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 hun-dred degrees Fahrenheit. Generally the vent will be a round sheet-metal pipe about six inches in diameter that leads upward from the furnace cabinet. 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. Once you have found your central heating equipment, you will notice ducts leading away from it. 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 .