Furnace Types

Furnaces can be classified into four general categories, based on efficiency and design.

Natural draft

The first category of furnaces are natural draft, atmospheric burner furnaces. These furnaces consist of cast-iron or riveted-steel heat exchangers built within an outer shell of brick, masonry, or steel. The heat exchangers are vented through brick or masonry chimneys. Air circulation depends on large, upwardly pitched pipes constructed of wood or metal. The pipes channel the warm air into floor or wall vents inside the home. This method of heating works because warm air rises.

The system is simple, has few controls, a single automatic gas valve, and no blower. These furnaces were originally made to work with any fuel simply by adapting the burner area. They have been operated with wood, coal, trash, paper, natural gas, or fuel oil. Furnaces that use solid fuels require daily maintenance to remove ash and “clinkers” that accumulate in the bottom of the burner area. These furnaces have been adapted with electric blowers to aid air distribution and speed moving heat into the home. Gas and oil-fired systems are usually controlled by a thermostat inside the home, while most wood and coal-fired furnaces have no electrical connection and are controlled by the amount of fuel in the burner and position of the fresh-air damper on the burner access door.

Forced air

The second category of furnace is the forced-air, atmospheric burner style with a cast-iron or sectional steel heat exchanger. The heated air is moved by blowers which are belt driven and designed for a wide range of speeds. These furnaces were still big and bulky compared to modern furnaces,and had heavy-steel exteriors with bolt-on removable panels. This style furnace still uses large, masonry or brick chimneys for flues and eventually designed to accommodate air-conditioning systems.

Forced draft

The third category of furnace is the forced draft, mid-efficiency furnace with a steel heat exchanger and multi-speed blower. These furnaces are physically much more compact than previous styles. They are equipped with combustion air blowers that pulls air through the heat exchanger. This greatly increases fuel efficiency while allowing the heat exchangers to become smaller. These furnaces may have multi-speed blowers and are designed to work with central air-conditioning systems.

Condensing furnace

The fourth category of furnace is the high-efficiency, or condensing furnace. High-efficiency furnaces can achieve from 89% to 98% fuel efficiency. This style of furnace includes a sealed combustion area, combustion draft inducer and a secondary heat exchanger. Because the heat exchanger removes most of the heat from the exhaust gas, it actually condenses water vapor and other chemicals (which form a mild acid) as it operates. The vent pipes are normally installed with PVC pipe versus metal vent pipe to prevent corrosion. The draft inducer allows for the exhaust piping to be routed vertically or horizontally as it exits the structure. The most efficient arrangement for high-efficiency furnaces include PVC piping that brings fresh combustion air from the outside of the home directly to the furnace. Normally the combustion air (fresh air) PVC is routed alongside the exhaust PVC during installation and the pipes exit through a sidewall of the home in the same location.

Single stage

A single-stage furnace has only one stage of operation, it is either on or off. This means that it is relatively noisy, always running at the highest speed, and always pumping out the hottest air at the highest velocity.

One of the benefits to a single-stage furnace is typically the cost for installation. Single-stage furnaces are relatively inexpensive since the technology is rather simple.

Two stage

A two-stage furnace has to do two stage full speed and half (or reduced) speed. Depending on the demanded heat, they can run at a lower speed most of the time. They can be quieter, move the air at less velocity, and will better keep the desired temperature in the house.

Modulating

A modulating furnace can modulate the heat output and air velocity nearly continuously, depending on the demanded heat and outside temperature. This means that it only works as much as necessary and therefore saves energy.

Heat distribution

The furnace transfers heat to the living space of the building through an intermediary distribution system. If the distribution is through hot water (or other fluid) or through steam, then the furnace is more commonly called a boiler. One advantage of a boiler is that the furnace can provide hot water for bathing and washing dishes, rather than requiring a separate water heater. One disadvantage to this type of application is when the boiler breaks down, neither heating nor domestic hot water are available.

“Octopus” furnace with oil burner.

Air convection heating systems have been in use for over a century. Older systems rely on a passive air circulation system where the greater density of cooler air causes it to sink into the furnace area below, through air return registers in the floor, and the lesser density of warmed air causes it to rise in the ductwork; the two forces acting together to drive air circulation in a system termed ‘gravity-fed’. The layout of these ‘octopus’ furnaces and their duct systems is optimized with various diameters of large dampered ducts.

Forced-air gas furnace

By comparison, most modern “warm air” furnaces typically use a fan to circulate air to the rooms of house and pull cooler air back to the furnace for reheating; this is called forced-air heat. Because the fan easily overcomes the resistance of the ductwork, the arrangement of ducts can be far more flexible than the octopus of old. Separate ducts collect cool air to be returned to the furnace. At the furnace, cool air passes into the furnace, usually through an air filter, through the blower, then through the heat exchanger of the furnace, when it is blown throughout the building. One major advantage of this type of system is that it also enables easy installation of central air conditioning, simply by adding a cooling coil at the outlet of the furnace.

Air is circulated through ductwork, which may be made of sheet metal or plastic “flex” duct, and is insulated or uninsulated. Unless the ducts and plenum have been sealed using mastic or foil duct tape, the ductwork is likely to have a high leakage of conditioned air, possibly into unconditioned spaces. Another cause of wasted energy is the installation of ductwork in unheated areas, such as attics and crawl spaces; or ductwork of air conditioning systems in attics in warm climates.

Central heating

A home furnace is a major appliance that is permanently installed to provide heat to an interior space through intermediary fluid movement, which may be air, steam, or hot water. Heating appliances that use steam or hot water are normally referred to as a residential steam boiler or residential hot water boiler.

The most common fuel source for modern furnaces is natural gas; other common fuel sources include LPG (liquefied petroleum gas), fuel oil and in rare cases coal or wood. In some areas electrical resistance heating is used, especially where the cost of electricity is low or the primary purpose is for air conditioning.

Modern high-efficiency furnaces can be up to 98% efficient and operate without a chimney, with a typical gas furnace being about 80% efficient. Waste gas and heat are mechanically ventilated through PVC pipes that can be vented through the side or roof of the house. Fuel efficiency in a gas furnace is measured in AFUE (Annual Fuel Utilization Efficiency).