Improving the Efficiency of Your Duct System

ENERGY LOSSES AND COSTS

Typical duct systems lose 25 to 40 percent of the heating or cooling energy put out by the central furnace, heat pump, or air conditioner. Homes with ducts in a protected area such as a basement may lose somewhat less than this, while some other types of systems (such as attic ducts in hot, humid climates) often lose more.

Duct repairs could be the most important energy improvement measure you can do if your ducts are in the attic. If only one half the typical loss of uninsulated and unsealed ducts that are in attics or crawl spaces were saved, it would amount to $160 off the total heating and cooling bill in a typical home. This savings is based on the national average use of natural gas and electricity for central heating and cooling at national average energy cost of 70 cents per them, and 8 cents per kilowatt-hour. With these savings, the cost to seal and insulate the ducts would most likely be paid for after three years. These estimates apply to retrofitting an existing home. For new construction more of the ductwork would be accessible to the installer and the potential savings would be greater; and with lower cost to install sealant and insulate, the payback would be less than one year.

Duct systems lose energy in two ways: by conduction of heat from the warm surface, and air leakage through small cracks and seams. For simplicity, we'll talk about warm air for heating, but the same information applies to cooling when the air conditioner is on.

Conduction

One way duct systems lose energy is for the warm air inside the ducts to heat the duct walls, which in turn heat the cold air outside the ducts. If the ducts are in an attic or vented crawl space that is nearly as cold as the outdoors, this heat is completely lost. If the ducts are in a basement, some of the heat lost from the ducts may be recaptured by warming the basement ceiling enough to reduce the heat lost from the house.

Air Leakage

Another way that ducts lose energy is through air leakage. Sometimes this leakage is from accidental holes in the ducts or poorly connected duct sections; but even if the ducts are sealed, their operation can cause the house itself to leak more air than would otherwise be the case.

An understanding of pressure differences in the duct system helps to better understand air leakage in the home. Air moves from high pressure to low pressure. To get air to move from the supply duct into the room it serves, the air in the duct has to be at a higher pressure than the air in the room. Similarly, to move air from the room into the return duct, the air in that duct has to be at a lower pressure than the air in the room. The registers are the openings through which this air is intended to move. The duct walls provide the barriers that prevent air from moving where we don't want it to go.

The fan of the central furnace creates these pressure differences. When the fan stops, these pressures quickly equalize and the flow of air through the duct stops, too.

Figure 2 shows a duct system that does not leak. The furnace fan produces a high pressure in the supply ducts and a low pressure in the return ducts. The high pressure forces warm air from the supply ducts to flow into the rooms, and low pressure draws room air back into the return ducts.

Leaky Supply Ducts

Figure 3 shows perhaps the simplest example of duct leakage. Here the supply ducts leak, but the return ducts are air tight. Even though half the duct system is good, two bad things still happen. First, some of the air that has just been warmed by the furnace is lost. Second, this air has to be replaced. If it isn't, the house would soon be pumped down to a vacuum, and we know that doesn't happen. What does happen is that cold air from the outside is drawn into the house through cracks and small holes in the outside walls. Usually these occur around doors and windows. Some houses have more of these than others, but no house is air tight. So we've lost some of the hottest air in the house (air that just came from the furnace), and replaced it with the coldest air around (air from the outside). In other words, a leaking supply duct is an energy loser in two ways: the energy loss that does not go to the rooms, and the extra energy needed to heat cold air that leaked into the house.

Leaky Return Ducts

Suppose the supply ducts are tight but the returns leak, as shown in Figure 4. The return duct is at a low pressure- lower than the house or the outside -so cold air from the outside is pulled into this duct. This cold air is heated in the furnace (along with air that came from the house through the return registers). The amount of air delivered to the house by the supply registers is greater than what the return ducts took from the house (the difference being the cold air that leaked into the return ducts). To equalize the flows, heated room air leaks out of the house through the same holes and cracks that, in the previous example, allowed air to leak in. So cold air is pulled in and warm air leaks out. In addition to creating energy losses, leaky return ducts can create health problems. (click here to see Figure 4.)

Zone Pressurization

Ducts can cause air leakage in the house even if neither the supply nor the return ducts leak themselves. Figure 5 shows how this can happen. Imagine that a home has a return register in one room but no supply (the room on the left in Figure 5), and a supply register in another room but no return. Now close the door between these rooms. The room with the supply duct (the room on the right in Figure 5) will have relatively high pressure. The supply duct will be trying to blow this room up like a balloon. Similarly, the room with the return will have relatively low pressure. So inside air will leak out from the room on the right, and outside air will leak into the room on the left. This places an added load on the heating equipment. The situation described here is somewhat simplified to show the basic idea, but variations of it are common in real homes. Most new homes built today do not have duct returns in each room. The problem can be avoided in rooms with no return register and doors that are often closed by installing an opening covered by a louvered grill in the door or in the adjoining wall.

Energy Losses When the Fan Is Off

So far, we've been talking about what happens when the central furnace fan is running. But even when it's off (which is most of the time) the leaks in ductwork add to the air leaks in the rest of the house. The cracks in duct-work typically have an area that is 10 to 20 percent of the leakage area of the house. Over the course of a heating season, the energy losses from ducts when the fan is off can be nearly as great as when the fan is on!

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Health Hazards
Inspection of the Duct System

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