Intro to Heat
•
Control of indoor environment has long been one of man’s primary
goals.
• It involves much more than making a warm indoor climate. Humidity
control, indoor air quality, cooling and efficiency are all parts of the
equation.
•
All energy we use comes from the sun, with the exception of
nuclear energy. The burning of fossil fuels such as fuel oil, natural
gas, propane and even hydroelectric all originate from the sun’s
heat.
• Temperature
• Temperature is a measurement of the intensity of heat. It does not
tell us how much volume of heat there is. If the temperature is very
high, we assume there is a lot of heat. This is not necessarily true.
For measurements of volume of heat, we use scales such as the
BTU.
British Thermal Unit (BTU)
• By definition, The BTU is the amount of heat required to raise the
temperature of 1 pound of water 1 degree Fahrenheit. From this we
can measure the capacity of heating and cooling equipment.
Generally, equipment is rated at the number of BTUs it will consume
(or absorb in the case of cooling) per hour. Also, structures can be
rated in BTUs per hour that they lose (or gain) to (from) the outside
environment. By this method, we can match equipment to
structures.
Heat Transfer
• There are 3 ways to transfer heat from one place to another.
Radiation is the heat we get from the sun. This is also part of the
heat we get from heating stoves. Radiation will travel great
distances virtually instantaneously. It travels through space or less
effectively through atmosphere. However, if an object is placed
between the source of heat and the destination, it will heat the object
and none will penetrate to the destination. Also, the amount of heat
that is transferred will vary with the distance to the heated object. If I
double the distance from a radiant heat source, I will only receive ¼
the heat. It is often said that radiant heat heats the person and not
the room. It is often used in spot heating applications where a
person is working in a warehouse. A small radiant heater is used
nearby the person giving him warmth without heating all the air in
the warehouse. Some homes have used radiant heat panels in the
ceiling or floor to supply heat for homes or businesses.
Convection
• Convection is the movement of heat from one place to
another by the density of the medium. Convection can
only work in a liquid or gas. When a fluid medium is
heated, its density is lowered. When its density is
lowered, it weighs less and tends to rise. An example of
this is the hot gasses from a campfire. Initially the hot
gasses rise very quickly due to the large difference
between their temperature and the temperature of the
surrounding air. As the gasses rise, they lose heat to the
cooler air around them. Thus the smoke from the fire
may rise then level off or even drop back down. Early
heating systems moved the heat through the structure by
convection. The efficiency of these systems was lower
than with fan forced systems, so most of these systems
are no longer in use.
Conduction
• Conduction is the movement of heat through a medium. An
example of this movement is a wood stove releases heat from the
wood inside burning. The heat warms the metal of the stove.
Metals are usually the fastest conductors of heat. You can tell how
fast a conductor a material is by touching the material during cold
weather. A fast conductor will feel very cold to the touch. A slower
conductor such as wood will feel warmer. There is a relationship
between fast conductors of heat and good conductors of electricity.
This relationship does not always hold true. Touch glass in very cold
weather and it feels very cold. However, glass is a very poor
conductor of electricity.
When heating structures, we use all 3 forms of heat
transfer
•
In the example of the wood stove
the heat released during the
combustion of the wood warms
the steel sides of the stove. The
then warmed steel sides radiate
heat to articles in the room (chairs,
people, walls etc). The warmed
steel also conducts heat to the air
next to the stove, warming the air.
As the air warms, it becomes
lighter than the cooler air around it
and rises toward the ceiling in a
convective current. When the air
rises to the ceiling, heat is
conducted to the material of the
ceiling and cooler air. As this air
gives up its heat it becomes cooler
and thus denser and moves back
down toward the floor and is
drawn toward the stove and
begins the cycle again.
.
Radiant Heating Systems
In some applications, systems are
designed to maximize radiant heat.
These may be spot heaters located
above checkout registers in home
supply warehouses. The temperature in
the warehouse is kept low but near the
checkout stations the need for more
heat is necessary for the comfort of the
customers and employees at the
checkout stand. These stands are
normally near doors that are opening
and closing numerous times, bringing in
cooler outside air. The value of these
systems is the radiant heat heats the
objects and people in the room more so
than the air in the room. This gives a
feeling of warmth when the air is cooler.
Considerable savings in energy usage
can be obtained by using these systems
in under the proper conditions.
Natural Convection Systems
•
•
When housing was small with one or two rooms a
heater located near the center of the room was
sufficient to give reasonable comfort to the entire
structure. However, when housing expanded to
multiple rooms with doors separating them, even
heating was less possible. This gave rise to
centrally located heating units with piping
connecting the heating unit to each individual
room.
Using the natural convection of warm air rising,
the pipes, if installed with a rise from the furnace
to the outlet, heat could be transferred to all the
rooms of the house. These systems offered even
heat throughout a multiple room structure with no
heating appliance located in the living space.
They could be sometimes operated during power
outages. Disadvantages were they had to be
located in a basement and their size filled most of
the space. They were less efficient than fan
forced systems and in large structures the longest
runs were cooler than the close runs. These
systems are sometimes called “octopus systems”
because of their large diameter piping emanating
from a central source. These systems were
sometimes equipped with a fan to assist air flow
Forced Air Systems
•
Even though fans could be
retrofitted to the octopus
systems, the quest for efficiency
led to heating systems designed
to have a fan to move the air
throughout the structure. This
led to much smaller heating
systems with ducting placed in
less obtrusive places and the
need for upward angled piping
could be eliminated. With
natural convection systems,
steady state thermal efficiency
was 50-60%. These systems
increased steady state thermal
efficiency to approximately
80%. This means that of all of
the heat released by the fuel,
80% was distributed to the
structure and 20% was lost
through the chimney or metal
vent. In modern systems,
thermal efficiency has increased
to as high as 96%.
Boiler Systems
• With a forced air system, air is
circulated through the furnace
to pick up heat. Then the
heated air is moved to the
conditioned space.
• In a like manner, water can be
used to transfer heat.
Advantages are that very small
amount of water can be used
to move a given amount of
heat compared to air. This
eliminates the need for large
ductwork. Water also absorbs
heat much more readily than
air making the heating unit
smaller than for forced air
systems.
Steam and Hot Water
• Boiler systems are of 2
types. Hot water and
steam. The hot water
boiler pumps cool water
through the boiler where
it picks up heat. The
water is then pumped
throughout the structure
and the heat is
transferred to the
conditioned space using
baseboard convectors,
pipes located in the floor
or cast iron radiators.
Steam Boilers warm the water to the boiling point and convert it to steam. The steam
then travels by natural convection through the piping to heat exchangers where the
steam is condensed back to water and the latent heat of condensation releases large
amounts of heat to the conditioned space. The water then travels back to the boiler to be
converted back to steam and the cycle continues.
The difference between a steam boiler and a hot water boiler may not be obvious at first
glance, perhaps not even after looking for a long time. They do tend to look like
plumber’s nightmares. Look at the 2 boilers shown. See if you can tell which is steam
and which is hot water.
Heat Sources
There are several sources of heat that
are commonly used.
o Natural gas
o Propane
o Fuel Oil
o Coal
o Electric
• With the exception of electric, all the
above fuels require combustion to occur.
• Combustion is the mixing of fuel and
oxygen at high temperature to release
heat from the fuel.
• Products of complete combustion are
usually CO2 and water with some trace
compounds.
• Incomplete combustion will produce CO, a
poisonous gas and other chemicals such
as soot and aldehydes.
Natural Gas
• Natural gas is formed as
a fossil fuel underground
related to oil deposits.
Under normal conditions,
it is a gas and is metered
in cubic feet. Natural gas
in this area contains 1000
BTU per cubic foot. It is
lighter than air, so it rises
when released.
Propane
• Propane is sold as a
liquid that boils into a gas
prior to use as a fuel. It is
also a product of fossil
fuel origin. Its BTU
content is approximately
2500 per cubic foot. It is
heavier than air and can
gather on the floor.
Fuel Oil
• Fuel oil is usually sold
as #2 fuel oil similar
to diesel. It is a liquid
that must be
vaporized or atomized
prior to burning. Its
BTU content is
140,000 per gallon.
Coal
• Coal is a solid fuel that
looks like a rock. It is
also a fossil fuel. It is
usually fed into the
furnace by a screw
mechanism. It is similar
to a wood fire in that it
must have a continuous
fire. It is used mostly in
power generation
although some is still
used for space heating.
Electric
• Electric heat is not an
energy source but a
form of energy.
Electric furnaces are
simply a converter
that converts
electricity to heat. It
is sold in kilowatts
with one kilowatt
equaling 3,417 BTUs.