A furnace is a device that produces heat. Not only are furnaces used in the home for warmth, they are used in industry for a variety of purposes such as making steel and heat treating of materials to change their molecular structure.
Central heating with a furnace is an idea that is centuries old. One of the earliest forms of this idea was invented by the Romans and called a hypocaust. It was a form of under-floor heating using a fire in one corner of a basement with the exhaust vented through flues in the walls to chimneys. This form of heating could only be used in stone or brick homes. It was also very dangerous because of the possibility of fire and suffocation.
Furnaces generate heat by burning fuel, but early furnaces burned wood. In the seventeenth century, coal began to replace wood as a primary fuel. Coal was used until the early 1940s when gas became the primary fuel. In the 1970s, electric furnaces started to replace gas furnaces because of the energy crisis. Today, the gas furnace is still the most popular form of home heating equipment.
Wood and coal burning furnaces required constant feeding to maintain warmth in the home. From early morning to late at night, usually three to five times a day, fuel needed to be put in the furnace. In addition, the waste from the ashes from the burnt wood or coal must be removed and disposed.
Today's modern furnace uses stainless steel, aluminized steel, aluminum, brass, copper, and fiberglass. Stainless steel is used in the heat exchangers for corrosion resistance. Aluminized steel is used to construct the frame, blowers, and burners. Brass is used for valves, and copper in the electrical wiring. Fiberglass is used insulate the cabinet.
The original gas furnace consisted of a heat exchanger, burner, gas control valve, and an external thermostat, and there was no blower. Natural convection or forced air flow was used to circulate the air through large heating ducts and cold air returns to and from each room. This system was very inefficient—allowing over half of the heated air to escape up the chimney.
Today's gas furnace consists of a heat exchanger, secondary heat exchanger (depending on efficiency rating), air circulation blower, flue draft blower, gas control valve, burners, pilot light or spark ignition, electronic control circuitry, and an external thermostat. The modern furnace is highly efficient—80-90%, allowing only 10-20% of the heated air to escape up the chimney.
When heat is requested from the thermostat, the burners light and throw heat into the primary heat exchanger. The heated air then flows through the secondary heat exchanger (90% efficient furnace only) to the exhaust flue and chimney. The average furnace has three heat exchangers each producing 25,000 BTUs for a total of 75,000 BTUs. A flue draft blower is placed in the exhaust flue to supercharge the burners and increase efficiency. The heat exchangers perform two functions: transfer heated air from the burners to the home and allow dangerous exhaust gases to escape up the chimney. The transferred heated air is circulated throughout the home by a large blower and heating ducts.
- The primary heat exchanger is formed from two separate pieces of 409 stainless steel sheet. Each half is formed into shape by a 400 ton hydraulic press. The two halves are then fused together by a 25 ton hydraulic press.
- The secondary heat exchanger is formed from 29-4°C stainless steel tubing and fins. The fins are welded to the tubing to form a radiator type configuration.
- The primary heat exchanger is crimped to the secondary heat exchanger through a transition box. The flue draft blower is attached to the secondary heat exchanger.
- The burners are constructed of aluminized steel and arrive at the plant preformed. They are then attached to a plate on the input side of the primary heat exchanger. There is one burner for each heat exchanger in the furnace.
- The vendor supplied gas control valve is mounted to the heat exchanger and burner assembly. It is connected to the burner through a pipe.
- The air circulation blower housing is formed through the same hydraulic press formation as the primary heat exchanger. The vendor supplied motor and squirrel cage rotor are connected and attached to the blower housing with brackets.
- A plate is then attached for mounting the blower assembly to the heat exchanger assembly. Another mounting plate containing the vendor supplied furnace control circuitry and transformer are attached to the blower housing.
- The air circulation blower assembly is then mounted to the heat exchanger assembly with screws and nuts.
- The cabinet consists of two doors and the cabinet housing. The cabinet housing is supplied as a flat pre-painted sheet of steel and placed in a hydraulic press to form a three sided configuration. Sheets of fiberglass insulation are glued to the sides of the cabinet.
- The cabinet is installed around the furnace assembly and secured with screws and nuts. The doors are installed on the front of the cabinet assembly. The completed assembly is boxed and prepared for shipment.
Each completed furnace undergoes an extensive series of tests. Checks for proper operation of the flue draft and air circulation blowers are performed. The gas valve is checked for proper operation. The heat output of the furnace in BTUs is measured. A dielectric test is performed for shorts.
Scrap metal from cutting and forming operations are collected and sent to recycling plants for reclamation. Any excess piping is either reused or discarded. Defective steel sheets can be sent back tot he manufacturer and reformed, depending on the extent of the damage. The majority of the components of the furnace are able to be recycled.
Furnaces have come a long way in the past 30 years. A primary focus by manufacturers is zone control of every room in the home. Each room will have a thermostat that will regulate heat flow to that individual room. As technology advances, these thermostats will be able to process voice commands or commands placed through a cell phone or computer.
Where to Learn More
"Hypocaust." The Romans in Britain Web Page. December 2001. <http://romans-inbritain.org.uk>.
Armstrong Air Conditioning Web Page. December 2001. <http://www.aac-inc.com>.
furnace, enclosed space for the burning of fuel. There are many kinds of furnaces, the type depending upon the fuel and the use to which the heat produced within it is put. Most familiar are the furnaces used in the heating of buildings. In the hot-air furnace, fuel is burned within an inner wall and air, led into a space between the inner and the outer wall, is heated and is led away to the various rooms of the building. Hot-water furnaces, by which water is heated to be led through pipes to radiators, and furnaces that turn water to steam for heating purposes are common. The kiln is a kind of furnace. In metallurgy, the separation of many metals from their ores is accomplished by the use of various kinds of furnaces, e.g., the blast furnace and the reverberatory furnace. The structure of these furnaces makes possible a good control of temperature. In the production of steel, however, the open-hearth furnace and the Bessemer converter are used in the treatment of cast iron. The electric furnace is extensively employed in the production of high-grade steels for use in making steel alloys and for the manufacture of high-speed tools. Heat may be generated in such a furnace by using an electric arc or by sending an electric current through resistive elements in the furnace. If the material to be processed is electrically conductive, heat may also be generated by creating an electric current in the material by induction or by inserting into it electrodes to which a voltage is applied. In the preparation of phosphorus from calcium phosphate, this compound of phosphorus is mixed with sand and coke and treated in an electric furnace. An electric current is sent from one electrode to another through the mass to create the extremely high temperature needed to bring about the chemical action that results in the production of free phosphorus. Graphite is produced from coal or coke in an electric furnace, and the extremely hard substance carborundum is made there by the combination of carbon and silicon (from sand). Nitrogen is obtained from the air (in the Birkeland-Eyde process) by passing a stream of air through an arc. The nitrogen and oxygen of the air combine to form nitric oxide.
fur·nace / ˈfərnəs/ • n. an enclosed structure in which material can be heated to very high temperatures, e.g., for smelting metals. ∎ an appliance fired by gas, oil, or wood in which air or water is heated to be circulated throughout a building in a heating system. ∎ used to describe a very hot place: her car was a furnace.