Heating Systems 321
is accomplished by passing an electrical current through wires or con-
ductors to the load device. By comparison, the heat pump operates by
circulating a gas or liquid through pipes that connect an inside coil to
an outside coil. Electricity is needed in both cases as an energy source to
make the systems operational.Resistance Heating in Buildings
When an electric current flows through a conductive material, it
encounters a type of opposition called resistance. In most circuits this
opposition is unavoidable, to some extent, because of the material of the
conductor, its length, its cross-sectional area, and its temperature. The
conductor wires of a heating system are purposely kept low in resistance
to minimize heat production between the source and the load device.
Heavy-gauge insulated copper wire is used for this part of the system.
The load device of a resistance heating system is primarily respon-
sible for the generation of heat energy. The amount of heat developed
by the load depends upon the value of current that passes through the
resistive element. Element resistance is purposely designed to be quite
high compared to that of the connecting wires of the system. An alloy of
nickel and chromium called Nichrome is commonly used for the heating
elements.
Resistive elements may be placed under windows, or at strategic
locations throughout the building. In this type of installation, the ele-
ments are enclosed in a housing that provides electrical safety and ef-
ficient use of the available heat. Air entering at the bottom of the unit
circulates around the fins to gain heat, than exits at the top. Different
configurations may be selected according to the method of circulation
desired, unit length, and heat-density production.
Resistive elements are also used as a heat source in forced-air cen-
tral heating systems. In this application, the element is mounted directly
in the main airstream of the system. The number of elements selected for
a particular installation is based upon the desired heat output produc-
tion. Individual elements are generally positioned in a staggered config-
uration to provide uniform heat transfer and to eliminate hot spots. The
element has spring-coil construction supported by ceramic insulators.
Units of this type provide an auxiliary source of heat when the outside
temperature becomes quite cold. Air circulating around the element is
warmed and forced into the duct network for distribution throughout
the building.