so can i put a damper in my flue just before it enters the ceiling and shut it down when my fire is about out. and shut the feed tube with two bricks. and then would it make my heater much more efficient by keeping heat in the flue going thru the mass bench???
You're wanting to do this when the fire is "about out"?
It seems that most folks want the fire thoroughly out, coals dead, before shutting the chimney damper, don't they? Of course, if your damper only closes 80% that may not be as big of a concern.
So, I'm thinking the bigger concern is that the gases from the coal burning stage need to be vented out of the house, and not saving some of the stored heat from escaping along with these gases.
I've also read of some folks who make a "plug" for the feed tube opening, complete with a gasket. I'm just guessing, but it seems like that would work pretty well, especially in combination with a 80% chimney damper in the "closed" position.
I've seen several feed tube plugs made. Some just use fire brick, or course. That's the easiest I think. Next easiest is just setting a salvaged lid over the opening (maybe with a gasket along it's rim).
Some folks form a plug just slightly smaller than the feed tube opening with castable refractory (like used to make capping stones/plates for masonry heaters, if you want to look that up).
For the "gasket" I've seen fiberglass rope used (like on a furnace door) and rockwool/ceramic wool.
I haven't gotten very fancy myself. First I just make my gap progressively smaller using fire brick. I can get down to about 1/8-inch gap using two bricks, and I lay a third over their seam (I don't think that makes much difference). If the fire has been out-out for at least a couple hours, to "seal" it, I begin by laying down over the feed tube opening, a piece of aluminum foil that is a few inches wider and longer than the opening, then lay a couple 12x12 ceramic tiles on top of that (left over from the tiles I stand on when feeding the fire), and then set the fire brick on top of that. So *if* the aluminum foil (which is somewhat crumpled by now) creates a gasket, I guess I have one, heheh, and if not, then I have just diminished the flow of air into the feed tube by a great deal. I no longer open and close my flue damper; I just leave it open all the time.
Bear in mind, this is the rocket stove/heater I am prototyping, and I will make a number of changes when I make the permanent build later this year. I have not yet decided if making a castable plug for the feed tube will be one of those changes. For that matter, I am leaning toward a batch-box, instead of a J-style feed, so I'm not really spending much thought in how to get a perfect seal over my closed feed tube opening. I am not yet of the opinion is is that big of a deal; at least not with my current build. I'm convinced that for now piling up the tile and brick on top of the aluminun foil makes a very good seal; if not perfect, certainly good enough. And honestly, I don't mind a small air leak there, as that encourages gasses to vent outside the house, which I consider important.
Plus, there is the theory that some express (such as Ernie and Erica Wisner) that the fire riser tends to form an air plug anyway (think along the lines of the U-trap in your sink). So unless there is a strong draft (stormy winds, perhaps?) the system may effectively seal itself after it cools sufficiently.
michael carman wrote:i'm curious why you don't shut the damper also. seems it would keep in heat....
This build is only a prototype. There's hardly any thermal mass, so I don't think it really makes any difference in my case. I'm going to tear it down after the winter passes, and then do the permanent build on my porch (after enclosing it). That's more than I could do before this winter came. So I began with outdoor builds, and then indoor builds (this is the second), while I collect temperature readings and get used to the feeding of the beast, and what not.
So after I close up the feed tube opening I just don't feel there is much loss, nor much stored heat to lose. So I don't bother with the damper. But I do cover the feed tube opening, because I still don't want what amounts to a 6x6" hole in the wall That seems a bit much, heheh
As I write this, it is July 2015, and obviously the winter heating season 2014/15 has ended. I never resumed the use of my flue damper. And I never noticed any trouble with just placing the fire bricks over the opening of my j-style rocket heater. I just kept closing the opening more and more, to 1.25 inch while the coals were dying down (the width of one fire brick split), and then just about an 1/8-inch or so open when I went to bed or to work. My thinking was this was enough to permit gases to escape, should any be present.
Other days, when the wind was from the "wrong" direction (I never got around to increasing the height of my chimney pipe, given this was a temporary and prototype build) I covered the feed tube opening first with a piece of heavy aluminum foil (still flexible, still for cooking use, just the heavy foil, not the very easily torn thin foil), then on top of the foil a couple of 12x12 inch floor tiles I had left over (which completely covered my feed tube opening), and then on top of the tiles, setting the three split fire bricks I used to moderate my feed tube opening when burning.
I cannot say that I ever detected a cold breeze flowing into the house using this set up. Thus, as of now, I am not convinced loss of heat is of great concern. Granted, I also did not simply allow a full 6-inch opening to remain fully open through my wall all winter either! But my opinion is that closing one end of a loop (feed tube, to barrel, to chimney pipe) is adequate to slow or halt the loss of indoor air to the atmosphere, to any appreciable degree.
I imagine a bottle, and trying to blow smoke into it, simulating a cold winter wind. How effectively does the smoke enter the bottle? How deeply? I am thinking, not so much. I'm not saying using a flue damper might not be a good idea; but my opinion is that it ought to be located in an obvious location, so that one remembers to *open* it before starting the new fire hehehe, having forgotten once or twice ...I can say the fire tends to draw much better when the damper is open
Now, bear in mind that I live in an older house and I *do* have old windows and doors, so I already have "heat leakage" in the house; this is *not* anything like a super-tight house.