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Sizing RMH for 7" exhaust

Dominic Lemoine

Joined: Jun 13, 2013
Posts: 7
Location: Wales, UK
Hi everyone,

This is my first post here. I wonder if you can help me with this.

I am planning to build a RMH in my lounge. I have been reading Ianto's book and am interested in building the 8" RMH. There is however an existing 7" exhaust flue situated at the top of an inglenook (where I have a standard stove at the moment and where I would build the RMH). I live in a 400 years house and I don't feel like opening the 3' walls to replace the 7" pipe for an 8".

My question is: what changes would I need to make in order to make Ianto's 8" RMH design work with a 7" exhaust flue?

Thanks a lot!

Forest Gardening, Short-rotation coppicing, Attachment parenting, Natural beekeeping and Yurting in beautiful Mid-Wales.
Kite Hill Yurts - Luxury Glamping in Mid-Wales
allen lumley

Joined: Mar 16, 2012
Posts: 2768
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
Dominic Lemoine : Welcome to Permies, and a BIG Welcome to the Permies Rocket Stove / Rocket Mass Heaters R.M.H.s Forum Threads ! You will always remember
your first few 'New Member' posts !

I can understand your not wanting to mess with 3' thick walls, and a 400 year old house needs to be treated with great respect ! I'm sure you are being mindful of
what you can and can't do to old Historical buildings !

First question is do you have a thatched or tiled roof ? The reason I ask is, we should physically examine the chimney on the roof, both for its present condition
and its overall opening/size . I am wondering if a closer look at the flue opening in your wall that receives the thimble from your current stove has not been re-
plastered / remade several times in the total history of your house ! A careful look here may show you that you need only minor changes at this location to modify
your receiver to accept 8'' pipe ! In any case, you want the services of a Good Chimney Sweep / Builder for inspection and possible upgrade !?!

Since you are giving your new R.M.H.- and its Dragon! - Pride of Place in your lounge I am sure you will be able to listen closely to your R.M.H., and attend to its needs
easily, And I see no problem with making the Built-in-furniture blend with your historical house, as long as you are happy with having the barrel as part of your R.M.H.

This is your decision and your decision only, So the only remaining obstacle is using enough of the remaining Heat Energy to allow for the Exhaust Gases, cooled and
slowed - compatible with your ( Internal ? ) Chimney. This is not an uncommon practice with normal wood stoves and I have word-of-mouth that this has been done
even as soon as part of the horizontal duct work near the end of your thermal bench ! The short internal vertical chimney can be Cob'd too, if additional Clean outs
are allowed for !

Here is where I stop pretending that I know more about this than I actually do, and tell you that while I might go ahead and try it anyway, You should get a second
opinion, starting with your Chimney Sweep ! The moderators of this Forum Ernie and Erica Wisner can be reached here at Permies, or through their own site -
- ernieanderica.info - , they have some personal knowledge of Ianto Evans necking down the flue size on at least one specific build ! They may be able to share more
information with you to put your mind at ease with this wrinkle in your Build !

For the good of the Craft ! be safe, keep warm ! As always, your comments, and questions are solicited and are Welcome ! PYRO - LOGICALLY Big AL !

Success has a Thousand Fathers , Failure is an Orphan

Dominic Lemoine

Joined: Jun 13, 2013
Posts: 7
Location: Wales, UK
Hi Allen and thanks very much for such a warm welcome!

I'll try to answer your questions. My house isn't historically graded (i.e. I don't have to ask permission to change it). At the moment, the house is covered with a slate roof.

I did my homework and consulted a chimney sweep a few months ago. The current flue has been redone some 30 years ago according to building regs. The flue is 7" all the way through and seems to be lined with pumice. There is no big gaping hole for open fires like we sometimes see in well preserved historical houses.

No, I don't mind the barrel being part of the house!

Has anyone heard of working 7" systems? I like the idea of the larger burn chamber which allows for larger logs. If not, I will probably go for a more common 6" system. Maybe buy the plans from Ernie and Erica Wisner. Would there be problems with a 7" exit flue? I can't see why, but I am new to the dynamics of RMHs.
allen lumley

Joined: Mar 16, 2012
Posts: 2768
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
Dominic Lemoine : I had to google Pumice chimney liners, this is news to me, We seem to follow along with the Europeans with vitreous clay liners for new construction,
and lots of restrictions on re-builds, this is interesting - I will want to research it more !

For a long time a 6'' flue was considered as small as any builder wanted to go and be sure that his Rocket Mass Heater R.M.H. and its Dragon would perform well, there
are some kits out there with 4'' square Heat Risers, but it would be for a very small space. I follow an old family rule about not buying the very first or the last of any
thing. A 7'' system is nice and safe, a middle of the road - Not a driving in the middle of the road, solution !

I did my First Rocket stove Mass Heater build with Ernie + Erica and found them extremely knowledgable and my wife Martha and I consider them
personal friends ! Using a set of their plans should give you a great feeling of comfort for your first time build, and they can be reached by e-Mail for follow ups !

There is a tremendous about of information available here through searching for and reading their old Posts !

We Americans are crazy about our 'happy snaps' and take pictures of every thing especially now with cameras on every phone, take a picture, take apart off, take a
picture, take a 'nother part off, repeat ! Reverse the order to put it back together and you know what it should look like when you get it all together-great piece of mind
Your own personal video log of your build will be a great personal souvenir of your build and something that all of the Rocket Stove Members here - Rocketeers - will
want to see. Please Keep in touch ! Good Luck ! Think like fire, flow like gas !

For the Good of the Craft ! Be safe, keep warm ! As always, your comments / questions are solicited and are Welcome ! PYRO - Logical Big Al !
Dominic Lemoine

Joined: Jun 13, 2013
Posts: 7
Location: Wales, UK
Allen Lumley: Thanks for the info! Yes, I am gathering information and bookmarking relevant forum posts. Lots of good stuff out there!

I will definitely follow your advice when I get to the build phase and take plenty of photos for posterity - and troubleshooting.

I do feel more confident knowing that a 7" RMH would be a middle of the road system. I don't quite feel like I am ready to start crazy amazing experiments.

I wasn't able to find what the burn tunnel's cross sectional area (CSA) would be for a 7" RMH. Any advice on how to get the numbers worked out? I'm seeing 30 sq. inches for a 6" system and 49 sq. inches for a 8" system. The ratios between the exhaust flue and the burn tunnel CSA is 5 for a 6" RMH and 6.125 for a 8" for a RMH. From this I am guesstimating a 7" system could have a ratio of 5.5625 ((5+6.125)/2). That would give me a CSA of 39 sq. inches. How does that sound?

allen lumley

Joined: Mar 16, 2012
Posts: 2768
Location: Northern New York Zone4-5 the OUTER 'RONDACs percip 36''
Dominic L. : I went to trade school so I wouldn't have to muck about with numbers ! We want pi (3.14) -X- 1/2 diameter (7/2=3.5)squared 12.2 X 3.14 = 38.3,
- Build it so the smallest dimension should be in your burn tunnel and you should be fine ! Think like fire, flow like a gas ! G'luck keep in touch.

A late thought, go to Permies sister site, - richsoil.com and click on rocket stoves - all ether professionally built or under the direction of a professional
P.S. Erica no longer lets Ernie cut his own Hair A. L.

For the good of the Craft ! Be safe, keep warm ! As always comment / questions are solicited and are Welcome ! Big AL !
I agree. Here's the link: http://permies.com/battery
subject: Sizing RMH for 7" exhaust
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