[Greenbuilding] Fireplace questions

[Keith Winston]

It seems to me there's still a hot debate about the outside air issue. 
Often, the point is made that the fire must warm indoor or outdoor air 
to the same temperature before combustion, and therefor the net energy 
is the same.

I wonder if that misses the point? As the fire burns, in the absence of 
an outside air feed it depressurizes the house somewhat, and fresh air 
is drawn in through whatever cracks and crevices can be found (which 
might include a cracked window or other explicit "air intakes"). There 
are a couple questions I have about this. Shouldn't the operator be 
adjusting that window, and finally closing it, as the fire dies out? 
That's unlikely to happen reliably with most operators, and the window 
or cracks will be a source of heat loss until closed. If the fireplace 
has glass doors and the air feed is inside the firebox, then this can 
either be more easily ignored (net heat loss will be much less) or 
thermostatically controlled. Second, in this first case cold air is 
streaming in from one or more points, through the living space. The very 
definition of drafty. If the house is too tight, then there are chances 
for backdrafting and introduction of combustion byproducts into the space.

The best argument against outside feeds is that negative pressure 
situations could result in fire being blown backwards through the air 
intake, which is typically made of plastic. I suppose either building it 
of metal (though until you build it like another chimney, 
clearance-to-combustibles and all, you aren't theoretically safe), or 
one might be able to fit some type of one-way damper on it (like a cape 
damper built to withstand high temperatures). I have no idea if there's 
any evidence of this problem actually occurring in a properly designed 
and installed system.

I think there's something else about the steady/non-steady-state 
assumptions, but I can't get it clear in my mind right now.

Keith


Corwyn wrote:
> On Nov 06, 2006, at 23:22, Bruce Donelson wrote:
>   
>> Any fireplace that lays any claim to efficiency will burn outside air
>>     
>
> Do you have data for this?  I don't see how outside air helps (in a 
> house where a furnace isn't running simultaneously).  Is the stack 
> temperature lower in an outside air fed fireplace?
>
>   
>> Big net energy loss.
>>     
>
> Again, only if there is a furnace running at the same time.  People 
> heated their houses for centuries with fireplaces.  If they were a big 
> net energy loss, they would have all frozen to death.
>
> I agree with most everything else.  A nice woodstove gives all the 
> ambience of a fireplace in my opinion.
>
> Thank You Kindly,
>
> Corwyn
>
>   

-- 
Keith Winston
Earth Sun Energy Systems
3927 Madison St.
Hyattsville, MD 20781
301-980-6325
keith at earthsunenergy.com
www.EarthSunEnergy.com