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Q: Household Appliances as Heaters ( Answered 5 out of 5 stars,   5 Comments )
Subject: Household Appliances as Heaters
Category: Science > Physics
Asked by: carla1-ga
List Price: $20.00
Posted: 08 Dec 2002 14:28 PST
Expires: 07 Jan 2003 14:28 PST
Question ID: 121507
This one stems from a dinner table conversation. Firstly, in
theoretical terms, the issue was distilled down to the following
experiment: take two identical sealed boxes which, for the sake of the
experiment, are perfectly insulated: they do not let heat, sound, or
other energy escape. In one box place a 100W heater, in the other a
fan that consumes 100W when running. After N minutes, measure average
temperature inside the boxes: are they equal? Does the difference
between convected / conducted / radiated heat affect the result?

Secondly, in practice: assume you have an electrically-heated house
with overhead fans providing excellent air circulation and curtains to
prevent loss of energy as radiated light. Discuss the claim that, in
winter, keeping the lamps and TV on is (virtually) as energy efficient
as leaving them off, on the grounds that lamps creates heat and light
(which is absorbed by dark surfaces and converted to heat) and a TV
produces heat, light and sound (which bounces around inside the house
until it is eventually absorbed and converted to heat by various
materials). Any heat thus provided reduces the requirement for
electric heating, the cost per watt of which is identical.
Subject: Re: Household Appliances as Heaters
Answered By: haversian-ga on 09 Dec 2002 20:00 PST
Rated:5 out of 5 stars
Hello Carla.

Izzard is partly correct.  Insulation in houses is designed to capture
heat energy.  Mostly it does this by containing the air (limiting
convection of heat), blocking light (limiting radiation of heat), and
by insulating (limiting conduction).  The only material difference
between your house and your hypothetical box is that insulation - the
box is assumed to be a perfectly insulated system.

Radio waves penetrate houses just fine, which is why your radio works.
 So, any energy radiated as radio waves (or X-rays, gamma rays, etc.)
will exit the house.  Sound waves also exit the house to some extent
(but how much quieter does your stereo sound just outside the house,
outside the room it's in? - that quietness is because your house
absorbed the rest of the energy).  Light can exit the house through
windows, but light which hits walls is absorbed depending on the color
of the wall.

Fortunately (for the purposes of this discussion), appliances whose
purpose is *not* to produce heat are hideously inefficient.  Consider
a computer, for example.  Your computer probably has a 300W power
supply in it, but the only work it does is computation (and moving
some air around with fans), which takes very little energy.  The vast
majority of the power consumed is wasted, converted directly into
heat, as all those electrons smash mercilessly into metal atoms in
tiny wires and make them vibrate faster.  Your stereo probably gets
almost painfully hot.  An incandescent light bulb, even a low-wattage
one, can burn you.  The two 15-watt fluorescents in my desk lamp here
even manage to be quite warm after an hour or so.  If you would like
to know about how efficient a sampling of appliances are, please
request an answer clarification and I'll get you one.

Now that we have covered the majority of the energy consumed, which
becomes heat directly, we need only consider what remains.  Light
bounces around and is largely absorbed by the paint on your walls, and
some goes out through windows.  Sound is absorbed by the insulation in
your walls, and some is let through.  There's not much radio or other
radiation emitted by appliances, as most that would be emitted is
absorbed by shielding in the appliances themselves.  In short, fairly
little energy is lost from your house through these means.  Vastly
more energy flows down the drain as hot water every time you take a
shower, do the dishes, or do your laundry.

The energy efficiency of heating with appliances versus heating with
gas burners is about the same.  The money efficiency though is
somewhat different - 100KW worth of electricity costs more than 100KW
of natural gas, coal, or heating oil.

You might also consider the environmental impact (which I assume
sparked this dinner discussion) of the way those 100KW are obtained. 
The electricity is generated by burning coal, oil, or natural gas, or
by solar, wind, tidal, geothermal, or nuclear sources.  The first
three are the big ones in terms of amount of energy generated. 
Burning coal to heat water to generate electricity to power a stereo
to heat a room is fairly inefficient - it's much better to burn coal
to heat the room.

If you would like elaboration on any part of what I have said, please
request an answer clarification, and I will be happy to oblige.


Request for Answer Clarification by carla1-ga on 09 Dec 2002 22:08 PST
Thank you very much, haversian. your answer is extremely lucid and
basically exactly what I was looking for. There are, however, three
small points I'd request you to elaborate on, and I'd like to add a
final comment as clarification, too.

Firstly, our dinner table "bet" (although that's too strong a word)
was actually about the first (theoretical) part of the question. I
realize that there seems to be a consensus on the issue, but a brief
formal answer to that part of the question would be appreciated.

Secondly, although I appreciate it can hardly be an accurate answer
without months of research, I would ideally like to know, within an
order of magnitude or so, how much less efficent you would guess an
"average" household appliance would be. In other words, would you
imagine a PC (for example) consuming 100W would be equivalent to a 90W
heater, a 99W heater, or a 99.9W heater.

Thirdly, I note both you ("the only work it does is computation ...
which takes very little energy") and Izzard ("There is an intrinsic
loss of some energy whenever it is converted from another form to heat
in the house") seem to refer to a sort of "generic" loss of energy. My
understanding is that all of these conversion losses *always* wind up
as heat, although I grant you that my computer could be emitting
energy in radio and other bands. On the other hand, this is exactly
what proper cabinets are supposed to supress, again presumably
converting them ultimately into heat.

On a different note, although I do appreciate the ecological concerns
raised by you and others, they are not, in fact, relevant to our
particular situation. In Quebec, where we live, virtually 100% of
electricity is generated hydro-electrically, and residential rates
work out to about 4 US cents per KWH, which is at least 40% cheaper
per BTU than oil or natural gas here. Our houses are *very* well
insulated with triple-glazing and R20-30 being the norm. In fact, it
was the old natural gas furnace in our house combined with the
advertising of the electricity company that got me started thinking
about this to begin with.....

Thanks again for the excellent answer.

Clarification of Answer by haversian-ga on 09 Dec 2002 23:19 PST
> Thank you very much, haversian.
You are quite welcome.
> ...a brief formal answer to that part of the question would be

The hypothetical box is a closed system.  By closed, we mean that no
matter nor energy can enter or exit the box except as specifically
noted.  In both the case of the heater and the electric fan, 100W of
energy are transferred into the box.  Temperature is a measure of
average energy (at very low temperatures, kinetic energy even becomes
important! - physicists have gotten atoms so cold they move only
inches an hour), and since the amount of matter in the box has not
changed, and the amount of energy has changed by the same amount in
both cases, the temperature is the same in both cases.
> how much less efficient you would guess an "average" household
appliance would be.
I would put a computer at 99.9% efficiency (a CRT monitor is perhaps
95% efficient, as the light it produces is work).  LCD monitors are
more efficient, say on the order of 10x.  Televisions are similar to
CRT monitors, but also produce work in the form of sound waves, so
proportionally less energy is wasted - perhaps 90%.  These really are
ballpark numbers though - real efficiency data tends to exist
primarily for things like internal combustion engines and solar
panels, where efficiency is extremely important.

> My understanding is that all of these conversion losses *always*
wind up
as heat
Pretty much.  As soon as any sort of radiant energy strikes an atom,
it pushes the atom around a bit, which makes the atom "hotter".  On
any scale smaller than cosmological, atoms are bound to get in the
way, so it's reasonable to consider all lost energy to be heat.
> On a different note...
Really?  That's interesting.  I suppose it makes sense though, what
with the differences in cheaply available natural resources in the US
and Canada.  That's pretty good insulation.  Not as fun though as
growing up in a house where a glass of water left on the bedstand
would freeze:)

I'm glad to have been a help,

carla1-ga rated this answer:5 out of 5 stars
Well, I conclude that I can safely leave my household appliances on in
the winter and my total energy bill should not go up more than a few
percentage points at worst. In fact, it may well go down because it
reduces the usage of the (for us) more expensive natural gas central
heating. Thanks very much to everyone -- very well done.

Subject: Re: Household Appliances as Heaters
From: dannidin-ga on 09 Dec 2002 01:18 PST
you are basically correct. heat is the ultimate form that energy takes
when entropy increases, so in your two boxes experiment, the
temperature should indeed be the same. as for using electrical
appliances for heating the house, this is as cost effective (but more
annoying - especially t.v.) as using electrical heating. however, it
is well known that electrical heating is NOT as cost effective as
other methods of heating, so perhaps you should reconsider turning
that damn TV off when you're not watching it? ;-)

Subject: Re: Household Appliances as Heaters
From: izzard-ga on 09 Dec 2002 06:24 PST
Well here is my “discussion” - which is all that you asked for.  No
references, though, so I don’t think I can post it as an “answer” (but
please tell me if I may).  I disagree with the claim that, in winter,
keeping the lamps and TV on is (virtually) as energy efficient as
leaving them off.

If I may summarise your notion: you say that you spend money on X
amount of electrical energy and, no matter what other form the devices
in your home turn that into, most of it will be eventually converted
into heat and so it is irrelevant if you consume 1 KW/hr for making
heat, or light from a TV, or a sound from a stereo system.

This simply isn’t true.  If you use all your electricty quota with an
electric heater, a good deal of that energy will warm the house.  If
you use the same amount of energy to play a lot of loud music, a lot
of that energy will be transferred _through_ the floor and be absorbed
by the earth, or pass through the walls and lost outside.

Yes, this also happens with the energy from a heater, but the
‘average’ house is designed to be efficient at containing _heat_
energy.  Not light, and not sound – and, as you know, these different
forms of energy pass through or are absorbed to different degrees by
different materials.  A heater will give you the most resulting heat
for a given energy cost.  I am saying that heating your home with
sound and light is going to cost more in lost energy to the outside.

So: the walls and structure of the house do a reasonable job of
containing the heat output from a heater.. ..but are rubbish at
converting light (and sound) and other forms of energy into heat and
then containing it.  There is an intrinsic loss of some energy when
ever it is converted from another form to heat in the house; else you
could call the house “100% efficient” at this job.  No house is like
the physically perfect, hypothetical boxes you described in your first
paragraph, and therefore we can only conclude that the claim is

Incidentally, we have to assume that we are measuring the efficiencies
of two or more methods of heating by the same metric.  I.e. raising
and maintaining the temperature of the air in the room by a certain
number of degrees in a given length of time.  That means we would have
to test with a _lot_ of stereo systems to give as much heat as one
electric heater in the same time.  Else we have to consider the fact
that a body loses heat at a rate which is inversely proportional to
the square of the temperature differential gradient.. (or something)..
..and let me tell you, that is no fun at all. ;)

To summarise:  I am saying that a house is never as good as the boxes
you imagined – in a real house it will always be better to reduce the
number of energy conversions and opt straight for the electric heater,
to minimize the effects of the house’s intrinsic inefficiencies. 
However, if by saying “ providing excellent air circulation and
curtains to prevent loss of energy as radiated light,” you are saying
there is no way for energy to escape this house, then it is as good as
the perfect boxes and therefore, no, it doesn’t matter what form the
energy takes.  You can turn the heater down a bit in your perfect home
and use some of your energy budget to watch the TV.. ..although if
there is no energy loss you don’t really want to turn anything on for
very long :)
Subject: Re: Household Appliances as Heaters
From: neilzero-ga on 09 Dec 2002 06:42 PST
Danniden made a brilliant comment. The fan/heater should reach exactly
the same temperature similtaneously until one of them burns out
because of excessive ambient temperature. If you heat your house with
resistance heating, then any applience heats with equal efficiecy.
Some details could make an applience less costly. ie if you don't like
cold floors, point a small fan (on the floor in the doorway) blowing
up at about a 45 degree angle out the door of the room. The fan will
pick up the cold air off the floor, and mix it with the warmer air in
the middle of the area outside the door. Warmer air will drift in the
reverse direction near the top of the door. You will want to shut off
or reverse the fan when some one is in the area outside the room, and
you may forget and fall over the fan.   Neil
Subject: Re: Household Appliances as Heaters
From: neilzero-ga on 09 Dec 2002 07:02 PST
izzard makes some good points, however, the electric heater is rarely
more than 1/10% better than a stereo, or light bulb and central
heating systems typically loose more than 1% of the heat to the
outside due air leaks in the ducts in the ceiling or under the floor. 
Subject: Re: Household Appliances as Heaters
From: craigengineer-ga on 09 Dec 2002 14:23 PST
Everyone seems to agree, as do I, that your theoretical example is
accurately stated.  On to the practical matter... FOR ALL PRACTICAL
PURPOSES, all electrical devices are equivalent for efficiency for
heating.  Electric heat is the only type that can approach 100%
efficency.  Gas cannot; it must expel some of the heat outside, in
order to bring in more air for the combustion process.  It is
generally a CHEAPER way to heat, in most areas, but that is due to the
cost/unit of the energy.

Let's talk about loud music: most of the energy that goes into your
stereo receiver comes out as heat.  The actual energy in the sound
waves is a small percentage, since the process of using electricity to
create high fidelity sound is so inherently inefficient.  And, of the
energy that goes into the actual sound, only a percentage of that
escapes.  Most of it is reflected, then absorbed.  It echoes of the
walls, and vibrates the fibers in your carpet, ultimately producing
heat, the lowest form of energy.

I had an apartment two heaters: one in the main living area, and one
in the bedroom.  They were both on the exterior wall, and both were
underneath windows.  In this case, I believe that running a loud
television would have been the MOST efficient way of heating the
house.  No direct conduction through the outside wall, and much less
convection at the windows, which serve as poor thermal insulators. 
Running a fan would even out the temperature in different parts of the
room, thus slowing conduction through the ceiling, and providing
nearly 100% heating efficiency at the same time.

Unless you are an appliance junky, you're not going to get enough heat
from running appliances.  So leave your thermostat set to a
comfortable level.  But if you have only electric heat, leave your
stereo on if you want.  Leave your lights on unless you want to sleep.
 They're all ABOUT the same efficiency.

Note: One thing that could make it more expensive to use your stereo
to heat your dwelling is the cost of replacement if you wear it out.

Electrical Engineer

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