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Gas-fired Forced-air Furnaces

High-efficiency and Standard-efficiency

Over the last 20 years, a new generation of higher efficiency gas furnaces and boilers has come to market. An essential difference in the design of these units is how they are vented, eliminating the need for dilution air. The combustion of gas produces certain by-products, including water vapour and carbon dioxide. In a conventional gas furnace, such by-products are vented through a chimney, but a considerable amount of heat (both in the combustion products and in heated room air) escapes through the chimney at the same time. Heat is also lost up the chimney when the furnace is off. The newer designs have been modified to reduce the amount of heated air that escapes during the on and off cycles and by extracting more of the heat contained in the combustion by-products before they are vented.

Furnaces with these design modifications use much less energy than conventional furnaces, so consider what this means to you in dollars. Our table comparing technologies and seasonal efficiencies can help you compare your possible savings. It can will help you decide which energy-saving features will give you the most for your heating dollar.

High-efficiency Condensing Gas Furnaces

Condensing gas furnaces are the most energy efficient furnaces available, with seasonal efficiencies of between 90 and 97 percent. The high-efficiency condensing gas furnace should be the furnace of choice for most Canadians. It is:

  • cost-effective for most climatic regions of Canada
  • not susceptible to some of the condensation and long-term vent degradation problems that can occur with the standard-efficiency furnace
  • better suited for the tight construction of an energy-efficient house

In Canada under the Energy Efficiency Regulations, the AFUE performance level for gas-fired furnaces was raised to 90% effective January 1, 2010. Until a more stringent ENERGY STAR specification is finalized, ENERGY STAR qualifications for gas fired furnaces are no longer recognized in Canada.

The ENERGY STAR specification for gas-fired furnaces is currently under revision. Materials related to this revision process are provided below. Participants and other interested parties with questions or concerns regarding the revision process can consult the U.S. Environmental Protection Agency website.

High-efficiency condensing gas furnace

Heating with gas

The burners are like those on conventional furnaces, and draft is supplied by an induced draft fan. However, additional heat-exchange surfaces made of corrosion-resistant materials (usually stainless steel) extract heat from the combustion by-products before they are exhausted. In this condensing heat exchange section, the combustion gases are cooled to a point at which the water vapour condenses, thus releasing additional heat into the home. The condensate is piped to a floor drain.

Because no chimney is needed, installation costs are reduced. The flue gas temperature is low enough for the gases to be vented through a PVC or ABS plastic pipe out the side wall of the house. Depending on the design, these furnaces can use up to 38 percent less fuel than older gas furnaces equipped with pilot lights. Furthermore, polluting emissions released into the environment are also reduced.

Some condensing furnaces or boilers differ only in that they use a pulse combustion technology to ignite small amounts of gas at frequent intervals.

Unlike conventional and standard-efficiency furnaces, whose efficiency decreases with furnace oversizing, condensing furnaces are actually slightly more efficient when they are oversized and run for shorter periods. Thus, if you are choosing a new condensing furnace, you can get a furnace that is slightly larger than the house heat demand without suffering an “efficiency penalty.”

Sealed Combustion Systems

In a sealed combustion system, outside air is piped directly to the combustion chamber, and the furnace does not draw air from inside the house for either combustion or vent gas dilution.

Although heating costs may be reduced slightly by decreasing the amount of heated air that is drawn from inside the house, the main advantage of sealed combustion is that it isolates the combustion air system from the house so that the furnace is not affected by the operation of other appliances in the home. The tight construction of an energy-efficient house, combined with the operation of exhaust fans (such as in the kitchen and bathroom) and clothes dryers, can cause spillage of flue gas and backdrafting. Sealed combustion units prevent this potential safety problem.

Most high-efficiency furnaces are designed as sealed combustion systems, and so are well suited to the tight construction of a modern energy-efficient house. Those that are not sealed typically have an induced draft that is powerful enough to overcome any house depressurization. Some standard-efficiency furnaces are also available as sealed combustion systems.

Standard-efficiency Gas Furnaces

Standard-efficiency furnaces have a seasonal efficiency of at least 78 percent; most have an efficiency of 80 percent. Standard-efficiency gas furnaces use mainly a naturally aspirating burner and do not have a continuously lit pilot light.

Newer furnaces have electric ignition systems, which can consume from 3 to 5 percent less energy than a furnace with a conventional standing pilot light.

Standard-efficiency gas furnace with induced draft fan

Heating with gas

Most standard-efficiency furnaces are equipped with a powered exhaust, usually consisting of a built-in induced draft fan. With more heat exchange, no dilution air and high resistance to flow during the off cycle, seasonal efficiency is much higher for today's standard-efficiency furnaces than for furnaces equipped with pilot lights. Energy savings are between 23 and 28 percent.

These systems can be vented through a chimney or out the side wall of the house using high-grade stainless steel. Some problems have been associated with the use of high-temperature plastic vent pipes with standard-efficiency furnaces. Regulations may forbid the use of certain vent materials in your area. You should discuss all options with your local serviceperson, approvals agency or gas utility.

Installation codes may require bringing a combustion air supply to the furnace from outdoors.

Maximizing efficiency

Cost considerations: operating cost, purchase price, installation cost

To accommodate different types of houses, there are three main furnace designs for use with gas-fired forced-air systems. The designs are named according to the way air travels through the system.
  • Upflow furnaces are recommended for basement floor locations.
  • Horizontal flow furnaces are particularly suited for crawl space installations.
  • Downflow furnaces are recommended for installations in mobile homes or on the main floor of houses on concrete slabs.

Older Furnace Types

Conventional Gas Furnaces

A conventional gas-fired, forced-air heating system consists of a furnace with a naturally aspirating gas burner. Unlike newer units, which feature electronic ignition, these systems use a standing (continuously lit) pilot light. The combustion gases pass through the furnace, where they pass heat across a heat exchanger and are exhausted to the outside through a flue pipe and vent. A draft hood serves to isolate the burner from outside pressure fluctuations at the vent exit by pulling heated house air into the exhaust as required. A circulating fan passes cooled house air from the return ducts over the furnace heat exchanger, where the air is warmed and passed through ductwork to all areas of the house.

A conventional gas-fired, warm-air furnace

Heating with gas

There are two entirely separate air movement paths:

  • The combustion path supplies air to the burner and to the draft hood and carries hot combustion gases through the burner, heat exchanger and flue pipe to the vent and out of the house.
  • The heat distribution and cold air return path circulates and heats the air inside the house.

Conventional gas furnaces have a seasonal efficiency of about 60 percent. Although most Canadian homes have “heat and circulate” type of equipment, conventional furnaces are no longer available, as they do not meet the seasonal efficiency standards required by the regulations of Canada's Energy Efficiency Act.

Converted Gas Furnaces

Another common type of gas-fired system is an oil-fired furnace that has been converted to natural gas, usually with either a power burner or a power-assisted burner. This type of unit has a fan with a burner to assist in the combustion process and to maintain an adequate draft. The dilution device is a double-acting barometric damper rather than a draft hood, but it performs a similar function.

Converted oil furnaces are generally more efficient than conventional gas furnaces, with seasonal efficiencies in the range of 63 to 68 percent. However, they are not nearly as efficient as new standard- and high-efficiency gas furnaces.

Gas Furnaces with Automatic Vent Damper

Some gas furnaces have a vent damper in the flue exhaust, downstream of both the furnace heat exchanger and the draft dilution device. A thermostat controls the damper. When the gas burner turns off, the damper is closed automatically after a period; when the thermostat signals to start the furnace, the damper opens before the burner ignites. By closing off the vent during much of the off cycle, the damper prevents some of the warm household air from being drawn up the chimney and lost to the outdoors.

These furnaces usually have an electric or electronic ignition and generally consume between 3 percent and 10 percent less fuel than a conventional furnace. However, some of the savings can be lost if a conventional gas-fired water heater is also connected to the same chimney. The water heater is still vented and is burdened by an increased draft, augmenting the heat lost through the water heater.

Vent-dampered gas furnaces do not meet current minimum standards for energy efficiency.

Other gas heating options

Several other gas heating options are available besides forced-air systems. Installing a central gas-heating system may not be practical or possible if your house is built on a concrete slab or if you live in a mobile home. Specialized gas heating equipment might be a good alternative. There are many kinds available, and you should consult your gas utility or a heating contractor for a detailed assessment. The following are some of the most common types.

Direct-vent Wall Furnaces

Direct-vent wall furnaces are self-contained, sealed combustion heating appliances that draw in combustion air and discharge combustion products through a vent to the outside. They are permanently attached to the structure of a building, recreational vehicle or mobile home, and are not connected to ductwork. These units circulate heated air by gravity or with the help of a circulating fan. Units with a circulating fan yield higher efficiencies.

Wall furnaces are compact and less expensive than central furnaces. They come in a variety of heating capacities, with efficiencies that range from that of a standard-efficiency unit with a pilot light to that of a high-efficiency unit with an electric ignition and induced draft. The annual fuel utilization efficiency (AFUE) can range from 70 to 80 percent, although high-efficiency central furnaces are generally much more efficient.

Room Heaters

Room heaters are self-contained, free-standing heating appliances with heat outputs much lower than those of central furnaces. They often resemble free-standing wood stoves. They are not connected to ductwork. Because they heat only the space in which they are located, most rooms require their own units. A vent pipe allows combustion by-products to escape to the outdoors.

Heat is circulated by natural convection or with a circulating fan. Units are available with AFUE ratings between 60 and 82 percent.

A recently developed direct-vent, gas-fired baseboard heater resembles electric or hydronic (hot water) baseboards. It allows the retrofit of existing electrically heated homes without the need for a chimney or a central distribution system.

Operating and Maintaining an existing gas heating system.