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do all electrical heaters put out the same amount of heat per watt used?

paul wheaton
Trailboss

Joined: Dec 14, 1998
Posts: 20644
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I recently saw a couple of people arguing about electric heaters. One claims that no matter what kind of heater it is, it is a scientific fact that the amount of heat per watt is going to be the same between all heaters. The other person said that they have tried a lot of different electic heaters in the same room and the room felt significantly warmer with some that used less power.

As long as we have a bunch of engineers here, perhaps we could get some enlightenment ....

Anybody have any info along these lines?


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Jeroen T Wenting
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Joined: Apr 21, 2006
Posts: 1847
The energy you pump into the thing can go to only 3 things: heat, light, or back into the grid as electricity (in which case you might as well not count it at all).
Light when it strikes a surface is transformed in part into heat as well.

So there is theoretically a difference in the direct heat output of those devices.
If they give off a lot of visible light and that light isn't absorbed by anything before going out windows for example the heat output would be lower.

But a more important factor in the different operation of these units is their basic design.
Some just convect heat away from themselves better than others. Inefficient ones might cause a large rise in temperature close to the unit with little effect in the short term at least elsewhere.
Efficient units would cause a more rapid rise in temperature at larger distances.

Of course over time each would have roughly the same effect (not taking into consideration that most units have mechanisms to cause the electrical power to cut off when a set temperature in the direct vicinity of the unit has been reached, which would cause inefficient units to actually pump heat into the room for a shorter period of time during any given (longer) interval).


42
Jim Yingst
Wanderer
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Joined: Jan 30, 2000
Posts: 18671
I agree with Jeroen's comments, and would only add that some heaters may be better at putting out heat in a particular direction - a desireable feature for spot-heaters; a person in the right spot can certainly "feel warmer", but of no real use if your goal is simply to raise the overall temperature of a room.
[ September 30, 2006: Message edited by: Jim Yingst ]

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Ernest Friedman-Hill
author and iconoclast
Marshal

Joined: Jul 08, 2003
Posts: 24187
    
  34

For an ideal emitter, the spectrum radiation that comes off of a heated body is a function solely of the temperature (so-called "black body radiation," not a great model for a heating element but good enough for government work.) Theoretically, different heaters would have different operating temperatures based on the (possibly temperature-sensitive) resistance of the heating element, right? Therefore I'd think different types of heaters would, in fact, emit different percentages of visible vs. thermal radiation. I don't have a feel for how much such variation would lead to a real appreciable difference in perceived warmth; anybody?


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Jeroen T Wenting
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Joined: Apr 21, 2006
Posts: 1847
It could have a very real effect.
Certain wavelengths are more readily absorbed by water (which effectively means the human body) than others, thus giving a more intense experience of heat, than are others.
 
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