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A furnace (American English), referred to as a heater or boiler in British English, is an appliance used to generate heat for all or part of a building. Furnaces are mostly used as a major component of a central heating system. Furnaces are 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 as the fluid are normally referred to as a residential steam boilers or residential hot water boilers. The most common fuel source for modern furnaces in North America and much of Europe is natural gas; other common fuel sources include LPG (liquefied petroleum gas), fuel oil, wood and in rare cases coal. 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. [1] Waste gas and heat are mechanically ventilated through either metal flue pipes or polyvinyl chloride (PVC) pipes that can be vented through the side or roof of the structure. Fuel efficiency in a gas furnace is measured in AFUE (Annual Fuel Utilization Efficiency).
The name derives from Latin word fornax, [2] which means oven.
Furnaces can be classified into four general categories, based on efficiency and design, natural draft, forced-air, forced draft, and condensing.
The first category of furnaces is natural draft, atmospheric burner furnaces. These furnaces consisted of cast-iron or riveted-steel heat exchangers built within an outer shell of brick, masonry, or steel. The heat exchangers were vented through brick or masonry chimneys. Air circulation depended on large, upwardly pitched pipes constructed of wood or metal. The pipes would channel the warm air into floor or wall vents inside the home. This method of heating worked because warm air rises.
The system was simple, had few controls, a single automatic gas valve, and no blower. These furnaces could be made to work with any fuel simply by adapting the burner area. They have been operated with wood, coke, coal, trash, paper, natural gas, fuel oil as well as whale oil for a brief period at the turn of the century. Furnaces that used solid fuels required daily maintenance to remove ash and "clinkers" that accumulated in the bottom of the burner area. In later years, these furnaces were adapted with electric blowers to aid air distribution and speed moving heat into the home. Gas and oil-fired systems were usually controlled by a thermostat inside the home, while most wood and coal-fired furnaces had no electrical connection and were controlled by the amount of fuel in the burner and position of the fresh-air damper on the burner access door.
The second category of furnace is the forced-air having atmospheric burner style with a cast-iron or sectional steel heat exchanger. Through the 1950s and 1960s, this style of furnace was used to replace the big, natural draft systems, and was sometimes installed on the existing gravity duct work. The heated air was moved by blowers which were belted 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. Energy efficiency would range anywhere from just over 50% to upward of 65% AFUE. This style furnace still used large, masonry or brick chimneys for flues and was eventually designed to accommodate air-conditioning systems.
The third category of furnace is the forced draft, mid-efficiency furnace with a steel heat exchanger and multi-speed blower. These furnaces were physically much more compact than the previous styles. They were equipped with combustion air blowers that would pull air through the heat exchanger which greatly increased fuel efficiency while allowing the heat exchangers to become smaller. These furnaces may have multi-speed blowers and were designed to work with central air-conditioning systems.
The fourth category of furnace is the high-efficiency condensing gas furnace. High efficiency condensing gas furnaces typically achieve between 90% and 98% AFUE. [3] A condensing gas furnace includes a sealed combustion area, combustion draft inducer and a secondary heat exchanger. The primary gain in efficiency for a condensing gas furnace, as compared to a mid-efficiency forced-air or forced-draft furnace, is the capture of latent heat from the exhaust gases in the secondary heat exchanger. The secondary heat exchanger removes most of the heat energy from the exhaust gas, actually condensing water vapour and other chemicals (which form a mild acid) as it operates. The vent pipes, also known as the exhaust system, are often installed using PVC pipe instead of metal venting pipe to prevent corrosion, but this will vary based on geographical location of the installation and local regulations. The draft inducer allows for the exhaust piping to be routed vertically or horizontally as it exits the structure. A typical installation arrangement for high-efficiency furnaces includes a fresh air intake (supply) pipe that brings fresh air from outside the home to the furnace combustion unit. Normally the fresh combustion air is routed alongside the exhaust PVC during installation and the pipes exit through a sidewall of the home in the same location. High efficiency furnaces typically deliver a 25% to 35% fuel savings over a 60% AFUE furnace.
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. However, the simplicity of single-stage gas furnaces come at the cost of blower motor noise and mechanical inefficiency. The blower motors on these single-stage furnaces consume more energy overall because, regardless of the heating requirements of the space, the fan and blower motors operate at a fixed-speed. Due to its One-Speed operation, a single-stage furnace is also called a single-speed furnace. [4]
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.
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.
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.
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.
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. In American practice, 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, whence 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.
Heating, ventilation, and air conditioning (HVAC) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. HVAC system design is a subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. "Refrigeration" is sometimes added to the field's abbreviation as HVAC&R or HVACR, or "ventilation" is dropped, as in HACR.
A boiler is a closed vessel in which fluid is heated. The fluid does not necessarily boil. The heated or vaporized fluid exits the boiler for use in various processes or heating applications, including water heating, central heating, boiler-based power generation, cooking, and sanitation.
A central heating system provides warmth to a number of spaces within a building from one main source of heat. It is a component of heating, ventilation, and air conditioning systems, which can both cool and warm interior spaces.
A recuperator is a special purpose counter-flow energy recovery heat exchanger positioned within the supply and exhaust air streams of an air handling system, or in the exhaust gases of an industrial process, in order to recover the waste heat. Generally, they are used to extract heat from the exhaust and use it to preheat air entering the combustion system. In this way they use waste energy to heat the air, offsetting some of the fuel, and thereby improve the energy efficiency of the system as a whole.
A fire-tube boiler is a type of boiler invented in 1828 by Mark Seguin, in which hot gases pass from a fire through one or more tubes running through a sealed container of water. The heat of the gases is transferred through the walls of the tubes by thermal conduction, heating the water and ultimately creating steam.
An air handler, or air handling unit, is a device used to regulate and circulate air as part of a heating, ventilating, and air-conditioning (HVAC) system. An air handler is usually a large metal box containing a blower, furnace or A/C elements, filter racks or chambers, sound attenuators, and dampers. Air handlers usually connect to a ductwork ventilation system that distributes the conditioned air through the building and returns it to the AHU, sometimes exhausting air to the atmosphere and bringing in fresh air. Sometimes AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork
A thermal power station is a type of power station in which heat energy is converted to electrical energy. In a steam-generating cycle heat is used to boil water in a large pressure vessel to produce high-pressure steam, which drives a steam turbine connected to an electrical generator. The low-pressure exhaust from the turbine enters a steam condenser where it is cooled to produce hot condensate which is recycled to the heating process to generate more high pressure steam. This is known as a Rankine cycle.
An air preheater is any device designed to heat air before another process, with the primary objective of increasing the thermal efficiency of the process. They may be used alone or to replace a recuperative heat system or to replace a steam coil.
A flue is a duct, pipe, or opening in a chimney for conveying exhaust gases from a fireplace, furnace, water heater, boiler, or generator to the outdoors. Historically the term flue meant the chimney itself. In the United States, they are also known as vents for boilers and as breeching for water heaters and modern furnaces. They usually operate by buoyancy, also known as the stack effect, or the combustion products may be 'induced' via a blower. As combustion products contain carbon monoxide and other dangerous compounds, proper 'draft', and admission of replacement air is imperative. Building codes, and other standards, regulate their materials, design, and installation.
A pellet stove is a stove that burns compressed wood or biomass pellets to create a source of heat for residential and sometimes industrial spaces. By steadily feeding fuel from a storage container (hopper) into a burn pot area, it produces a constant flame that requires little to no physical adjustments. Today's central heating systems operated with wood pellets as a renewable energy source can reach an efficiency factor of more than 90%.
A forced-air central heating system is one which uses air as its heat transfer medium. These systems rely on ductwork, vents, and plenums as means of air distribution, separate from the actual heating and air conditioning systems. The return plenum carries the air from several large return grills (vents) to a central air handler for re-heating. The supply plenum directs air from the central unit to the rooms which the system is designed to heat. Regardless of type, all air handlers consist of an air filter, blower, heat exchanger/element/coil, and various controls. Like any other kind of central heating system, thermostats are used to control forced air heating systems.
Condensing boilers are water heaters typically used for heating systems that are fueled by gas or oil. When operated in the correct circumstances, a heating system can achieve high efficiency by condensing water vapour found in the exhaust gases in a heat exchanger to preheat the circulating water. This recovers the latent heat of vaporisation, which would otherwise have been wasted. The condensate is sent to a drain. In many countries, the use of condensing boilers is compulsory or encouraged with financial incentives.
Economizers, or economisers (UK), are mechanical devices intended to reduce energy consumption, or to perform useful function such as preheating a fluid. The term economizer is used for other purposes as well. Boiler, power plant, heating, refrigeration, ventilating, and air conditioning (HVAC) uses are discussed in this article. In simple terms, an economizer is a heat exchanger.
Forced-air gas heating systems are used in central air heating/cooling systems for houses. Sometimes the system is referred to as "forced hot air".
An electric boiler is a device that uses electrical energy to boil water instead of fossil fuels used in traditional gas or oil boilers.
HVAC is a major sub discipline of mechanical engineering. The goal of HVAC design is to balance indoor environmental comfort with other factors such as installation cost, ease of maintenance, and energy efficiency. The discipline of HVAC includes a large number of specialized terms and acronyms, many of which are summarized in this glossary.
A package boiler is a factory-made boiler. Package boilers are available in a range of standard designs. Package boilers are used for heating and act as a steam generator for small power purposes such as self-powered industrial plants. Package boilers are low pressure designs. A low pressure means low temperature water in the heat exchanger. The large difference between the flame temperature and the heat exchanger discards most of the available entropy. Discarding most of the entropy caps the thermodynamic efficiency below the range needed to make a low pressure boiler suitable for a co-generation plants even when the available capacity is adequate for the application. Advantages of package boilers are that they can be delivered and installed as a complete insulated assembly that doesn’t require a large exclusion zone around itself. The required steam, water, fuel, and electrical connections can be made rapidly. These boilers are inexpensive to operate because their automatic burner management system doesn’t require continuous supervision and they have low scheduled maintenance costs.
Pellet heating is a heating system in which wood pellets are combusted. Other pelletized fuels such as straw pellets are used occasionally. Today's central heating system which run on wood pellets as a renewable energy source are comparable in operation and maintenance of oil and gas heating systems.
A pellet boiler is a heating system that burns wood pellets. Pellet boilers are used in central heating systems for heat requirements from 3.9 kW (kilowatt) to 1 MW (megawatt) or more. Pellet central heating systems are used in single family homes, and in larger residential, commercial, or institutional applications. Pellet boiler systems run most efficiently at full load and can usually be regulated down to 30% of full load. Since the warm up phase of pellet boilers usually takes longer than for oil or gas firing systems, short burning phases have negative effects on the fuel efficiency. In order to improve energy efficiency and reduce harmful emissions, pellet boilers are usually combined with buffer systems, such as insulated water tanks.
An industrial furnace, also known as a direct heater or a direct fired heater, is a device used to provide heat for an industrial process, typically higher than 400 degrees Celsius. They are used to provide heat for a process or can serve as reactor which provides heats of reaction. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. Heat is generated by an industrial furnace by mixing fuel with air or oxygen, or from electrical energy. The residual heat will exit the furnace as flue gas. These are designed as per international codes and standards the most common of which are ISO 13705 / American Petroleum Institute (API) Standard 560. Types of industrial furnaces include batch ovens, metallurgical furnaces, vacuum furnaces, and solar furnaces. Industrial furnaces are used in applications such as chemical reactions, cremation, oil refining, and glasswork.