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re: need help w/fireplace problem
20 nov 2000
phxbrd@home.com  wrote:
 
>> >> > ...how would these inlets save energy?
 
>> >> by using unheated makeup air.
 
>> >which, as a bonus, is also more efficient in the combustion process.
 
>> oh? a woodstove delivers more useful room heat per pound of wood when
>> using, say 30 f vs 70 f inlet air? i'da thunk the other way round...

>you'da thunk wrong.  cold air has more oxygen than expanded hot air.

how does this prove that a woodstove delivers more useful room heat per
pound of wood when using 30 f vs 70 f inlet air?

sure, cold air contains more oxygen by volume, but it also has a higher
density and specific heat by volume, ie it requires more heat to warm 
a cubic foot of cold air 1 degree f than a cubic foot of warmer air...
a pound of air is a pound of air, like a pound of feathers or lead,
regardless of temperature.

say burning some particular chunk of wood takes 1 pound of air, ie about
0.23 pounds of oxygen, and that air or the combustion products thereof
need to enter the chimney at 300 f in order to provide sufficient draft
for the woodstove. we can either put 1 pound of 30 f air into the stove
and heat it to 300 f, or warm the same pound of 30 f air to 70 f first
and put that pound of 70 f air into the woodstove and heat it to 300 f...

ashrae says the enthalpy of 30 f air is 7.206 btu/lb with respect to
0 f air, ie heating a pound of 0 f air to 30 f requires 7.206 btu. the
enthalpy of 70 f air is 16.818 btu/lb, so heating 1 pound of 30 f air
to 70 f requires 16.818-7.206 = 9.612 btu. the enthalpy of 300 f air
is 269.74 btu/lb, so heating 1 pound of 30 f air directly to 300 f takes
269.74-7.206 = 262.534 btu, vs heating a pound of 30 f air to 70 f and
then heating it to 300 f, which takes 9.612+269.74-16.818 = 262.534 btu,
ie exactly the same amount of heat. there is no miracle here.

all this is by way of saying that a certain fuel mass requires a certain
oxygen mass for combustion, and heating that certain oxygen mass to 300 f
requires the same amount of energy whether it's heated from 30 to 300 f
entirely inside the woodstove or heated from 30 to 70 f in the room and
then heated from 70 f to 300 f in the woodstove.

>> >a good fireplace should also seal off indoor heated air from being
>> >sucked up as part of the draft.
 
>> sure. we may be depending too much on natural draft for woodstoves,
>> with a serious energy penalty. we might take a clue from the pellet
>> stove people, and use a fan to ensure a draft, with a short double-
>> walled horizontal heat exchanger "chimney" to preheat incoming air
>> with outgoing air. a fan could also help get a fire started, and
>> help with room temp control.

>you doan need no steenkin fan, only a fresh air vent providing cool
>outside air for combustion.

it seems to me that a woodstove with a fan can be more efficient, since
it can exhaust a cooler flue gas, because it doesn't have to rely on the
bouyancy of hot air for the draft. an analogy: a fan with a motorized
damper can move more air than a fan that needs to use some of its air
pressure to hold open the springs of a damper.

>> >most traditionally 'open' fireplaces suck away as much or more room heat
>> >than they produce.  that's why you often have a cold back even when your
>> >front is uncomfortably hot, while sitting in front of that fire.
 
>> the latter phenom is "radiant heat," unrelated to "sucking away room heat"
>> with excess draft air.

>you're half right.  radiant heat was already described, as was draft by
>convection, which  sucks heated air out of a house.  by the
>way, since no real vacuum is created inside the house from such a draft,
>it's infiltration that allows heated air to leave - yet another unsolved
>problem.

what you are trying to say here?

nick

---it's okay to disagree with me. however, once i explain where you're
   wrong you're supposed to become enlightened & change your mind.
   congratulating me on how smart i am is optional.




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