Why Do We Need Furnace?
Furnace heating elements are electrical resistance heaters used to produce hot air for home heating or industrial process heating. The term may refer specifically to the electric coil itself or the complete assembly (including the housing).
The first type of modern furnace introduced into residences was central heating via forced hot air as early as 1820. Hot water and steam as heat sources were first employed around 1900.
An electric-resistance heater warms the oil in an oil furnace, which is then circulated through a heat exchanger. Heat rises from heated surfaces because of their lower density and tendency for convection and radiation due to their temperature difference with surrounding materials; this is also in reverse: hotter objects emit radiant energy in the form of light and infrared waves.
A blower moves air across the heat exchanger to distribute the warm air through the home in a forced-air system. To assist in this process, a draft inducer can be used. A draft inducer creates an area of low pressure outside the furnace; thus, more oxygen is drawn into it for combustion. Another option is electric-resistance heat strips on each duct of forced hot-air heating system, which allows one input temperature control to drive different fan speeds at various coil locations according to thermostat settings; it also eliminates problems with dusty lint accumulation inside ducts due to higher velocity airflow associated with “fast fan” speeds, etc.
Typically, the blower speed is variable in oil-fired furnaces to ensure proper airflow at various system loads. The draft inducer provides the most efficient means of giving this control. Many new forced-air furnaces also include a permanent electronic damper attached to an induction fan mounted on the furnace’s heat exchanger; it senses when airflow is required and closes if high airflow is not needed. Such alternative control strategies, with either electromechanical dampers or an electronic damper, are less expensive than using variable speed fans for this purpose.
Forced-air furnaces for central heating typically have two rotary dampers, one on the blower outlet and another on an air inlet manifold. A draft inducer motor is mounted between the outlets of these dampers. The inducer motor increases the pressure drop across the rear muffler to provide adequate airflow. If it fails mechanically, its internal thermal protection will trip; this causes a lockout on high limit control and shuts down heater operation until manually reset by using circuit breaker or fuse box disconnects for power supply to the furnace unit
When forced-air furnaces are used in large commercial buildings, they often have two blowers to provide sufficient airflow when the furnace is operating at a low load (idle). It also reduces the risk of electrical overload when one blower fails because it draws more than the usual current.