Good morning Mr Thomas,
The frustrated engineer in me has two questions, one of which might be for your mason friend:
Is there a standard for the size/weight of a freestanding chimney? IE for one so tall it must be so wide by so deep?
And are the external air intakes for combustion air? or do they exit into the metal faced void and pass warmed air back into the room?
I don't see how you get anything done. I'd always be ensconced in my recliner in front of that...............
Steve aka 71MKIV
Good early morning Steve, it just so happens that later this morning I'll be seeing Steve, my fireplace genius friend, and I'll ask him about your questions regarding freestanding chimneys. I'll post what he has to say about them.
Now, about the external combustion air. That's an excellent question! First a little background and theory. Some of this is a repeat, but many of the following supporting pictures are new.
As we all know, a fire needs air (oxygen) to burn. If you don't supply air from outside the home, a fireplace will take room air and burn it. As it burns room air, replacement outside air will find its way into the home through all the various cracks and crevices in the exterior of the home. That creates drafts in the home and you've also burned nice, warm, room air that your house heating system has warmed up. Not very efficient, so we supply outside air for the fireplace to burn. That leaves warm, room air in the house and not burned in the fireplace; much more efficient. Here's how we did that.
Here is the fireplace under construction before the concrete pad that supports the stone and hearth is poured. At the top of the picture you see the ends of the combustion air intakes where they are outside the home. They then pass through notches in the concrete sill and connect to the combustion air openings on the hearth. Those openings are plugged with wood here to prevent concrete from getting in them during the pour.
Here's a side view of the same thing. Note the re-bar to reinforce the concrete.
And here's a view from outside the house looking in. You'll notice the concrete pour for the hearth has begun in the background.
Here the concrete pour is all completed from inside and...
...outside. I might also point out that the back of the fireplace is not outside the home as is commonly done. Rather the fireplace is completely within the heated envelope of the house. That greatly helps efficiency also.
Squint hard and you can see the two round combustion air duct openings, now surrounded by brick.
In time those ducts were finished off by installing...
...manual opening/closing louvers.
Once the generator and decks were completed you can barely see the left hand louver.
Here that louver is seen from the deck by the bottom step. The right hand louver is hidden under the deck but an access panel for it was built in while the deck was under construction.
Inside the house on the hearth, the combustion air openings (here located under the blue painters tape) are covered with opening doors and have a manual, adjustable butterfly valve within the duct. When those doors and butterfly valves are all closed, no outside air can leak in. They are only opened when a fire is burning.
So all that my combustion air ducts do, is provide outside air directly to the front of the firebox opening. Virtually no room air is burned by the fire, only outside air is used by the fire. It's a very simple system, but it greatly enhances the efficiency of the fireplace. There is no metal faced void anywhere.
I hope that wasn't a too long winded response Steve. It should give you an excellent idea of what, why and how my external, combustion air system works.
Thomas
