Help to design my new RMH please

Hi Folks,

I'm working to design my first RMH for my 2 story stone house (2 floors, ~100 sqm) in Ireland.

I don't have a definite design yet, I'm still going through books, see what's available here in Ireland/UK, etc.

1. Based on my house size, I would go for an 8'' CSA
2. I have no liner in my current exit chimney, would a flexible flue liner be good enough? I know the temp is not supposed to be high if you have a decent mass heater size. I still don't have the exact size, something like ~5m x 1m (length of the wall in a L shape). I'll put a diagram soon.
I was thinking something like this ebay flue liner even if it more expensive than I thought and I think the stainless 904 is overkill, 316 is good enough. It's not available on my hardware shop, nothing in Ireland ... !
3. For the mass heater, what do you use? Is it something like this? Example mass heater pipe
4. I still need to read more but one difficult point for newbie like me is to find what material to get, like the one to insulate the raise heater (I've already bought my drum, my firebrick (124 total, prob too many but I'll prob too another rocket stove later on). For the mass heater filling, I was planning to use concrete and stone, I got many from my various DIY job + Cob for the inside. Would normal Cob be safe for the inside? Or do you need fire clay for the mix? Things like that is very tricky especially when your hardware shops have no clue about this stuff.
5. As my wall is in stone (it is not bare stone, there is a layer of something, prob cement), it should also be warm up if I have the mass heater close to it, touching it?

Thanks for your help, I'll post more details when I'm progressing my design.

All the best,
Julien

Hi Julien;  
I'll try to answer some of your questions.
You want an 8" Rocket, are you thinking of a J tube or a batch box?
The eBay flue liner is much more than you need unless you must meet local installation regulations to use a wood burner.
The pipes in your mass can be a very light gauge, You will cover them in cob. No need for stainless or even standard black stove pipe.
I used 5' long sections of HVAC pipe in my mass, which was very cheap.
I used one piece of black pipe for durability where it comes out from the mass and switched back to the HVAC pipe to go out thru the roof.
To insulate your riser it depends on what you build from.  
A firebrick riser should be wrapped with an insulating blanket (rock wool - mineral wool )
A home-built fireclay perlite riser is insulated all by itself.
A five-minute riser made with Morgan super wool is one of the best, needing no extra insulation.
An insulated firebrick riser is the absolute best.
Your mass is best created from large stones, using local clay/ sand mortar to fill any air gaps between rocks. Rubble works, but many tiny air gaps do not hold heat.
Do not let your mass touch the stone foundation. It will steal your heat. You want a 4" gap or you want insulation between them.

Fireclay/sand mortar is only necessary around the core.
Local clay is fine for sealing the mass.

Thanks a mill for your answers Thomas, very useful to me!

It will be a J tube in fire bricks so I should go with rock wool - mineral wool.

HVAC pipe is a ventilation type pipe, right? No regulation here.

I found this for the exit chimney flue, I need a flexible one as there is at least one bend and I don't know what else, it can support -30/+150 degree, I think it should the job  flexi item - What do you think?

I found those pipes for the mass heaters, metal pipe - What do you think? They have T pipes and 90 all in 200mm, it is not super cheap but so far I couldn't find better.

Cheers,
Julien

Hi Julian;
I have a question, what is wrong with your existing chimney?
Why does it need a liner?

Yes, HVAC is ventilation pipe.

The flex pipe you linked to is not acceptable.  
Ideally, RMH exhaust temperatures are around 150F but can/could run over 300F at times.

The round metal duct in your second link would work fine in a mass.
Is common single-wall black stove pipe available?

Are you aware of stratification bells?
With one, you would not need any piping or a solid mass at all.

Hi Thomas,

Thanks again!

There is no liner on the existing chimney, maybe I've got the wrong terminology, I currently have only a bare large rectangular ~11'' x 27'', pic below showing it.

The flex pipe I linked is in Celsius (Ireland ) - So it's 280F. So it should be good, I'll try to find one better if possible.
 
I'll search for a "common single-wall black stove pipe", I don't know. To give you some context, I'm French living in Ireland and many things are not available here, a lot is imported from the UK or used to be with this Brexit.

I never heard of "stratification bells" - I tried to search on the forum, no luck.
But not needing a mass and pipe would be great, my wife is not too happy about the space required by the RMH mainly because the current setup of the room, we will have to make a good few changes to fit it!

I've attached a pic of the room I'm planning to install the RMH, on the left there is the door and on the right there is some storage. Behind the old stove I have an oil aga.
I was considering knocking the storage space (it was an old door replaced by storage, on the bottom there is some pipes to I'll prob cut only half of it) for air circulation but again my wife doesn't like too much the idea!

Cheers,
Julien

Hi Julian;
Let's talk about your existing chimney.
11x 27" wow that is large!  Is it inside your home or on the outside?
Is it intact?  Safe to vent into?

Now let me explain "bells" to you.
A traditional RMH like you are thinking to build, uses horizontal pipe sitting on insulated cob, surrounded by more cob and large rocks with no air gaps.
Often this mass is finished in cob shaped as a bed/bench.
A different approach is to "contain" your horizontal pipes and cob /rock mass within clay brick walls.
This can also be a bed /bench shape.
This adds another layer of heat-holding material and eliminates cob dust/repairs. In my opinion, it looks more traditional rather than hippyish.

A stratification bell is a large airtight container with a chimney exit located at the bottom.
It can be a metal tank or a clay brick box.  It has no specific shape. It can be tall and skinny or short and long like a bed/bench.  It can fit in whatever spot you like. There is specific size (ISA) specifications for the internal surface area of the bell.
Your Rocket core sits with the riser inside this "bell"  Hot air coming from the riser goes to the top of the "empty box" as hotter air arrives it heats your "bell" and the cooler air falls down towards the outlet on the floor level.

Here are threads about my 8" J tube into a brick bell and then my conversion from a J tube over to a 7" batch box.

https://permies.com/t/94980/Brick-Bell-Shop-Heater

https://permies.com/t/138779/Breaking-News-Shop-Dragon-admitted

Hi Thomas,

Yes this is very large, it is inside the house (in the center of the house ish), it looks safe and it was used before with a wood stove, the wood stove was connected to it with only a black pipe and then going like this, very bad job ... So if I clean it, it would be safe to vent especially temperature would be much lower.

I don't fully understand the different of the stratification bell and the bell used for a RMH (when using an airtight metal drum).
I have attached the diagram. Isn't it the same?

Your solution looks great but I think it is out of reach for me, it is my first one I'm building and I'm so DIY yet ... and I've got the book "the rocket mass heater builder's guide" to help.

Cheers,
Julien

Also I know RMH can heat water (and it is dangerous). As I have already in place a central heating working currently with Aga + oil.
I would ideally like an RMH that could also heat up my central heating. I don't know how good the RMH will heat up the whole house this way.

Julien Vailles wrote:
I don't fully understand the different of the stratification bell and the bell used for a RMH (when using an airtight metal drum).
I have attached the diagram. Isn't it the same?
Julien

The metal barrel is not a true bell and therefore can be somewhat misleading. For that reason, here on Permies, a bell (borrowed from masonry heater lingo) has been called a stratification chamber. A larger space where the incoming hot gasses have more time to separate out through density differences.
For more reading on this subject go to:
Bell theory
Stratification chamber

Thanks Gerry! Now I get it, this stratification chamber is super cool and actually fix a few problem! Matt was mentioning a smaller footprint design, I guess it would be to do a vertical stratification bell? Also what about the CSA? is the stratification bell's outlet the same than the feed? Exit chimney, etc.?

As I mentioned a few time I'd like to use it to heat my central heating water.

Main issue with the fire is the burst of temperature. If I was having the coil pipe on the top of the stratification bell or lower if it is too hot, I could have a constant temperature with a max temperature (ideally if I could more the coil pipe, I could set the temp) for it without having to worry about boiling it and going boom!

I've seen this design too before, it's great (because safe) but you still boil water and the

Does anybody here have any exp with this?

Cheers,
Julien

I had a drop tube (pipe penetrating the top and extending to about 3” from floor height) acting as my outlet from my stratification chamber since there was no room to come out from the side.
On the end of the drop tube I made a funnel to help direct the cooled flue gasses into the exit pipe. The funnel allowed me to be closer to the floor to maximize the most heat extraction.

For a side exit, a funnelled end would help just as well to direct the gasses into the opening easier. The gasses are moving pretty slow at this point but anything you can do to help alleviate drag is helpful.

If you will be using a metal barrel in your RMH project, this excerpt form the Rocket Oven video is worth watching: Repurposing a Metal Barrel