Ask A Scientist Physics Archive

name         Linda M. M.
status       educator
age          50s

Question -   One of my students claims that his father always makes
the family turn off the fans when they are not in the room.  He claims
that they give off heat from the motor, therefore unless you are in the
room the fan is not doing any good, just giving off more heat.  He also
says that lighting candles in warm weather gives off heat that warms the
room (when the air conditioning is not on, or even if it is.) The student
and his mom disagree with him. I say that the amount of heat is so
inconsequential it makes no discernable difference in the room (from the
motor of the fan and the candle.)
Help me please. (The dad is adamant about being right!)
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Hello,

The father is right.

A fan generates heat and warms up the room. Depending on the size of the fan
and the room and presence or absence of air flowing in and out of the room
this heating may be consequential.  In a small room, with windows and doors
shut, the heat of the fan is perceptible.

As a side point, one can heat a room without a heater by running a
refrigerator (best at the coolest setting or better yet by leaving its door
open.)

The reasons fans are used for cooling are (a) blowing air over the skin
increases evaporation which cools the body, and (b) blowing air replaces the
stagnant air in a room by forcing turbulence and perhaps bringing in fresh and
cooler air if the outside air is cooler.

Cooling by evaporation can be experienced by inserting a finger in water (or
better yet an alcohol) and then blowing on it. One feels the finger getting
colder.

AK

--
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory
=========================================================
The thought of a fan on in a room without a person in the room strikes
me as one of the imponderables of life like "if a tree falls in the
woods and no one is around to hear it, does it make a noise?"  So, if no
one is in a room with a fan on, does the air feel cooler?  Even that
statement "does the air feel cooler" is not a correct statement all in
all.  The only way to cool the air is to take heat out of the air, and
that the way that is normally done in the home is with an air
conditioning unit.  Fans only move the air about to make it "feel"
cooler.  Ceiling fans really work in two ways: they help circulate the
air in the room so that it is more evenly tempered and they increase the
flow of air over the body.  Regular oscillating fans do more of
increasing the flow of air over the body.  When air is moving over the
body, it "feels" cooler because the rate of heat dispersion of one's
body is increased.  So, if there is no body (literally) in the room, is
the room cooler?  I agree with you and I doubt that the fans are adding
any measurable heat to the room, but I would argue that it is probably
better to turn off the fans to save electricity if no one is going to be
in the room for an extended period of time.  If people are moving in and
out of the room, then I would leave it on.
As for the heat of a candle, one candle in a large room will not
significantly increase the heat of the room, unless it is one big candle
about the size of a fireplace.  Yes, the candle is adding thermal energy
to the room, but if the room is large enough and the candle is not very
big, it would take many candles to heat the entire room.
If this still doesn't help, let us know and I'll actually churn out some
numbers on heat dissipation and temperature rise of a room (or maybe one
of the other scientist has already done so).
Hope this helps.

Chris Murphy
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Linda,

"Inconsequential?" A lot depends on how well sealed and insulated the
room is. On this issue, I must side with dad. He is correct about
the heat output -- and besides, it wastes electricity. Regarding the
candle: it, too, adds heat -- and hydrocarbon pollution.

Regards,
ProfHoff 359
========================================================
Of course the dad is right.  Candles obviously have flames, which produce
heat. You also are right.  Compared to other power sources,  burning candles
do not produce much heat, after all.  A light bulb produces much more heat.

As for the fan, the dad certainly is correct that the motor produces heat,
and so running the fan increases the average temperature of the room.  Fans
make the air more comfortable for two reasons: (1) by moving the air, they
blow away body heat and evaporate sweat, cooling the body, and (2) they also
mix the cooler air near the floor with the warmer air by people's faces.
The first effect is obviously meaningless if nobody is in the room.  The
second effect might produce some benefit if nobody is in the room, IF
someone will be coming into the room fairly soon.

The amount of heat a fan adds to the room depends directly on the power (in
watts) it draws.  By my calculations, if a 40-watt fan runs in a small (12'
x 12' x 8') room for one hour, it generates enough heat to raise the
temperature of the air by about 9 degrees Fahrenheit.  I would think that
this is enough of a temperature increase to justify turning off the fan if
nobody is going to be in the room for a while.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois
========================================================
First, the electric fan -- the heat given off by the electric motor can be
calculated from the wattage of the motor, that is volts x amps = joules/sec;
however, there are other factors entering here. The flow of air blowing
against the skin evaporates perspiration, which absorbs 10 kcal/gm of
water -- that is quite a large heat loss. Also if the fan is positioned in
such a way to blow cooler air from the rest of the house (assuming it is air
conditioned) that cooler air will compensate partially for the heat
generated by the electric fan motor.

The candle is another story. It generates the heat of combustion of a
hydrocarbon of the formula approximately C(n)H(2n+2) with no compensating
cooling; however, for small candles and large rooms the temperature increase
would be very small.

Vince Calder
========================================================
Linda,

The purpose of a fan is to move air, rather than to cool a room.  If you are
in the room with a fan blowing at you, the hot air around your body is blown
away.  This allows some cooler and dryer air to move next to you.  Your body
can then release heat and moisture faster.  A fan can also help keep air
moving throughout an entire house.  A fan can covert one very hot room and
one very cool room into two comfortable rooms.  If a room is enclosed (no
place for cool air to come in or hot air to go out), then your dad is right.
A fan can redistribute heat, but it cannot make it disappear.

Still, the engine does produce heat.  One way to find out involves running
it for an hour.  Turn it off, unplug it so nobody gets hurt.  Open it up so
you can reach the motor.  Touch it with a few drops of water.  See how fast
they evaporate.  If they sizzle away quickly, your fan produces significant
heat.  If not, you can try using a damp finger.  Do not let little kids try
this.  Usually, newer fans with smaller motors produce less heat, but all
produce some heat.

As for candles, they too produce heat.  The amount of heat is related to how
bright the candle is.  I expect you have almost burned yourself on a candle
glass.  Most heat from the candle will rise upward.  Again, if it has a
place to go, it will not matter much.  In an enclosed room, burning an
entire candle can heat the room a little bit.  Several candles at once can
start to matter.

Dr. Ken Mellendorf
Illinois Central College
========================================================
The amount of heat produced by the fan is more than just the heat you
can feel from the motor.  ALL of the energy used by the fan becomes
heat, and this amount of heat is comparable to that produced by a light
bulb or a person -- say, around 60 to 150 watts, depending on the fan.
Now, whether or not this rate of heat production is consequential
depends, of course, on what it is compared with.

If a fan is being used to exchange cool air from outside with warm air
from inside, or cool air from near the floor with warm air from near
the ceiling, then the heat produced by the fan is relatively
negligible.  But if the temperature difference between the "cool" and
"warm" air is small, the net effect of the fan is merely to blow a thin
layer of warm, moist air away from your skin and replace it with other
air from the same room.  In this case the fan serves no purpose when
you leave the room, and if you should certainly turn it off.

I do not know how much heat a candle produces.

Tim Mooney
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