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A furnace, referred to as a heater or boiler in British English, is a heating unit used to heat up an entire building. Furnaces are mostly used as a major component of a central heating system. The name derives from Latin word fornax,which means oven. 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 boiler or residential hot water boiler. 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. 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). Furnaces primarily run on natural gas or electricity. Furnaces that are used to boil water are called boilers.
The term furnace also refers to the various types of metallurgical furnaces, used for smelting and other metalworks, as well as industrial furnaces, which are used in various industrial applications such as chemical plants and providing heat to chemical reactions.
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, 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, 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 includes 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. 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.
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.
A furnace room is a mechanical room in a building for locating a furnace and auxiliary equipment. Such a room minimizes the visual impact of the furnace, pipes and other gear. A modern compact furnace for a single family home can readily fit into a small closet. However, care must be exercised to provide adequate ventilation as the exterior of the furnace unit emits a significant amount of heat, and a natural gas or any other fueled furnace will need an adequate amount of combustion air.
A garage should never be used as a furnace room for a number of reasons. Air leakage around the connecting ductwork and other needed passages could act to transport potentially dangerous contaminants (including carbon monoxide) from the garage into the main body of the house, ductwork and other passages between the garage and the living areas of the house could be breaches of the required fire resistant barrier between these two areas, a furnace or other such appliance would need to be protected from potential vehicle impact by bollards or some other means, and any ignition source in a garage is required to be at least 18 inches above the floor level due to the potential of explosive gasoline vapors in any garage. In the picture, the airflow is from bottom to top, with an electronic air filter on the bottom, followed by a natural gas high efficiency or condensing furnace (98% efficient) in the middle, and air conditioning coils on top.
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Heating, ventilation, and air conditioning (HVAC) is the technology of indoor and vehicular environmental comfort. 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 combined cycle power plant is an assembly of heat engines that work in tandem from the same source of heat, converting it into mechanical energy. On land, when used to make electricity the most common type is called a combined cycle gas turbine (CCGT) plant. The same principle is also used for marine propulsion, where it is called a combined gas and steam (COGAS) plant. Combining two or more thermodynamic cycles improves overall efficiency, which reduces fuel costs.
A central heating system provides warmth to the number of spaces within a building and optionally also able to heat domestic hot water from one main source of heat unlike heating, ventilation, and air conditioning system 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 improves the energy efficiency of the system as a whole.
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, heating or cooling 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 AHUs discharge (supply) and admit (return) air directly to and from the space served without ductwork
A thermal power station is a power station in which heat energy is converted to electricity. Typically, water is heated into steam, which is used to drive an electrical generator. After it passes through the turbine the steam is condensed in a steam condenser and recycled to where it was heated. This is known as a Rankine cycle. The greatest variation in the design of thermal power stations is due to the different heat sources: fossil fuel, nuclear energy, solar energy, biofuels, and waste incineration are all used. Certain thermal power stations are also designed to produce heat for industrial purposes, for district heating, or desalination of water, in addition to generating electrical power.
An air preheater is any device designed to heat air before another process (for example, combustion in a boiler 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 fueled by gas or oil. They achieve high efficiency by condensing water vapour in the exhaust gases and so recovering its latent heat of vaporisation, which would otherwise have been wasted. This condensed vapour leaves the system in liquid form, via 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 oil burner is a heating device which burns #1, #2 and #6 heating oils, diesel fuel or other similar fuels. In the United States ultra low #2 diesel is the common fuel used. It is dyed red to show that it is road-tax exempt. In most markets of the United States heating oil is the same specification of fuel as on-road un-dyed diesel.
The annual fuel utilization efficiency is a thermal efficiency measure of space-heating furnaces and boilers. The AFUE differs from the true 'thermal efficiency' in that it is not a steady-state, peak measure of conversion efficiency, but instead attempts to represent the actual, season-long, average efficiency of that piece of equipment, including the operating transients. It is a dimensionless ratio of useful energy output to energy input, expressed as a percentage. For example, a 90% AFUE for a gas furnace means it outputs 90 BTUs of useful heating for every 100 BTUs of natural gas input. A higher AFUE means higher efficiency.
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.
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, vacuum furnaces, and solar furnaces.Industrial furnaces are used in applications such as chemical reactions, cremation, oil refining, and glasswork.