rocket stove manifold with 7 exhaust lines.

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.

If your chimney is located such that heating it will help heat the house, and is large in cross section, I might consider using it as another bell - let its whole volume contain the exhaust until it approaches the roof, at which point you channel it into a regular system-sized chimney liner (insulated) so it is not trying to heat the outdoors or drafting in an unhelpful way.

I presume 7 3" diameter pipes have about the same cross section as one 8" duct, but due to surface friction effects, you would actually get much less flow through them than through a single 8" duct.

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.

 

People have built RMHs incorporating an existing fireplace, not entirely inside a fireplace that I know of. It looks like you could build a good bell on the hearth, which leads to a second bell inside the chimney. Technically, using a chimney space as a bell would involve running an insulated pipe down to take exhaust from near the base of the space. What are the internal dimensions of your chimney? It would need to be really big to do it the "proper" way. As long as you have a first bell whose exit to the chimney space is at fireplace floor level, I think you could benefit from simply using the big chimney space as a slower rising column of gases, without trying to make a real bell of it.

Making the fireplace cavity part of the main bell, and running a "plunger tube" down from the sealed off chimney cavity to 6" (adjustable) from the fireplace floor, would give you the maximum main bell space. The important thing about a bell, aside from exiting near its base, is to have the right internal surface area (above exit level) to absorb most but not all of the heat produced.

As far as layout, it looks like the hearth is a bit narrow to have either a J-tube or batch box opening on the main face; it would have to extend back into the fireplace cavity, which would make the plunger tube trickier. With the width and side access available, I might turn the core parallel to the long wall and feed it from one end, whichever works best with room layout. If you brought the bell back 18" from the end of the stone mass, you would have a fireproof hearth below the feed for safety, and still be able to make a good sized bell.

What is your climate? If not too harsh, you could probably go with a 6" batch box and have easy chimney connections and transitions. If you want to use a J-tube, you would probably want an 8" system.See  http://batchrocket.eu/en/building#bellsizing  for detailed information; it seems to be that an 8" J-tube has around the same power and ISA requirements as a 6" batch box.

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.

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.

Glen!

A sideways batch!

http://www.ecologieforum.eu/viewtopic.php?f=3&t=4610

And yes, definitely a plunger tube land.

I think, the firebox could be knocked off, to make the whole fireplace a bell. May be!

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.

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.

The other problem with the split design as proposed is that while the cross sectional area of 8 3" ducts is similar to a single 8" duct, but the drag due to the increased wall surface area is hugely different.  Here are the calculations. I am going to assume that the lengths of both systems the 8 3" ducts and the single 8" duct.  The volume of a single 8" duct is 16*pi*L cubic inches and the eight 3" ducts are 18*pi*L cubic inches.  So the 8 ducts are about 12% larger than the single 8" duct.

Now lets look at surface area.   The single 8" duct has 8*pi*L square inches of surface area, but the 8 3" ducts have a surface area of 24*pi*L square inches.  Thus the surface area of the walls of the duct are what create drag are three times as much on your proposed design.  Thus your design will have a lot more drag.

If you used six 4" pipes you would have a volume of 24*pi*L cubic inches which is 50% more than the single 8" duct.  The surface area is still 24*pi*L but the ratio of the volume to the area is one to one.  While this is worse than the three to one volume to surface area of the 8" duct, the 50% of additional volume in the six 4" pipes should compensate for the incresed drag
This is not an exact calculation and i would quickly bow to someone with a deeper engineering knowledge of fluid mechanics than this electrical engineer has.

Best wishes on your design.

Thanks for the calculations, Ralph. Another way of figuring is that the layer of air next to the wall is relatively useless for flow (something like 1/2" or so), so you need to reduce the effective radius by that amount. So a 3" duct actually has maybe 2" diameter of fully flowing gas, while an 8" diameter duct has 7" of flow. A ratio like (7 x 2 x 2 = 28) vs. (1 x 7 x 7 = 49) shows how much difference that makes. Note that the surface drag effect is not related to the diameter but to the character of the surface, so has a much bigger detriment for small diameters.

Glenn Herbert wrote:Thanks for the calculations, Ralph. Another way of figuring is that the layer of air next to the wall is relatively useless for flow (something like 1/2" or so), so you need to reduce the effective radius by that amount. So a 3" duct actually has maybe 2" diameter of fully flowing gas, while an 8" diameter duct has 7" of flow. A ratio like (7 x 2 x 2 = 28) vs. (1 x 7 x 7 = 49) shows how much difference that makes. Note that the surface drag effect is not related to the diameter but to the character of the surface, so has a much bigger detriment for small diameters.

So using your rule of thumb with my guess of 6 4" ducts would yield the following:

One 8" duct would have 12.25*pi square inches of use-able cross-sectional area.

Six 4" ducts would have 6*2.25*pi or 13.5*pi square inches of use-able cross sectional area.

So my guess agrees very well with your rule of thumb!  if I had guessed five 4" ducts it would have only been 11.25*pi square inches which is  0.92 of the 8" duct.

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.

Hi Sam,

Just to clarify, are you contemplating keeping your existing stove insert and simply trying to capture some of the heat that is going up your chimney to improve efficiency?  I know that many different ideas have been bantered around, but on reading again your first post I began to wonder what I asked you above.

Sincerely,

Ralph

If you do take out the existing fireplace and build a RMH core in its place, you would have access to the interior of the chimney cavity. Assuming it really is just a hollow concrete block box inside (which seems likely), I think you would get the best results AND the least work by just making sure the cavity is sealed so that gases cannot leak out into the house, and letting the RMH core dump its heat straight into the cavity. Then you would run an insulated duct down from the existing chimney at the top, to 6" or a foot from the floor of the cavity so that only the coolest and heaviest gases leave for the sky.

Building a system of small ducts to split the exhaust would be a lot more work, and less effective.

The stone facing looks to be around 4" thick from the corner stonework, and with the probably 8" block structure, you probably have an interior cavity two feet less in length and width, so maybe 2' x 6'.16' perimeter x 8' high would give 144 square feet of internal surface area, which is half again as large as recommended for even an 8" batch box, per the chart at batchrocket.eu. I think that given your very favorable conditions (chimney in center of house, a plunger tube carrying the exhaust up through the middle of the bell) your setup could support that amount of ISA.

I would advise to insert a top to the cavity about 8' above the cavity floor. (You don't really want most of the heat gathering at the very top of the chimney mass.) I don't know what would work best in your case, maybe pieces of rebar inserted in holes drilled in the block walls and mortared in place to support refractory slabs, all sealed with refractory cement... Looking at what is there would be the first step. Technically the top third at least of the interior should be lined with firebrick, but you might not need that if the heat can disperse enough after it hits the top of the cavity.

Are there any other flues that come up from the basement through your chimney mass? That would be a hindrance to using the cavity. It might be possible to reroute or protect another flue, but it would need separation in some way.

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.  

Your current plan sounds quite practical, except that a metal fireplace shell bears not the slightest resemblance to a batch box configuration and would be unlikely to work without significant modification. You would probably do much better just building a firebrick or other refractory core in place of the original fireplace location.

Do you know what the chimney liner currently is (metal or flue tile)?  If flue tile, and currently supported by the fireplace shell, you will probably have to replace it, as resupporting it would almost certainly lead to cracks in the joints. If the flue is supported independently of the fireplace shell, you only have to run a new stovepipe from there down to 6-8" above the cavity floor.

With your house layout (central chimney), you may not have any problems establishing draft on startup, but if you are concerned, I would install something like a tee in the stovepipe above the batch box riser height, with a handle to the outside so you can open the tee for starting. Joints would have to be well protected from fume leakage into the room, of course.

A point of terminology, if not understanding: the batch box firebox is part of the core, as is the heat riser, so you would build a heat riser and firebox inside your block cavity. The riser, and all other parts of the core, need to follow the specifications in batchrocket.eu. The developer Peter van den Berg has tried about all reasonable variations of the concept, and found that the system needs to be close to these figures to work best.

After building the core, I would make a removable metal panel to seal it to the original fireplace opening, so that you can have access to the interior for future inspection and possible maintenance.

To seal the block cavity joints, I would consider surface bond cement in a continuous layer. It would be cheaper than sealing all the joints with silicone caulk, possibly faster, and probably more reliable and protect the blocks from the hot atmosphere inside. With a cavity as large as yours, I would think that the heat would not be too severe except on the ceiling. Speaking of cavity size, the internal surface area of the cavith walls and ceiling need to be in balance with the batch box heat output, so that neither too much nor too little heat is extracted from the gases. Too much extraction and there may not be enough heat left for good draft, and too little extraction wastes heat up the chimney. Again, batchrocket.eu has info for calculations.