[Strawbale] [SB-r-us] re: Passive Heating

[Sherwood Botsford]

activism98201 at verizon.net wrote:

snip
> I'll still state that there's more energy required to move air than you might think.  
Consider a cubic meter of air.  At standard temperature and pressure it 
weighs 1.3 Kg.
Now, lets heat it up 20 degrees C.  So it's temperature is now 320 K 
isntead of 300K.
It's volume is now 320/300 of a cubic meter. This hunk of air has a 
lifting force on it of 320/300 of 1.3 Kg. = 0.85 newtons.


Now, lets drag that air, kicking and screaming out of the attic, and 
shove it into a duct under the floor.  Call it 4 meters. Work = force * 
distance.  Hmm. about 4 J.


Specific heat of air is roughly 1 kJ /kg/C  So our hunk of air has about 
20,000 Joules of extra energy compared to the cold air.

So the intrinsic cost of moving the air into the living space is not high.

On the flip side: 20,000 J = about 20 BTU.

Our typical household furnace puts out 100,000 BTU/hour. 
So if we were heating our house by dragging lumps of warm air out of the 
attic,  we'd have to spend 4 * 100,000/20 = 4 * 5000 or about 20,000 J.  
(Coincidence..)  in an hour of running the furnace.  A KwHr is 3,600,000 
J.  So roughly 1/200 of a kWHr would correspond to the heat transfer for 
an hour of furnace running.

Hmm.  I've not seen any furnaces with 1/200 Hp fan motors on them.

So moving air efficiently doesn't seem to be the way things are done.

The key concept here, however, unless I've muffed something, is that the 
elevation of the source of your hot air doesn't have much effect on the 
energy cost of getting it where you want it.