Dan Pagan

I'm not sure if the oven if the best test, as my hunch is that highers temperature will have an effect.  This is just a hunch though, and doesn't really hold with the previous example I shared, as in that case I think heat was put into the system using a heat pump.  I don't know if it was air or ground source and at what temperatures though.   As you say, the way to be sure is to do tests. So I will, and I'll report back here once I have.

Yep, I agree about the cost savings at scale, and the physical practicalities and benefits of using sand for such a large project.  My point was more that sand can't be a 'bad' material to store heat in.  It must work to at least some degree. Stone or cob might be more efficient, but by how much.  Some of the posts and info I have seen online seem to suggest that sand is a bad idea, and that it won't really work.  Others that it is the best thing since sliced bread and that you'd be mad to use anything else.  I'm stuck in the middle struggling to reconcile these two seemingly opposing points of view :/

As mentioned I'm not sure how accurately I'll be able to measure it, but I plan to test between using sand, stones, and a sand and stone mix. On the small scale I am working on it's relatively easy to remove and replace both of those. Cob, not so much, so I'll leave that one for now

@Jon Norton
It's all just ideas at the moment, but I am thinking to have several elements. The rocket stove, the insulated heat battery, a heat exchanger and a water tank.  My idea is to heat the mass of the heat battery to several hundred degrees and then control the release of that heat energy to the heat exchanger to keep the water tank at the desired temperature. The mass is a better place to store the heat than the water as it can be more compact, and therefore easier to insulate... with the added benefit of not turning to steam and exploding.  The water tank will also be an open system as well in case something goes wrong and too much heat is transferred into the water tank.

@Stephen Greenwood
They are excellent points, but I am reading so much conflicting information when it comes to sand as a heat store. I understand your points, but in your example I think heat rising, reflecting and radiating away might have more to do with the depth at which the heat transfers. The one thing I can shake though is why the worlds first commercial heat battery is being made using sand.
https://polarnightenergy.fi/news/2022/7/5/the-first-commercial-sand-based-thermal-energy-storage-in-the-world-is-in-operation-bbc-news-visited-polar-night-energy
You can even see some of the piles of spare sand in the article, and it's just regular sand.  I remember reading somewhere that the type of sand, even really fine kiln sand didn't effect performance much. I can't believe that they haven't done their research for the material to use as their heat mass.  I'm sure costs can be a factor, and maybe a mass of solid rock would be more efficient, but impractical, but I think there is more to this, perhaps to do with the higher temperatures involved.  Since yours and other comments about sand on here, I have been planning to try using a mix of sand and stones to see how that works. At the moment, I am still building the rocket stove section, so I have a long way to go before I can test heat battery materials.  

Thanks again for your comments and sorry for the tardy reply. Life got in the way again.

You're right it is, but it's also really pricey here, stove pipes especially. That said, I'm happy to pay if I can find the right sizes for the insulated stuff to go through any roofs.

Saving aside, I am keen to use second hand if I can.  No chimney pipes have come up but as luck would have it  I just found some 200mm smooth ducting locally. It's old and is a decent thickness. I snagged ten 50cm sections for €5 each. Happy days.

Hi Thomas, Thanks for the information. I was wondering about using brick channels, but thought that would be prohibitively expensive with refractory bricks, although I've not done the math. So it's OK to just use 'standard' smooth clay bricks in these channels.  That was on my list of things to ask about.

Related to that I was also going to ask about using an existing chimney.   I am thinking about building a second RSH as a heat battery for heating water and I have a currently unused brick chimney. This house was built in the 70's before the oil crisis and had been heated using heating oil. That boiler has been removed but the chimney is still there are looks to be in good condition. It also looks to have been built using refractory bricks, but I need to check that. I also need to check what the exhaust temperatures would have been for an oil heating system.  

Increasing the size of the flue could be an option, however that creates another problem on getting large enough insulated pipe to exit through the roof.  I understand that the ideally the pipe size should be consistent, and that would be tricky to do if the pipe in a bench was already large to begin with. At least if I used brick channels I have more control over the dimensions. I imagine getting the channels to gradually slope upwards could be tricky, but I do like the idea of using bricks. They are readily available.

Yes, I am planning to start with a J tube design. I have started reading about batch boxes though, but only briefly. That is one of the two areas that I really need to read more about at the moment. The other being bell designs, and their pros and cons vs a bench design.  So much to learn

Thanks again.

Yes, I understand that we are always creating a heat sink. But all the designs I have seen so far gradually release that heat into a space. The larger the mass, the longer it takes to heat but also the longer it releases heat into the space.   My point was that I am trying to minimize this, as I am not wanting any of the heat from the mass to go into the space, but to be available when I need it.  A heat battery if you will, not a new term I know.  This is still built around a RMH and a heat sink but the difference is that the sink is designed to be insulated rather than to radiate heat into a space.

So I think the type of heat sink does matter, as you want to minimize external surface area. So I think a bell design naturally suits this better. That said if you want a big mass, then it has to be above the RMH, which I guess can be quite an engineering challenge.  The bench, although more area to insulate has the advantage of being easier to construct on the ground.  There is also the issue of where you capture your heat from the RMH, directly from above the main insulated riser, or from the longer chimney like the one is a bench, or both.  Then it also needs to be considered if any areas being insulated will effect the draw or the efficiency of the burn.

This is all new to me and I don't have the answers yet.  I am still working on a design for what I want to do.  As this is such active community with so many interesting idea already tried and shared, I am assuming that this has probably been done already, or at least similar things. So I am trying to find as many as possible and learn as much as I can before I start building.

Hi Jon, Great post. I was about to ask the same question when I found your post. I am looking to do something very similar. The sand topic is really interesting. I live in Finland and have been keenly following the sand battery news.  One often over looked benefit of the sand, it that it can be very easily emptied and reused if the design changes.  Cob can also be reused, but not as easily.  The point made above about cost on a smaller scale is a good one. Maybe the better mass for a heat battery on the home scale is a combination of rocks and pebbles with the gaps filled with sand.  If the sand trapping air causes an insulating effect maybe finer sand is a better option on a smaller scale.

Heat storage medium aside, the most unique thing about using a RMH in this way is that we want to eliminate as much heat loss as possible. There is no benefit to heating the space, we want to insulate the RMH and store as much heat as possible.

I am currently leaning towards a bell design, as a bench is just more surface area to lose heat.  How to pack as much mass around that bell, how to insulate that mass and then how to safely exchange the heat in that mass to water as still very much work in progress

Good luck and keep posting your finding on the forum please as it sounds like we are working towards a similar goal.

I was talking about a couple of links in those 9 year old forum posts, which is pretty understandable.  

There does seem to be one broken link on the makers list though. The Walker Stove link is to an index page that is no longer there. It should link to : https://walkerstoves.com/

Thanks. I will check those links out. Contacting other builders in Europe makes a lot of sense.  If I find any product links to suitable ducting I'll post them back here, in case others are looking for similar, as a few of those links are dead now.  +1

Hello all, I am new here and looking to build my first RMH. I am starting to gather materials and draw up plans. So far so good, and reading this forum had provided me with some great tips and advice already.  

My main problem so far is sourcing the ducting.  I notice that most of the American builds have lovely smooth duct pipes. But all I have found so far in Europe have been the twisted type of duct pipes, and usually too small (125mm is common, ~5"). I remember reading a post on here somewhere where somebody said not to use those, because they slow air flow.

Is that really the case, are they really a bad idea?
Has anyone used them? How did you get on?
If not, and you're in Europe what did you use instead.

Thanks
Dan