A lot of people end up wasting hundreds of dollars in energy, each year, due to their ducts. Air leaks and poor duct insulation are the direct causes. But the problem can go deeper; it may also be due to the design, type, size and length of the ductwork.
Obviously, there are problems with existing ducts that can't change without replacing and changing the whole system. But whenever possible it will be worthwhile to consider a few basic principles, especially in new home construction and large home renovations.
type of ducts: metal vs flexible ducts
Consider round, rigid metal ductwork: these ducts develop less static pressure (the velocity at which the air moves within the duct) and have a longer life and are more resistant to rupture. They are the best for performance and energy savings..
Avoid flexible duct. Do not let yourself be fooled by lower prices and short-run advantages.
Flexible ducts have a few advantages: they are less expensive and are quick and easier to install (it bends easily); and may come covered with insulation and a vapor barrier, which makethem advantageous for joist bays, or to get around some tight corners, or to connect branches to register boots.
But there are downsides. Flexible ducts are much more prone to rupture and damage, have a shorter lifespan, and their turns and curves are a cause of airflow obstruction. This and their inherent higher air resistance - a penalty that can be up to twice that of metal round ducts – make them a less advantageous choice.
Some experts recommend flexible insulated duct, because of concerns involving condensation problems in the inside of non-insulated metal ducts - and the mold and mildew problems that that may cause. They are assuming that contractors will not insulate metal ducts. But that should not happens. The best is to use metal ducts and to insulate them properly.
Building cavity ducts and duct boards
Also avoid cavity ducts. Stud spaces or joist bays are not duct runs, as some contractors and builders seem to think; they perform poorly, and are a cause of serious problems.
It’s almost impossible to seal and insulate them properly, and if your house has this type of “ducts” consider replacing them, especially if they communicate with the outdoor air (floor cavity duct returns use to draw most of the air from outside, causing thermal fluctuations and pressure over the house’s shell).
Also avoid duct board; it’s too leaky and prone to damages.
If you are installing a new duct system, consider carefully its length. And do not use rules of thumb. To accept rules of thumb is to ignore the complex dynamics of heating and cooling systems, and their impact on safety, indoor air quality, comfort and moisture issues.
The size of ducts should be assessed taking into consideration the type of equipment (heat pumps require larger ducts), the type of ducts (flex vs. metal), the design (elements like curves and T's instead of Y-joints at the corners) or the length of the system; and also by considering climate issues and the size of the windows and rooms, and their insulation and sealing levels.
Many duct experts claim that the size of the ducts should involve room-by-room calculations, assuming sophisticated systems dependent on different room conditions, which is true for conventional central heating and cooling systems in homes with conventional levels of insulation and sealing.
But that’s not the case for highly insulated and air-tight homes, where the heating needs are reduced to very low levels, and where temperatures do not fluctuate significantly from room to room, and the air duct system can be simple, short and straight. And that's how it should be...
Duct design and length
The most efficient way to deliver heated and cooled air using ducts is by using short and straight runs. Complicated designs with long runs and many turns and curves, or 90º/T's pieces of ducts (instead of Y-joints) at the corners will always be a source of air-resistance and low performance.
The layout should consider a central location of the heating/cooling equipment, and the ducts should run within the living space. Reject proposals that do not conform to these rules.
See, for details: Duct Systems for New Homes
One of our videos about Ducts