Energy conversion efficiency of electric heaters is 95%? Shouldn't it be a 100%?

[Femme_physics]

The excuse given to why energy conversion efficiency even exists, is because that some of the energy is converted to heat. Then how come in electric HEATERS not all the energy is converted to heat? To what does the rest of those 5% is being converted to?!?

http://img691.imageshack.us/img691/7126/heaters.jpg

 

[uart]

Yes it should be approximately 100%. They're probably just being a bit conservative and allowing for some of the electrical energy being lost as heat outside of the room being heated (eg in the wires carrying the current to the heater).

 

[Femme_physics]

I see, uart. Makes sense :) I actually had somewhat suspected this could be the answer, I'm glad to see it is the case. All makes sense now! Thanks!

 

[vk6kro]

Also there is useful heat and wasted heat.

The heater may waste a lot of energy just heating itself up.

There may be a fan to blow hot air to the outside of the heater and this takes power.

There may be control circuitry and digital displays of temperature and time. These use power.

Water heaters often make noises as the water is brought up to temperature. This is sound energy.

Heat may escape through the heater to the outside of the building if it is wall mounted.

So, that 5% seems like an understatement.

 

[MATLABdude]

You also frequently get waste light energy as well, since many elements glow. Come to think of it, the oft-cited figure for the efficiency of an old-school light bulb is 5% light energy, and 95% heat energy!

 

[uart]

Guys, remember that most of those "losses" you speak of will actually still convert back to heat within the room, and thus do not necessarily detract from efficiency.

Take the blower electric motor for example. It's not 100% efficient but where do the losses go - into heat within the room. At the energy that is converted to moving air will generally be "randomized" back to heat as it moves through the room. Sure there might be tiny amount of sound energy that escapes the room but this is miniscule (and even so much of this will be absorbed by soft furnishings etc and convert back to heat before it can escape the room anyway).

Take the EM component (IR and red light) from a glowing "radiator" type element. Again this will be mostly absorbed within the room and thus converted to heat, particularly if the radiator is appropriately situated within the room. Sure if you had a radiator type element pointing directly out of a non-curtained window then you would loss a lot of radiant energy, but normally you would never position a radiant type heater in this manner.

 

[vk6kro]

Sure.

It depends on your definition of efficiency. If I have a room heater, I don't care how hot the walls get. I want it to heat up the air in the room.

So, power used in heating the heater itself, or the carpet, or the walls and ceiling is lost power and subtracts from the possible 100 % efficiency of the conversion from electrical power to heat.

Incandescent light bulbs have been banned here because of poor efficiency and yet they were contributing heat to the total heating of a room. So, it all depends on how you define efficiency.