[Grovenet] Electric Heating

[Gregory Gritton]

At 05:02 PM 7/29/2008 -0700, you wrote:
>Electrical efficiency is the economic factor. Heaters convert
>electricity into convective and/or radiant heat. Their measure of
>efficiency is sometimes printed on the packaging.

>"Cadet" wall heaters offer fast convective heat. They blow warm air
>when they're on, but offer little residual heat when the element goes
>off, so the temperature in the room can zigzag. Be sure to check the
>model numbers on the Web! Some Cadet heaters with defective switches
>have been implicated with electrical fires in the past.
>Oil-filled heaters offer slower but more efficient convective
>heating... they radiate heat even after the element goes off.
>Heat lamps or "crystal heaters" provide radiant heat only to the spot
>where they are directed, and aren't terribly efficient.
>Radiators offer the most even and pleasant heat, but would need a
>solar water heater on the roof or a south-facing slope to be really
>efficient and cost-effective. And that would require a backup on
>winter days.


While the quality of heat, such as whether it is
slowly and continuously produced, or produced in
bursts, may vary, the efficiency of most electric
heaters doesn't vary much.  In fact, most forms
of electric heating are 100% efficient.


Most transitions from one form of energy to another
produce waste heat, and are therefore not perferctly
efficient as heat is not what is desired.
However, where heat is desired, the "waste" can
be used, thus making 100% efficiency possible.


The reason gas furnaces are less than 100% efficient
is that some of the exhaust from burning the gas
carries some of the heat up the flu.  In addition,
the furnace may radiate some heat to a garage
or wherever it is installed.  Thus, efficiencies
of 80-90+% are typical.


However, with electric heaters, there generally
isn't any other place for the heat to go.  The heat
is produced indoors, and there is no exhaust that
is expels.


The only possible differences in efficiency might
be:
  - Underfloor units will radiate some of the heat
    down into the ground.  Thus, not all heat enters
    the home and therefore they aren't 100% efficient.
  - If you aim a heat lamp at an exterior wall, or
    worse, a window, some of the heat will make it
    through the wall or window to the outside.


There are a couple of ways of getting better than
100% efficiency in your heating (or, to put it another
way, you can get more heat out of your heater than
the electricity you put in).  One way that was already
mentioned was to use the sun to preheat the water
going to a radiator.  Another way is with a heat
pump, which is effectively an air conditioner run
in reverse.  It transfers some heat energy from
the outside (despite it being cold) to the inside,
just like an air conditioner transfers heat from
a colder inside to a warmer outside.  It does take
energy to do this, but with heating, most of the
extra energy ends up being deposited on the inside.
Thus, you get some energy from the outside, plus
most of the energy used in the process.

Heat pumps can multiple the energy by 2-3x, depending
on the outdoor and indoor temperatures.

Finally, while electric heating may be 100% efficient,
compared to gas being about 90% efficient, that doesn't
mean it makes economic or ecological sense to go electric.
While most existing electric power is generated from
coal, nuclear, and hydroelectric plants (mainly the
latter in the northwest), most power plants being built
to handle additional demand are powered by natural gas.

If you use the natural gas to generate electricity
(or any other fuel source), only about 1/2 of the
energy in the gas gets converted to electricity,
the rest ends up as waste heat, which is rarely used.
Thus, your overall efficiency is about 50%, compared
to 80-95% from burning the gas in a furnace.
(But, conversion to electricity which runs a heat
pump may still be worth it.)

Sincerely,

Greg Gritton