Sam Sewall

Mr. Herbert,  Thanks so much for your patients. I think we are on the same page now. Sorry I'm not as skilled at describing my intentions as I should be. I think you have a very good understanding now of what I'm trying to do. Here's what I got from your last couple of posts. I can remove my current firebox, Seal the entire cavity. (thinking of using high temp caulk on all seams in the block). Then using angle iron construct a steel frame to create a "ceiling" inside the fireplace cavity at 8'. The ceiling will be constructed of firebrick probably 4" thick with an insulating material above that. (Maybe loose vermiculite?). I then will run insulated pipe from the top of the flue to 8" off the floor of the cavity. That would be the "plunger tube" you speak of, correct???. After this is done I will need to build the core. How close to the ceiling will this have to be do you think? Then I will have to build a batch feed type firebox and get it completely sealed at the cavity opening. I'm thinking of utilizing the existing steel fire box with some steel fabrication and lining it with firebrick much like some I've seen. I am going to work on some drawings/plans and will post them once I get them done. Right now I have only one concern....draw from the plunger tube..Will I have to preheat the cavity prior to attempting to start a fire. I most likely won't be using it until temps get into 40 degree highs so I could quite possibly be able to do this by simply opening the intake/batch box for a while before attempting to start the unit. Looking forward to hearing your input and once again thank you and the others who have replied to this post. P.S. I have decided not to attempt multi-flue..no sense in trying to re-invent the wheel.  

Glenn Herbert wrote:Yep, Max, that's another possibility, though I think sticking to a standard batch box build for someone's first try might be safer.

I do see that the OP is suggesting gutting the fireplace and using the whole interior of the chimney mass. The idea of making a bunch of little ducts and building them into the walls of the mass would be far more effort, and much less effective, than simply lining the big cavity with cob/plaster/refractory to ensure no gaps that could leak CO, and running a plunger tube down in the cavity. An 8" square chimney which it sounds like what is there would not allow much "bell" action, but if the stone mass is essentially empty inside, and there are no other flues in it (or any other flues are properly isolated from the cavity), the whole mass would make a fantastic bell. Just set the combustion core into the former fireplace opening and let it exhaust up into the cavity. Sized properly, that would work excellently, and allow all of the beautiful existing stonework to be appreciated.

Unfortunately I'm still learning how to use a forum.  It has taken me a while to digest the info you all provided. First let me say I'm still a little confused to what a plunger tube is. From what I'm getting Glenn you are recommending a batch "heater" within the fireplace and use that to heat the existing mass and be happy with that. I am not ruling that out it's not a bad idea imo. My original thought was to find away to harness the heat from the exhaust in a vertical mass thinking that a manifold would be more effective in doing that than one 8" vertical pipe incased in a cob mix I really want the advantage of the stored heat. I have not tore into the fire place but I believe the inside behind th stone is lined with 8" cinder block. What is your opinion after reading Mr.Kettell's last post. After reading that I would be apt (based on my inexperience w rmh combine w/ my experience with wood stoves) I would be inclined to go with 5 x 4" ducted manifold system if I go that direction. I feel that that would make for a stronger draw up the flue. That being said do you feel if I simply lined the inside with 8 or more inches of cob do you believe it would absorb enough heat to accomplish the same outcome. It seems to me it would take more wood to heat the cob from the outside in. It would definately be a lot easier that way. Really appreciate everyones help. BTW I do have a cy of The Rocket Mass Heaters Builders Guide.

Back to my original post - Yes 7 3" manifolds would allow me to maintain my CSA.  My thought was to distribute them up the inside of the fire place cavity on 3 to 4 walls and incase them in a clay & cob mix to harvest the heat out of them. They would be manifolded into 8" pipe on each end. One manifold coming out of the bell the other out the very tp of the chimney stack. Not sure if they would all draw equally at start up but the capability is possible. MayBe???  It's not far off a regular chimney with two wood burners tied into it.

The fire box dimensions are 35" w  x 34" h x 25" d.  

I've never been involved with building a fire place so there are a few things I'm not quite up to speed on.

I'm assuming the Chimney is 8" square based on the flue coming out the top which is an 8" square terra cotta liner. It's my belief that it runs to the basement for some reason the knuckle heads that built it (over 20yrs ago) did not install a clean out. The distance from my fire box to top of chimney is approx 16-18'

I am not familiar with the term "plunger tube".

I believe if I removed the firebox the inside dimensions would be rectangular approx 70"W x 32"Dx 16'H.

I live in NE Pa..  It occasionally gets down to -10 in the middle of winter with average highs around 30. Down right balmy compared to upstate.  I spent many years in northern Maine and very familiar with wood heat.

So what you are saying, I believe, is to build a RMH within the fireplace.  Then seal the rmh within the cavity of the fireplace thus creating a large bell over the enclosed rmh and just ducting the exhaust up through an insulated flue.

Has this been done that you know of?

BTY my outside dimensions of the fireplace  front is 7.5' wide approx 12' from heath to ceiling sides outside dimension (depth) approx 4'.  I believe the outside is cultured stone wit 8"block within and a block foundation for the entire thing in my basement which is essentially a big hollow room directly beneath my fireplace.

 

I'm another one of "those"guys trying to make use of my Existing fireplace. I have an idea and want to run it by you folks.  Considering an 8" exhaust out of the bell and then making a manifold with 7 3" vents to the top of my chimney then tying back into another 8" pipe and exiting the chimney cap. This will let me maintain my CSA and let me use the insides of three or four of my fireplace walls to create a mass to heat up. Its a huge Fireplace centrally located in my relatively small house. Am I nuts or what?

PS came here looking for rmh info but have just been checking out some other areas on the sight. Just Awesome.