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Post by richard on Nov 18, 2011 20:56:33 GMT -8
I am planning to assemble a rocket mass heater in the basement where there is already a seldom used fireplace on an outside wall. This fireplace has a brickwork chimney extending up about 30 feet. I've searched this forum for discussion about using an existing chimney to vent the exhaust gasses but not found much. (Although I could have missed it completely as I've been sidetracked many times by more interesting topics.)
I assume that this is commonly done.
The cross sectional area of my flue is larger than the planned RMH system size. Will the increased size matter to the gasses after they start to rise out of the house? I was thinking about running the ducting into the smoke chamber of the fireplace and sealing around it at the damper to see what happens. I guess I could also try blocking off the whole front and dumping the exhaust right into the firebox.
Is there anyone who would like to share their experience. Thanks.
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Post by Deleted on Nov 19, 2011 6:50:26 GMT -8
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Post by richard on Nov 19, 2011 14:57:12 GMT -8
Hi karl,
Thanks for the link. Interesting but I'm still a bit fuzzy about the difference between a conventional chimney and that of a rocket stove.
I can see that having an insulated chimney liner matching the system size, or even smaller, would make this a non issue and I'd just connect it up.
Where I get baffled is that the flue functionalities of a conventional wood stove are separated in the rocket stove. The draft component is contained within the rocket itself. The heat riser produces its own stack effect pulling in combustion air and pushing the hot gasses into the heat storage. The final chimney in the rocket only needs to remove the exhaust fumes without plugging up the works. I'm hoping that the hot exhaust, even though its had some of its heat collected in the mass, can still rise and find its way out of the house.
I am assuming that the design of my existing flue accounts for backdraft issues and that attaching a RMH would only increase flow resistance, lessening the problem.
From what I can tell, a properly running RMH should have little problem with creosote and chimney fires. Since the combustion should be more complete, few deposits should be left. And, the temperature of the exhaust gasses should be well below the ignition temperature of creosote.
I also worry about the exhaust cooling too much. Either the stove may be hard to start or let smoke out of the feed without a strong pressure difference. I did try lighting a fire in the fireplace last night and easily got a draft started with a sheet of newspaper burning on the smoke shelf. So maybe having a priming port could eliminate problems here. The other issue I see with cooling exhaust is that condensation could flood the works. I would like to hear from others if this has been a problem.
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Post by homesower on Nov 19, 2011 20:23:43 GMT -8
If you are exhausting into a chimney you need to know what else enters that chimney. If you can't guarantee that nothing else does you need to run a separate liner to your stove. Its not a matter of draft for the stove its a matter of not gassing your family. For my wood furnace I ran a separate liner. Since nothing else entered the chimney for until the third floor I poured perlite around the liner for twenty feet to insulate the liner. As your chimney is an outside one something like that might help. You should also have a damper between the stove and the chimney. This is one of those weighted dampers that allow a little air to get sucked in when the chimney is trying to pull more than your stove can provide. It reduces the pressure fluctations on the stove, so it will help keep the fire steady. www.house-design-coffee.com |
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Post by grizbach on Nov 19, 2011 20:27:45 GMT -8
Richard, I have an example of what you want to do. Take a look here "http://donkey32.proboards.com/index.cgi?board=discuss&action=display&thread=197"
With a standard chimney the warmth of the house naturally wants to exit the chimney. Inversly, if it happens to be hotter outside than it is inside, the chimney will backdraft. So an open damper is like having a window open.
I just made a plate to block and seal my chimney and then piped my flue to it. I still have a working damper that is usually half open when i'm having a burn.
Advice that I give people that have draft problems with their rocket is to put a vertical stack on it. As long as the temp of your exhaust is warmer than the temp outside, all should be good.
Grizbach
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Post by peterberg on Nov 20, 2011 2:06:34 GMT -8
An outside chimney which is larger than system size of the rocket stove? Please, place a stainless steel liner, flexible or rigid and preferably the same as system size. Fill it around with vermiculite or perlite and incorporate a primer box into the plans. Together this will solve most if not all of the common problems associated with such a chimney.
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Post by grizbach on Nov 20, 2011 8:50:47 GMT -8
My mind just clicked into "standard chimney". My standard chimney is a 10" flue. I did build my sealing plate with with a primer hole. I never have had to use it.
Your set up being brick is probably larger and would suck more of the heat out of the exhaust. It is a good sign that you got a draft going easily.
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