. Any portions of the system, old or replacement, that need insulation can now receive it.
Correct zone pressure imbalance problems. After all of the above repairs have been completed, if any zone pressure imbalances remain, they should be corrected. The final part of this step will include a worst-case depressurization test for any zones containing combustion equipment.