Until recently, most people who bothered to think about ducts at all considered them a mere medium for transporting air. Problems with ducts were as invisible as the air they carried. Now researchers and practitioners are discovering the complex ways ducts interact with other components of heating and cooling systems, with building envelopes, and with the surrounding environment. The field is exploding with new ideas and groundbreaking technologies.

This special section of Home Energy is an up-to-the-minute and comprehensive look at this rapidly expanding field. Our guest authors are among the nation's leading experts, including the inventors of the newest diagnostic techniques, and specialists who have developed guidelines for using them. Here, in one handy reference, readers will find

• How the pros use state-of-the-art technologies to find and fix leaky ducts.

• Never-before-published studies on ducts in homes with basements.

• Which utilities have duct-sealing programs, and how they are best managed.

• A guide to installing ducts in new homes.

• A preview of what's to come, and just how much sealed and insulated ducts can save.

How Does an Air Distribution System Work?

A typical forced-air distribution system delivers air from the furnace, air conditioner, or heat pump to a home's living space, then back to the equipment to be heated or cooled again (see Figure 1). The system is designed to be a closed, pressure-balanced loop, with the same amount of air entering and leaving the conditioned zones (or thermal envelope) through a network of ducts. When the equipment's air handler is turned on, a fan forces air at high pressure into supply ducts that deliver the air to rooms in the house through supply registers. The conditioned air, ideally at neutral pressure in the thermal envelope, exits through return grilles, where negative pressure from the fan's intake pulls the air through return ducts back to the air handler. If there are leaks in the supply or return ducts, this balance is disrupted, and zones will become pressurized or depressurized, depending on where the leaks occur.

How Ducts Lose Energy

Even in a perfectly balanced system with no leaks, ducts may lose energy by conduction since they usually run through hotter or colder unconditioned zones, like attics and crawlspaces. Studies show that ducts typically lose about the same amount of energy through conduction as they do through leaks. Wrapping ducts with insulation helps reduce conductive loss.

Ducts are really extensions of the conditioned zone, so when they leak, they act like any other leaks in the thermal envelope (around windows and through bypasses, for example), and will lose energy by convection. This will happen even when the air handler is off. During the off cycle, hot air inside the ducts can rise and exchange heat with the unconditioned spaces in a cyclical movement. This is known as the thermosiphon effect.

When the air handler is on, air leakage can increase threefold due to the high air pressure produced. In supply ducts conditioned air is pushed out through holes, while leaky return ducts suck in air. Figure 2 illustrates how ducts leaking in unconditioned zones influence the pressure inside the thermal envelope. Supply ducts that leak to a crawlspace, for example, send less air to the rooms inside, causing depressurization (Figure 2A). Outside air will then rush to infiltrate the envelope through any existing leaks. Energy is lost in several ways. In winter, heated air will blow uselessly through supply leaks into the crawlspace and the warm ducts will lose heat to the cold surrounding air all along the duct run. At the same time, cold air will seep into the depressurized living space at an accelerated rate and the furnace will be working harder and longer to maintain a comfortable room temperature.

Leaks in return ducts outside the thermal envelope cause just the opposite effect (Figure 2B). The fan draws in unconditioned air through holes in the ducts, allowing less air to exit the room through the return register. Pressure builds up in the living space so that the conditioned air seeks other escape passages and squeezes its way to the outside.

Imagine the energy losses in an air conditioned home that has leaky ducts in the attic. In summertime, attic temperatures can be as high as 120deg.F, and in some climates, as high as 150deg.F. This extremely hot air is pulled into the return ducts at the same time that supply ducts are fast losing coolness by conduction. Inside, high pressure in the living area is forcing conditioned air outside. Under these conditions, the cooling efficiency of the air conditioner can be cut in half.

No Two Homes Leak Alike

In most homes, ducts leak in both the return and supply sides of the system, complicating pressure balances (and sometimes even balancing the system). They also leak anywhere--in both conditioned and unconditioned zones, and in areas that are partially conditioned, like many basements in the Northeast and Midwest. It is also typical for houses to have supply registers in every room but only one central return grille. Closing a bedroom door in one of these homes can create a separate pressurized zone in the bedroom. Then there are houses that have return grilles, but no return ducts at all, just open (and some not-so-open) passageways behind walls, ceilings, and floors.

The location of ducts and of the heating and cooling equipment can also have health and safety consequences (see "Ducts, Health, and Safety"). It's extremely important that repairs are done by trained technicians who have the appropriate tools and materials for testing and sealing ducts. As the articles in this section demonstrate, a whole-house approach to duct repair is the best guarantee for an energy-efficient, comfortable, and safe home. n

Figure 1. Typical duct system.

Figure 2. Supply leak (A) and return leak (B).

Energy savings have motivated the new wave of interest in duct sealing, but health and safety are other compelling reasons to get a duct check-up.

Leaky return ducts draw in air from the immediate environment that is then distributed throughout the home. That air can contain:

• Dust, pollen, and other airborne particles.

• Humid air that can cause condensation, mold, and mildew.

• Vapor from household cleaners and other toxic materials stored in attics, basements and garages; vehicle exhaust fumes.

• Soil gases such as radon and methane; herbicides and pesticides.

• Fumes from combustion appliances.

Sealing ducts incorrectly can make a bad situation even worse. Untrained technicians may fix only the obvious leaks, and create serious pressure imbalances. With today's technology, leaks can be sealed and tested so that the homeowner is assured both energy efficiency and air quality improves.

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