The transfer of heat is the next problem to consider
after the heat has been produced. It must be moved to the
space where it is to be used. Heat always flows from a
warmer to a cooler substance; consequently, there must
be a temperature difference before heat can flow.
Naturally, the greater the temperature difference, the
faster the heat flow. Two objects that have different
temperatures, when placed together, tend to equalize
their temperature. Heat travels in heating systems from
one place to another by three different methods. All
three of these methods are used in most heating systems.
They are discussed in the paragraphs that follow
Conduction is the flow of heat from one part of a
substance to another part of the same substance or from
one substance to another when they are in direct
When one end of a stove poker is held in a flame,
the other end will soon be too hot to hold. This
indicates that the heat is being conducted, or
transferred, from one end of the poker to the other end.
Such a transfer of heat is called conduction.
Conduction is used to transfer heat through the walls of
a stove, furnace, or radiator so that the warmth can be
used for heating. Some materials do not conduct heat
as well as others. For example, if a piece of wood had
been used instead of the poker, the end of the wood
away from the fire would have remained cool. Those
materials that offer considerable resistance to heat
flow are referred to as insulators or poor conductors.
Convection is the transfer of heat by means of
mediums, such as water, air, and steam. When air is
heated, it expands, becomes lighter in weight, and
rises. The cooler air, which is heavier, then flows in to
replace the warm air. Thus a convection current is set
up. Water, when heated, acts in the same way as air.
The water next to the heating surface becomes warmer,
lighter, and rises. This action allows the cooler water to
flow in next to the heating surface and become heated.
Convection is a very important factor in a heating
system. It is this force, developed by heating the
medium, which circulates that medium to the space to
Radiation is the transfer of heat through space.
When a hand is held in front of a stove, it is quickly
warmed by means of radiation. In this same manner,
the earth receives its heat from the sun.
Radiated heat is transferred by heat waves, similar
to radio waves. Heat waves do not warm the air through
which they pass, but they must be absorbed by some
substance to produce heat. For example, when you
stand in the shade of a tree, you feel cool because the
leaves and limbs are absorbing the heat waves before
they reach you.
When heat waves strike an object, some are
reflected, some may pass through, and the object
absorbs the rest. Polished metals are the best reflectors
known; therefore, they are a poor absorber of heat. A
poor absorber is also a good radiator. Rough metal
absorbs heat more readily than a highly polished metal,
and it also loses heat faster by radiation.
The color of a substance also affects its absorbing
power. A black surface absorbs heat faster than a white
one. That is why light-colored clothes are cooler in
summer than are dark-colored clothes.
Q1. Heat can be produced or generated by what
What two types of measurements directly affect
the safety, efficiency, and reliability of heat plant
Temperature is the measurement of what?
Q4. Convert 82 degrees Fahrenheit to degrees
Q5. Heat travels in heating systems by what three
Learning Objective: Understand the types and
characteristics of combustible gases and fuel oils used
in heating systems.
If electricity and coal are disregarded, the fuels
most commonly used with heating equipment are
either gas or petroleum. Next, we will take a brief look
at the types and characteristics of combustible gases
and fuel oils used for heating.
TYPES OF GASES
Gaseous fuels are usually classified according to
their source that, in turn, determines their chemical
composition. The heat valve (Btu per cubic foot) varies
with the types of gas and determines the quantity