Electric heating or
resistance heating converts electricity directly to heat. Electric heat is
often more expensive than heat produced by combustion appliances like natural
gas, propane, and oil. Electric resistance heat can be provided by baseboard
heaters, space heaters, radiant heaters, furnaces, wall heaters, or thermal
Electric heaters are
usually part of a fan coil which is part of a central air conditioner. They
circulate heat by blowing air across the heating element which is supplied to
the furnace through return air ducts. Blowers in electric furnaces move air
over one to five resistance coils or elements which are usually rated at five
kilowatts. The heating elements activate one at a time to avoid overloading the
electrical system. Overheating is prevented by a safety switch called a limit
controller or limit switch. This limit controller may shut the furnace off if
the blower fails or if something is blocking the air flow. The heated air is
then sent back through the home through supply ducts.
In larger commercial
applications, central heating is provided through an which incorporates similar
components as a furnace but on a larger scale.
are systems that circulate a medium
for heating. Hydronic radiant floor heating systems use a boiler or district
heating to heat up hot water and a pump to circulate the hot water in plastic
pipes installed in a concrete slab. The pipes, embedded in the floor, carry
heated water that conducts warmth to the surface of the floor where it
broadcasts energy to the room.
Hydronic systems circulate
hot water for heating. Steam heating systems are similar to heating water
systems, except steam is used as the heating medium instead of water.
Hydronic heating systems
generally consist of a boiler or district heating heat exchanger, hot water
circulating pumps, distribution piping, and a fan coil unit or a radiator
located in the room or space. Steam heating systems are similar except no
circulating pumps are required.
Hydronic systems are closed
loop: the same fluid is heated and then reheated. Hydronic heating systems are
also used with antifreeze solutions in ice and snow melt systems for walkways,
parking lots and streets. They are more commonly used in commercial and whole
house radiant floor heat projects, while electric radiant heat systems are more
commonly used in smaller "spot warming" applications.
In mild climates a can be used to air condition the
building during hot weather, and to warm the building using heat extracted from
outdoor air in cold weather. Air-source heat pumps are generally uneconomic for
outdoor temperatures much below freezing. In colder climates, can be
used to extract heat from the ground. For economy, these systems are designed
for average low winter temperatures and use supplemental heating for extreme
low temperature conditions. The advantage of the heat pump is that it reduces
the purchased energy required for building heating; often geothermal source
systems also supply domestic hot water. Even in places where fossil fuels
provide most electricity, a geothermal system may offset production since most of the energy
furnished for heating is supplied from the environment, with only 15–30%
From an energy-efficiency
standpoint considerable heat gets lost or goes to waste if only a single room
needs heating, since central heating has distribution losses and (in the case
of forced-air systems particularly) may heat some unoccupied rooms without
need. In such buildings which require isolated heating, one may wish to
consider non-central systems such as individual room heaters, fireplaces or
other devices. Alternatively, architects can design new buildings to use
which can virtually eliminate the need for heating, such as those built to the standard.
However, if a building does
need fully heating, combustion central heating offers a more solution than electric-air central heating or than other direct devices. This stems from the fact
that most electricity originates remotely using , with up to two-thirds of the
energy in the fuel lost (unless utilized for ) at the and in . In proposals exist to phase out direct
electric heating for this reason (see ).
Nuclear and hydroelectric sources reduce this factor.
In contrast, hot-water
central heating systems can use water heated in or close to the building using
high-efficiency , , or . Wet has proven
ideal. This offers the option of relatively easy conversion in the future to
use developing technologies such as and , thereby also providing .
Typical efficiencies for
central heating are: 85-97% for gas fired heating; 80-89% for oil-fired, and
45-60% for coal-fired heating.