[Greenbuilding] Fireplace questions

[Keith Winston]

Hi Corwyn,

>> 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.
>
> Why?
Mmmm, I guess I thought my point was obvious. Alas, not an infrequent 
oversight on my part ;-)

What I meant was, wind whistling through an open window and up a 
chimney, and wind whistling through an open fireplace outdoor air feed 
and up a chimney (behind closed fireplace glass doors) will be very 
different in how much heat they steal from the house. That seems obvious 
to me, but I could beat that horse if you're feeling sadistic.
>> 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 back-drafting issue is, as you say, contested.  The only data I 
> have seen showed no difference.  A decent fire is only drawing a few 
> CFMs, I would feel hard pressed to notice it.  Drafty would, I think, 
> be in the 100s of CFMs (more like my kitchen vent fan).
As Norbert went on to state, 500 CFMs can be more like it. One of the 
papers I was just reading mentioned 20-50 l/s, which is around 40-100 
CFM, not a trivial amount of air or energy. Also, the amount of air 
varies considerably by the stage of the fire, so unless someone is 
paying attention, the air supply will be wrongly adjusted more often 
than not. Which is exactly why you might like to keep it all behind a 
pane of glass.

Reading through the literature that Norbert pointed us to, even though 
some of it is labeled "Outdoor combustion air was a good idea . . . 
until it was studied", it's not at all obvious to me that the literature 
actually says that. It looks to me like they set up straw men and knock 
them down, like the issue of heat lost up the chimney in heating up the 
outdoor air vs. indoor air (which then gets the caveat of "unless you're 
using another heat source, like a boiler or furnace", which most users 
will be doing, further degrading the point, but I digress), instead of 
looking at net efficiency, which seems to me would have more to do with 
all the heated air that goes up the chimney that has essentially nothing 
to do with the operation of the fireplace. The only exception MIGHT be a 
perfectly operated fireplace run 24/7.

I will also note that I am paying little attention to the spillage 
issue/CO. It looks like it has more to do with other depressurizing 
appliances than anything with fireplace operation. I'm fairly confident 
that an outdoor air supply properly installed won't increase that risk. 
I'm not sure there's sufficient evidence to do more than speculate in 
that regard.

Well, I'll stop there. It's chilly tonight, too bad I don't have a 
fireplace...

Keith