Home & Science:
Understanding Heat Transfer
By Bruce W. Maki, Editor
What Is Heat?
Intuitively, we all know what heat is. It's that warm feeling
from the sun, from your furnace, from your toaster. When it's
cold outside we turn up the heat, on the thermostat, and
a machine somewhere creates it.
But some of us want to know more.
Engineers and scientists, for example, need to understand
heat. Designing and building a furnace, a stove, a clothes dryer,
or an air conditioner requires a full understanding of the science
of heat transfer.
Some of us hate paying so much for home heating. Some of us
know that inefficient home heating causes avoidable environmental
side effects, such as air pollution and greenhouse gases like
Heat vs. Temperature:
A form of energy.
Fully transferable from one object to another.
When an object has heat, (and they all have some heat)
it contains a form of internal energy.
A property of matter. A block of metal or a pond of
water will always have a certain temperature.
A measure of the amount of heat in a certain
object, such as the air mass covering your city, or a pot of
water on your stove.
Consider a few things about atoms and molecules:
Atoms and molecules don't just sit around. In liquids,
they are constantly jostling about. In gases, they are
flying around quite fast, slamming into each other and into the
container they are held in.
In solids, the atoms don't migrate much, but they vibrate.
Constantly. Atoms also vibrate when they are in liquid or gas
When atoms (and molecules) receive more heat, they vibrate
more. Molecules of liquids jostle faster. Molecules of
gases fly around faster, collide harder, and exert more of a
push on their container.
Also, all atoms have electrons which rapidly orbit
When an atom receives more heat, it's electrons orbit faster,
and may even orbit farther away from the nucleus. The electrons
could collide with those from a neighboring atom, causing them
to orbit faster.
Supposedly, at the coldest possible temperature, around -454
degrees Fahrenheit, which is called absolute zero, all
atomic motion stops. Atomic vibration stops, and maybe even electron
orbiting. This sounds weird to me. What happens to the electron
when it stops orbiting? Does it fall down, no longer held in
place by the centripetal force of orbit? Of course, reaching
absolute zero has never been done, and later you'll understand
But this atomic-level knowledge is not critically important
to understand heat and heat transfer.
||What The Average Person
Needs To Know
||"That which makes things go."
|First Law of Thermodynamics
||Energy cannot be destroyed or created. Only it's
form can be changed. *
|Second Law Of Thermodynamics
||Heat moves from warm objects to
colder objects. No exceptions.
||The rate of heat transfer across a given
barrier (such as your ceiling) is proportional to the difference
in temperature between the two sides.
|* All right,
you wise-guy advanced physics types, you caught this one. True,
energy can be created, by destroying matter. That is Albert
Einstein's principle contribution to science. E=mc² means
that tiny amounts of matter can be converted to enormous amounts
of heat, as in thermonuclear fission (Nuke plants and atomic
bombs) But in all non-nuclear reactions and systems, a trivial
amount of matter is destroyed and the total quantity of energy,
for all practical purposes, is constant.
There are three methods of heat transfer: conduction, convection,
||Heat energy travels linearly along a material,
from high temperature to low temperature. Example: Hold a metal
rod in a flame. The hot gases in the flame heat the end of the
rod, and eventually your hand will feel a temperature rise.
When a fluid (liquid or gas) is next to a solid surface of
a different temperature, heat will be transferred.
Natural convection: Heated fluids become less dense
and rise. This is why flames (unless forced) tend to travel upwards.
When people state the common misconception that "heat rises",
this is what they mean.
Forced Convection: A circulating pump forces the fluid
through a chamber such as a long pipe. Heat is transferred to
the pipe, and then to the air at all points around the pipe.
Special devices (such as metal fins) amplify the rate of heat
transfer in some places, namely, at the radiators.
Long-wavelength (infra-red) light emitted by all objects.
The amount of heat given off depends on the object's temperature.
The heat our dear planet Earth receives from the Sun
comes only in the form of radiation.
Infra-red radiation, being a type of radio wave, travels in
a straight line, and can pass through air, glass or a vacuum
(such as outer space). It is absorbed when it strikes an object.
Radiation heat transfer can be greatly reduced by a shiny, reflective
barrier, such as aluminum foil. Some home insulation panels come
with a shiny foil surface. Dark surfaces absorb more radiation
than light surfaces.
About Your House:
Heat transfer in a residence is a combination of all three
But I will make an assertion based on my technical education.
The radiation heat transfer from a typical house in winter
is small, so small that is can be considered trivial. However,
the radiation heat transfer to a house in summer can be
very significant, especially in the South.
Conduction occurs through the walls, floors, and ceiling of
your house, especially through the wallboard and wooden framing.
Concrete is a fairly good conductor of heat, so concrete floors
pose a challenge in colder climates.
Convection may be the biggest factor in heat loss. Inside
an un-insulated wall, the wallboard is constantly warmed by the
room. Just inside the wall cavity, the air is heated by the wallboard,
and rises up. At the top of the cavity the air transfers heat
to the cold exterior wall surface, which conducts the heat to
the cold outside air. Kiss your money goodbye.
So What Does Insulation Do:
Some insulation products, such as foam plastics, have
low coefficients of conduction (they don't transfer heat too
well). This is good.
But glass is a very good conductor of heat. How does fiberglass,
the most popular insulation, do any good? Because the glass fibers
are tiny, and they create tiny pockets of dead air space. Convection
barely occurs in these. Tiny pockets of air are actually a decent
insulator (I'll explain later) When there are billions of tiny
pockets of dead air space surrounding your house, heat transfer
Fabrics (i.e. your clothes) work in the same way. Cotton would
be a great home insulator, but it is flammable (a big
problem) and will rot if it gets wet (an even bigger problem).
Fiberglass does not have these problems.
Cellulose insulation, which is just finely shredded
newspapers, is another excellent product. But it has to be treated
with fire retardant and rot-resistant chemicals. Cellulose is
usually blown into place. It has a habit of settling over time,
which makes it a poor choice for wall insulation. But for ceilings,
it's economy can't be topped.
The Great Freebie:
There is something called a thermal air film that surrounds
all objects. This means that the molecules of air that are very
close to your house tend to want to stay put, and not move much
even when they are warmer than their neighboring air molecules.
This is caused by plain old friction. The net result is
that you get a small, extra amount of insulation value just from
having a simple barrier, such as your wall or windows. And you
get not one but TWO air films, inside and outside.
Now about that fiberglass insulation: it's this air film that
makes it so effective. Replicated many times over, it adds up
to a good insulation value, made from a good conductor.
Some people say that single pane windows get more R-value
from their air films than from the glass. I believe it. I'll
try to dig up some books on this.
For years I have been building "Interior Storm Windows".
I use that shrink-film plastic that 3M makes, and wants you to
discard every year. But I install the film on a custom-made wooden
frame that fits snugly inside the window opening, so the whole
unit can be removed in the spring and replaced in the fall. And
these things work! My intention was to reduce air infiltration,
which they do, but I know there is an added benefit: two more
air films. I can tell by the reduction in condensation on the
windows. What condensation forms is always frost and always on
the outer panes of glass, never on the plastic "interior
storm window". The inner layer stays warm, which is what
That "Delta-Tee" Thing:
Near the beginning I stated that the rate of heat transfer
across a barrier is proportional to the difference in temperatures
between the two sides. This is very important, because the
rate of heat transfer is also proportional to the rate at which
your money flies out the window.
|delta T = T(in) - T(out)
- delta T is the difference in temperatures (I can't make the
Greek symbol, so I'll spell it out)
- T(in) is the indoor temperature
- T(out) in the outdoor temperature
Right now, the outside temperature is 12 degrees. My office
is 72 degrees. The difference in temperature is 60 degrees. (Hey,
this is Northern Michigan in February... it could be worse)
I used to live in Northern Ontario. It gets much colder there.
Minus 18 would not be uncommon. That would be a delta T of 90
degrees. The same building on such a night would consume 90/60
or 1.5 times as much energy.
Or consider this: When I finally call it quits for the night,
I'll turn the thermostat down to 62 degrees. That will lower
the delta T to 50 degrees. The rate of heat loss (and fuel consumption)
will be 50/60 or 83.3% of the current rate, after the system
Your home's energy consumption is directly affected by your
behavior, namely, how high you set the thermostat.
Without making any changes to your home's insulation, you
can reduce your heating bills by:
- Setting your thermostat to a lower temperature at night or
when you are away. The lower the better. But don't let your house
get below freezing (or you'll say goodbye to your pipes).
- An automatic, programmable thermostat can do this for you.
These cost a few bucks, but they pay for themselves quickly,
possibly within a month or two.
- Shutting off the heat registers in unused rooms. Keep the
doors closed too. Be careful with bathrooms: frozen and burst
pipes will cost more to repair than any heat savings.
- Engineering Thermodynamics by William C. Reynolds
and Henry C. Perkins,
- 1977, Publisher: McGraw-Hill
Home What's New
- Fundamentals Of Heat Transfer by Lindon C. Thomas,
- 1980, Publisher: Prentice-Hall
Copyright © 2000
Written February 17, 2000