Improving the Efficiency of Your Duct System

HEALTH HAZARDS

Leakage in the duct system can be hazardous to your health. Especially problematic are leaky returns in an enclosed space such as a basement or garage that also contains the furnace. This was the situation shown in Figure 4. If the return ducts leak, their low pressure can pull down the pressure in the basement or garage as well, and this can suck flue gases from the furnace and radon gas from the soil surrounding the home. The flue gases can be hazardous to health if they contain carbon monoxide. Exposure to radon gas from the ground is the second leading cause of lung cancer (after smoking).

Although experts disagree about how common these hazards are, by upgrading the energy efficiency of the duct system you have an opportunity to avoid these potential problems in your home.

INSPECTION OF THE DUCT SYSTEM

You probably wonder how you can know if your system is losing large amounts of energy. Although it is often difficult to be sure without testing, some tell-tale signs, if present in your duct system, should make you have it checked by a professional.

It will help to make a simple diagram of the system. This can be a rough sketch. There is no need for blueprint quality here.

The first thing you need to do is find the central heating unit. That should be no problem if it is located in a basement. It is probably something you pass by almost every day. However, it may be located in an attic or crawl space.

Safety Tips

To get near the ducts, you may have to look into spaces you have never been in before, especially if the duct system is located in an attic or crawl space. Therefore, be sure to follow common-sense safety rules:

• Guard against falls, cuts, and other personal injuries.
• Do not open up or probe into any electrical devices, wires, or connections.
• Wear an approved mask if you go into an area with fiberglass or loose fill insulation.
• Before you touch any uninsulated duct, hold your hand about an inch from it to check if it is hot. This is especially important in furnaces fueled with gas or oil, because the vent pipe that carries the flue gases to the chimney looks like a small duct yet may have a temperature of several hundred degrees Fahrenheit. Generally the vent will be a round sheet-metal pipe about six inches in diameter that leads upward from the furnace cabinet. (If the furnace is one of the new high efficiency models, it might be vented horizontally.)

Filters

While you are at it, you might want to locate the filter, which is usually within the central fan unit or at the return register. The filter removes dust and other small particles that otherwise could interfere with the operation of the blower and the furnace heat exchanger. When it fills up with dust, it cannot do its job and needs to be changed.

Experts recommend that the filter be changed several times during each heating season. At about a dollar each, frequent filter changes are a low-cost way to save energy, protect your heating equipment, and remove some of the dust that otherwise would be delivered to the living space.

How to Distinguish Between Supply and Return Ducts

Once you have found your central heating equipment, you will notice ducts leading away from it. There is a supply duct and a return duct, but which is which? In one common type of installation, the return duct leads down from the basement ceiling to enter the furnace near the floor. The supply duct runs out from the top of the furnace. This kind of system is shown in Figure 1. There are many types of installations and it is not always easy to tell which is the supply and which is the return just by looking at the ducts, particularly if you are not familiar with them.

You can wait until the furnace comes on, or the thermostat can be pushed up for a short while to force the furnace to come on. If the ducts are made of sheet metal, you should be able to feel the supply ducts getting warm. The returns will not change much in temperature. Another possibility is to follow what you think are the supply ducts to one of the registers. If this is in fact the supply side, you should be able to feel air coming out of the register.

How to Identify Trunks and Branch Ducts

After you have identified the supply and return sides of the duct system, you can follow the supply side. Most often, you will see something like what is shown in Figure 1- a long, boxlike structure called a trunk which sprouts smaller branch ducts that lead out toward the rooms of the home. Another common installation has all the supply ducts branching directly from the furnace like the arms of an octopus.

Building Spaces Used as Ducts

So far we have assumed that the duct system is completely separate from the other components of the home. Often this is not so. To save money, builders sometimes use the building structure itself as part of the duct system. One common tactic is to use the spaces between basement or ceiling joists as ducts. (Joists are the horizontal-running boards-generally 2" x 10" or 2" x 12"-that support the floor above.)

Although this type of construction can be made to operate efficiently, it often leads to significant energy losses. One reason is that joist-space ducts are likely to be uninsulated. Another problem is that they may have unintended leakage paths to the outside, typically through the end of the joist cavity.

With returns, it is even more common to see portions of the building structure used as part of the duct system. Some homes have no return at all; the furnace simply has an intake grille through which basement air is drawn in to be warmed and distributed to the home.

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Tell-Tale Signs of Problem Ducts

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