A quality furnace provides inviting warmth and comfort when the seasons change and winter’s chill is in the air. Over time, furnace technology has continued to advance, and manufacturers produce a wide array of different furnace models that are energy efficient, dependable and priced competitively.

Fuel Options

Furnaces are configured to burn fuel oil, natural gas or propane. At one time, a furnace could be converted in the field by changing orifices, but modern appliances are fuel specific. The selection of a particular type of furnace often depends on fuel availability, installed costs and maintenance. In general, most homeowners prefer natural gas because of the convenience and relatively low cost per therm.

Propane is often the choice for those living in rural areas without a natural gas infrastructure. Propane burns clean, but a storage vessel must be installed to hold the fuel. Oil furnaces are usually replaced whenever possible since underground tanks can develop leaks with age. Since oil is a solid, internal furnace components must be regularly cleaned to eliminate the buildup of residue byproducts.

Furnace Components

A furnace is comprised of five essential components outlined as follows:

* Heat Exchanger: A heat exchanger is a core furnace component constructed from stainless steel or aluminized steel. It features hollow chambers or specially designed tubes that are used to transfer thermal energy into the surrounding air.

* Burner: The burner mixes air and gas in the controlled combustion process. This results in the release of heat energy that is absorbed by the heat exchanger.

* Vent Pipe: Also known as b-vent, the venting system provides an unobstructed pathway for waste gas to leave the combustion chamber and exit the building through a roof termination.

* Blower: A blower is a specially designed fan that pulls cooler air through the return side of the system and pushes heated air back into the living area through the supply side.

* Ignition System: Modern furnaces use an electronic ignition instead of a standing pilot for improved reliability and better safety. When the thermostat calls for heat, an intermittent pilot employs an electrical spark generator to ignite the pilot flame, which subsequently lights the burner. Conversely, a hot surface ignition system uses a controlled-resistance heating element to ignite the burner.

* Filter: Many furnaces incorporate a tray designed to hold a media filter. As air passes through the closed system, the filter traps harmful contaminants that can degrade indoor air quality and reduce system efficiency.

Furnace Operation

No matter how they are designed, all furnaces incorporate a thermostat that constantly monitors the indoor load. When heating is required, a relay is opened, and an inducer fan is activated. The blower is engaged and ramped up to a predetermined threshold while the ignition system is activated. A small, measured amount of gas is introduced into the combustion chamber and subsequently ignited.

Once the gas begins burning, the igniter shuts off. The combustion process begins to heat up a stainless steel heat exchanger. At a specific temperature, the blower fan draws indoor air across the heat exchanger where thermal energy is transferred. The heated air is then forced through the duct system back into the living area.

Furnace Types

There are three essential furnace designs that take slightly different approaches to heat generation. The different technologies were introduced to address the challenges of various climates, budget constraints and efficiency expectations.

* Conventional Warm Air Furnace: A conventional furnace has a single-stage burner, a single-speed blower and a heat exchanger. It operates at one speed regardless of the varying indoor load requirements. Single-stage furnaces are the least expensive to purchase and install, but they have the lowest efficiencies and highest operating costs.

* Induced Draft Furnaces: An induced draft furnace incorporates a fan to pull combustion waste gas directly into the venting. The action of the fan helps promote the draft process, which results in improved efficiency.

* Condensing Furnace: A condensing furnace contains a second heat exchanger that compresses moisture within the hot flue gas to pull out additional heat energy. The waste gas is vented directly outside through a short piping run that terminates on an exterior wall. Condensing furnaces can be purchased in efficiencies up to 98.5 percent, and virtually every Energy Star rated furnace uses condensing technology.

Other Furnace Enhancements

Furnace comfort and efficiency can be improved by adding two additional enhancements to basic models.

* Two-Stage Operation: A two-stage furnace features a modulating gas valve, which provides both high-fire and low-fire settings. The lower stage operates during relatively mild weather when the heating load is light. The high stage engages when the weather becomes extreme and the outdoor temperature drops below freezing. This combination helps save energy and provides better humidity control and air mixture.

* Variable-Speed Blower: A furnace equipped with a variable-speed blower affords better comfort and enhanced efficiency, especially when paired with a two-stage furnace. Longer fan cycles promote better indoor air quality and help reduce stale air pockets and drafts. A variable-speed motor can be up to six times more efficient than a single-speed model.

Finding the Right Furnace

Advanced furnaces offer exceptional comfort, enhanced efficiency and low noise when compared to models installed less than a decade ago. Improvements in furnace technology help maintain an even indoor temperature no matter how severe the outdoor conditions may be. Reputable contractors offer a variety of quality products that are designed to provide many years of dependable operation.