Mass material - Help from overseas.

Hi.
I live in Yorkshire in the UK. I’m intending to build a RMH and have been looking for some advice, but RMH’s are far and few between in the UK, and experience & advice is even rarer.

I’m about to build a garden office. Along with planning to insulate it better than the house, I’m wanting to install a RMH. Because the garden office will be small (only about 3m x 4m, or 10ft x 13ft), there will be little affordable space to be taken up by a mass bench. In addition to this, I do not have access to cob. It’s harder to come by than you would expect when you are living in a uk city.
I therefore intend to run the flue underfloor.

I’ve considered a number of ways to do this and have settled on it being within the floor mass.


My first question is, what material should I use?
I’m intending mass-fill concrete (on top of insulation) with a thin screed on top and finished with a floor tile so to minimise any insulating effect.
Will concrete be a good mass?
Will it be good for absorbing the heat?
Will it release the heat slowly after the fire has died?

All my internet searches assume using cob. I can’t find any reviews on concrete.



I’m considering on branching the flue into multiple smaller diameter flue pipes under the floor so to dissipate the heat, and then merge them again to an 8inch chimney.
My second question is, Will splitting to multiple smaller flue pipes and re merging again disrupt the flow?
The feed cross sectional area will match the chimney csa, but I will struggle to get the multiple smaller pipes under the floor to match the same csa. I intend to increase the csa under the floor, but will bringing it back down again impact the flow?


Because I’m casting it all in concrete, I’ll only have one attempt at this.


I believe this will be the first RMH in my city.

Any advice will be really appreciated.

Thanks.

PS: yes, I have purchased and read the RMH builder’s guide.

Hi Chris, Welcome to the Rocket Mass Heater forum!

From my understanding, if you are to make a manifold with many pipes coming out of one, then join them back together again, what will happen is that  one pipe will end up acting as the main flow route for the exhaust gasses and the others will hardly do anything at all.

A Roman hypocaust could be attempted instead or what some people do is heat water and run it through pipes buried in the floor as another option.

Hi. Thanks.
Multiple flies are a bad idea then. Best sticking with 1 larger glue bent around the space?

I did consider the Roman hypocaust design, but didn’t go far with it due to the amount of excavation I would need to do (planning limits the height I can build) and I felt there would be a lack of heat retention due to lack of mass.

*flues

Hi Chris;
The UK has plenty of bricks readily available.
Have you considered a brick bell ?
All above the floor in any shape that fits your tight space.

Concrete is a big investment. It works with liquid piping, which has a limited high temperature.

My worry is that the differential temperature within a contiguous concrete slab will cause cracking, and as the piping degrades, exhaust leaks into the living space. Perhaps I'm over-reacting, but frankly, I think it's a big gamble.

There must be a better way. What is your subsoil made of?

thomas rubino wrote:Hi Chris;
The UK has plenty of bricks readily available.
Have you considered a brick bell ?
All above the floor in any shape that fits your tight space.

I understand the bell needs to displayed heat as quickly as possible in order to create a downdraft. A brick bell would retain the heat which would interfere with this.
Or have I misunderstood your suggestion?

Douglas Alpenstock wrote:Concrete is a big investment. It works with liquid piping, which has a limited high temperature.

My worry is that the differential temperature within a contiguous concrete slab will cause cracking, and as the piping degrades, exhaust leaks into the living space. Perhaps I'm over-reacting, but frankly, I think it's a big gamble.

There must be a better way. What is your subsoil made of?

This has been one of my concerns too. I’ve considered expansion joints, but these would interfere with the thermal mass. But then why would it be any different to any modern underfloor heating? That doesn’t break up.
Also, I understand cob would crack exactly the same and that I t is the stove pipe that is relied on as the exhaust is at a lower temp when exiting the manifold, and therefore shouldn’t degrade.

Like I say, RMH are far and few between here in UK, and so I’m working on internet research and my own theory only. A gamble.

The sub-soil is a decent top-soil to quite a depth. It’s old farm land with lots of organic matter. On top of that, the location it will be on is where previous owners mounded more topsoil when excavating for a pond.
Being a uk city location, there is very little space to locate it.

Hi Chris;
You are familiar with the design of the original piped mass.
Its barrel (Not known as a bell) indeed shed its heat allowing it to cool and fall.
The riser is placed within 3-4" of the barrel lid creating a vortex of hot gasses that then sink down to your pipes and heat your mass.

The RMH builders guide was published prior to stratification chambers (bells) becoming a more popular design.
With a stratification chamber, the riser is 12" - 36" or more from the roof.  A barrel is not needed at all unless more radiant heat is desired.
The super-heated air from your riser rushes to the roof of your brick bell. as soon as it gets there it has cooled and is replaced by the hotter air coming from the riser.
That cooler air sinks down to the floor of your (bell) stratification chamber.
At the bottom of your bell is where you place your exhaust outlet.
All that hot air is now soaking into your bricks.

Strat chambers do not have ash buildup issues like a pipe system.
Depending on the size of the chamber, a bypass arrangement can be placed near the top to allow hotter gasses to start the draft up your chimney.

In my opinion stratification chambers are easier to build.
Cob as a filler is completely done away with.  
Bells do not have to be brick, they can be a large metal tank with a mass around and inside.

Here are a few links for you to check out.
https://permies.com/t/94980/Brick-Bell-Shop-Heater
https://permies.com/t/99519/Baby-Dragon-Roars
https://permies.com/t/138779/Breaking-News-Shop-Dragon-admitted

Hi Chris, I built a 4 inch batch box using a brick bell.  I used it for three winters, but each winter I had a riser failure (first riser was clay slip/perlite, next was insulation board, the last was 5 minute riser) and in the fourth winter when an acquaintance was giving away a multi-fuel stove I allowed myself to be persuaded to dismantle the rmh for good.  I think it was my lack of expertise which caused the failures, not necessarily the rmh design itself.  

So that's my disclaimer.

I will comment that the bell was excellent at holding heat;  I could burn for about an hour or less in the afternoon (using basically sticks and twigs from my garden and the local country park, all found wood) and it would stay warm all evening.  The bell itself was single chamber (initially double but the second half didn't rev up like the first so I made the internal division into a pillar instead) and about 4'x5'x2'.  I used bricks as I mentioned, and clay/sand mortar so it was relatively simple to erect and also take apart.

Also bear in mind that for insurance purposes, rmhs are not legal!  If a neighbour makes a claim against you for anything (not necessarily fire related) you could be personally liable.

So, I’ve been researching brick bells (thanks for the tip), and they are certainly on the radar for a future project. I can certainly see the benefit.
However, I simply won’t have the space in my small office build. I think on this occasion I need to utilise an under floor mass.

Having done more research, I’m now uncertain about 2 things…
1) I still don’t know what material to use for the floor mass given cob isn’t a feasible option for me.
2) will an 8inch system be too much for such a small space (12ft x 10ft)? Would I be better downsizing to a 6inch system?

I really wish there was more interest in RMH in the UK so there would be more advise over here.

Stick with an 8",  if it gets too warm then burn it less often.
I'm not sure what your best fill would be beyond cob, I guess dirt would be my next choice rather than concrete.

thomas rubino wrote:Stick with an 8",  if it gets too warm then burn it less often.
I'm not sure what your best fill would be beyond cob, I guess dirt would be my next choice rather than concrete.

Hi Thomas.
Thanks for the reply.
I can’t help but think that the mass must be solid enough to be the floor itself otherwise I’d be putting a deck on top which would act as an insulator, which I don’t want.

Hi Chis, I live in Guernsey so almost England!
A well insulated small cabin wont need a huge stove to keep it warm, I would consider a small batch box or even a vortex stove.
Both will chuck out heat for around 1.1/2 hours on a single load and all you need is some mass around the fire.
You don't need a huge amount of mass  especially if  you are prepared to light a few loads a day.
Concrete blocks are not very good because they are full of air but vibrated concrete is as good as it gets for a solid mass.
I have a small concrete mass under my 6” J tube and it can hold heat for 8 hours.
You can try piping under floor mass but the stove will work better and start better with a small bell and a more direct exhaust chimney.