RMH to replace a wood stove with a 4inc flue

Hi this my first post and my grammar is not good so be kind.

We live in England in a 650sq ft cabin, which we heat with a 3.5kw wood stove, feeding a 4inc double walled chimney.

I would like to replace this with a RMH, I know 4inc RMH are not good, so I thought a 5or 6inc feeding a 1 1/2 barrel bell mass then going to the 4inc chimney.

I have a 14inc tube for the barrel and was going to use a 5 or6inc stainless heat riser with 2inc off insulation around it.

I have a foot print of 96inc by 30inc.

Your input would be great.

Thanks Paul

Paul Carter : You are giving yourself a tall challenge for a first build, and it will be hard for an American to help you score some of the materials that are common on this side
of the pond and scarce over there !

We have several members that have successfully manufactured D.I.Y. 'Cast-able cores' most of them with square or modestly rectangular shapes, Question #1) can you get
insulated 'backer board'?

The traditional route that we have followed is to maintain a ' Constant Cross Sectional Area' striving to keep it, Whole-y! ( pun intended), recently we have come to realize
that an extra amount of "room' between the exterior wall of the Heat Riser and the barrel is not a bad thing, finding where more space becomes a negative will take time and
experimentation! 2.5'' and 3'' being comfortably within reason !

The Transitional Area where the hot exhaust gases turn from their falling vertical flow-into a right angle horizontal flow and past the clean out, then into the horizontal ductwork
is now often treated as an opportunity for a Second Bell, ( if the barrel itself is called the 1st bell ), that and a short area of Thermal Mass to further collect, cool, and slow the
flow of the exhaust gases, may indeed allow you to choke down your exhaust gas flow to be successfully handled by a 4'' round exhaust pipe of sufficient height !

Another way to say this is there are two (2) chances to increase the exhaust gas flow, 1st with a high Heat Riser, and 2nd with the height of the vertical chimney !

The only absolute here is - Try it out doors first, and then try it inside. You will have to live with your resulting build for at least a year before you can be sure that it will probably
serve you well in the future ! I hope this helps a little ! This is where I ask if you have gone to cobcottage.com for your PDF Copy $15.ooU.S.D. of Evans' and Jackson's
Great Book - '' Rocket Mass Heaters '' there isSTILL no other book in any language that contains more Rocket Stove / Rocket Mass Heater family information -
( and I don't make a dime ! ) For the Good Of the Craft ! As always your comments and Questions are solicited and all are Welcome ! Think like Fire!, Flow like a Gas!, Don't be the
Marshmallow ! PYRO - Logically BIG AL !

As always, your comments, and questions are solicited and are Welcome ! Think like Fire !, Flow like a Gas!, Don't be the Marshmallow !! PYRO - Logically Big AL !

Thanks I have the rocket mass heater book.

Where does the castable core go? and what is the 'backer board for.

Can I not make the burn tunnel out of fire bricks?

Can I not use 1/2 oil drum in place off the ducting to help with the flow.

We do have clay so cob is not a problem.

Sorry for all the questions.

Paul Carter : In the upper right hand corner below the 'Permies Banner of the Day', is the 'permies toolbox',- select Search, and on the new page scroll down to the
Google Search and enter Castable core, or Castible Core, This will take you to a File where you can hunt for/click on a Thread by that name and then you can hunt
for a couple of videos of successful D.I.Y. Cast Cores ! As long as you will be building your own system from scratch you could consider this option !

Yes you can use Fire Bricks For the Feed Tube, Burn Tunnel, and the Heat Riser, this would actually be the simplest build with a tried and true system that works !
A second big plus with fire bricks is you can dip your bricks briefly in water, followed by a dip in a Clay Slip, and Stack your fire bricks without mortar, this is an
improvement that is not possible with old house brick !

I have not tried the 1/2 barrel system and am not confident enough to advise you on that adaptation of the system, It will work very much like a Bell, radiating a
lot of heat quickly, this will slow the speed of the hot exhaust gases, the total length of thermal mass bench will probably be cut about in 1/2 and greatly
reduce the number of elbows you can use ! I hope I have helped you some And pointed you in the right direction ! For the Good of the Craft !

As always, your comments and questions are solicited, and are welcome ! Think like Fire!, Flow like a Gas!, Don't be the Marshmallow ! PYRO-Logically BIG AL !

I would agree with allen that changing CSA (cross sectional area) for your heater, putting a 6 inch system and then plugging it into a 4" exhaust is asking for serious trouble. I think you will have major issues with your stove.

With the 1/2 barrel, it is unlikely you need any flue. Matthew Walker has built them and has post here. I would suggest you check them out. The 1/2 barrel works as a bell, and if you cover your barrel with cob or other thermal mass, it will not radiate heat especially quickly. How quickly it radiates or looses heat is dependent, like a flue system, on the materials used, it just needs less space. Bells also do not slow the exhaust speed, but they do reduce overall friction. A good insulated chimney should give you all the draw you need.

Here is a explanation of how bell systems work vs flues.

http://blog.dragonheaters.com/wood-heat-storage-flues-vs-bells/

FYI Also I would say your barrel is not big enough, even for a 4". We need an 18" barrel for our 4" dragon heater cores.

Paul Carter : I have gone back and edited my post to make it a little more readable an hopefully removed a ambiguity.

Check out the (d) paragraph on pg 36?, following the 'Dementia and Proportia' picture pg 35 ? Your Tube that you want to use for a barrel will give you a whole bunch of trouble,
If you can get the system to work, physics requires the smaller barrel will radiate the same mount of heat energy as a larger barrel, in order to do this it Must do so at a
Higher Temperature.

This is why when your 3 yr old wants to climb up in your lap for a cuddle, It feels so good! Their Mass to Surface ratio is such that to maintain an internal Temp near 98.6 F
their skin has to radiate excess Heat Energy at a higher temp !

I feel confident that, when Sandy Mathieu said- "With the 1/2 Barrel you will not need any Flue '' he meant any additional Horizontal flue, but check with him it is never correct
to put words in someone else's mouth post/thread ! I do not understand how a system like a Bell could be said to have less friction when by their nature they
increase turbulence, the scrubbing of turbulence might have the effect of slightly increasing laminar flow but not without some decrease in the flow of gas, I have only observed
short, abbreviated Rocket Mass Heaters R.M.H.s with 1/2 barrels as compared to more traditional builds, but that is just what I have observed, not science !

Obviously you have seen a working system with Half barrels, I wonder how well they will draft when cold, obviously that can be overcome with a newspaper fire started in your
final clean out !

Over all I think that you are trying to accomplish to much and deviate a little too far for a first build and other than general information I am probably not the best person to help
on this build- if you run into problems, which is likely, I am afraid that trying to make things right would prove difficult ! I do hope for the Best ! Big AL!

Sandy does your 4" Dragon have a 4" chimney? and can you connect it to a bell? ie 1/2 barrels

I have a 30 gal barrel so could use that instead of the tube or in fact I could roll any size to suit, so I can get round that problem. I just thought that a full size barrel would radiate to much heat.

Yes it can be run with a 4" chimney and 1/2 barrel. The 30 gallon barrel would be a better fit. Smaller than this, the barrel will be too hot.

will try a 4" with a half barrel and see if it works , nothing to lose if not will replace chimney with 6" pipe.

Will post pictures as I go.