New here, first post, I have been researching all different designs, and I came up with my own.

Miles,

I understand that you would like to use the existing ducts, but the principle of heating with masonry heaters is radiation of the mass heated by hot exhaust and by radiation of the burning/burnt fuel. Radiation constitutes 50-70% energy output of wood burning. For this reason heating rooms in which with the masonry heater is not located is either difficult or just impossible if the walls are masonry. That's why in part of Europe where masonry heaters were (or still are) popular, each room had a separate unit. It is partially possible if using water as the medium. That's why you can not find anything that would meet your idea. Like I said before - in theory it would be possible, but you would have to develop such system.

Cristobal Cristo wrote:Miles,

I understand that you would like to use the existing ducts, but the principle of heating with masonry heaters is radiation of the mass heated by hot exhaust and by radiation of the burning/burnt fuel. Radiation constitutes 50-70% energy output of wood burning. For this reason heating rooms in which with the masonry heater is not located is either difficult or just impossible if the walls are masonry. That's why in part of Europe where masonry heaters were (or still are) popular, each room had a separate unit. It is partially possible if using water as the medium. That's why you can not find anything that would meet your idea. Like I said before - in theory it would be possible, but you would have to develop such system.

Hmm, I may end up going with something that is at least sound in design for the heater, like a bell for example, and work with that to start with.

As I have said in another post, this place was built on tinkering, and it seems like no-one has done this exact thing before, so testing is very limited. I might just build the damn thing so I can test how well it actually works! You never know, it might surprise us all and become a new design, or it might fail miserably and be taught as a reason not to do this in the end. As long as it doesn't explode and I get some kind of use out of it, I would like to try something a little new.

Not too out of the box, fitting as many of the principles already learned into it as I can, but unless someone has done the testing to say this is not a good idea, I am willing to be the one to do that testing and find out.

I am now thinking a double masonary bell as the mass could be a better idea, maybe even run the air direct through the gap between the bells to maximise contact with the hot bricks, I do have a physics background, but doing calculations is never the same as seeing something in action.

I may take what I have and go with it, but I'd rather work on the design as much as possible before I go ahead, unless someone here gives me a good reason to stop of course.

So far though, just being something new is not reason enough for me, so I'll keep drawing for now.

Cheers.

Just so I'm covering all bases, my own research told me that this would be the best way to go with the existing constraints I have:

1. I have a ready supply of small sticks every year that I can dry out, but getting larger wood will cost me money every year and I might as well just use the reverse cycle and save the time and effort of lugging it all over the place, so I would like something that burns the small stuff as efficiently as possible.

2. I have existing ducting under the floor and it's on stumps and a fair gradient (front of the house is walk under, back is crawl) so putting something like this inside is not going to work for me, I would have to stump down the whole mass etc, and I'm not willing to go that far with my foundations! But I would like to use the existing ducting

3. I don't have infinite backyard, so I have cordoned off a 2mx1m area to build in, and I would like to keep the height in the 1.5m area so as to not give the neighbours an eyesore out of their kitchen window too. The area I have is close to the house to allow for the ducting to be as short as possible. I have not done any actual building yet though, only digging so far.

If there is a way that anyone can think of to convert my small sticks into hot air to pump around the ducts in my house that is not a rocket mass heater with air ducting through it, I am absolutely open to hearing about it. This is just the best idea I could come up with after a lot of looking into it.

Cheers, I'll happily end the thread right now if there is another way to do this and not cost me more than a couple of grand or so in aussie dollerydoos.

My intuitive idea that sparked all of this was thinking about how much warmer the house got when I was a kid and we had one of those huge wood heaters in a very large room, with the fan off, the area around the fire got hot, but the other end of the house stayed cool, but turn that fan on and the whole house would be toasty in a few minutes. There has to be something to running air over a hot fire and heating a space with it, doesn't there?

After a fresh google search, these guys are doing essentially what I want but with an existing fireplace, they just place the hot box against the hot chimney and duct the rest of the house up with a fan running the whole thing from what I can tell:
https://chazellesfireplaces.com.au/fireplaces/hot-air-ducting-system/
I want that, but contained in a rocket mass heater if that is possible, but once again, if there is a reason no-one does this, I am absolutely all ears, if it's just that no-one has tried it, I am willing to try.

Sorry for the edits, but the ideas keep coming. Since hot gasses rise, I could theoretically do away with the ducting and seal up the whole gap under the house to use as one giant strat bell, with the top being the floor of the house being constantly heated and the gas escaping from the front where the hill is lower naturally.

It would involve making the under house gap airtight though, which may cost more than the heater build in the end... but ideas are still coming.

Yes, I think you are right about building something now.  Let us know when you are producing heat as you desire.  Perhaps that time gap could let you firmly grasp the next step, i.e., transfer of heat to the house.  If you have time, look at two tiny designs: the micro RMH at the Love Shack, and Cyclone by Kirt Mobert. If you could live with that idea it might be easier and foolproof, and give back your back yard space.

One last plea Miles go with a proven design, then use water circulation into your house or under the floor. Water's specific gravity gives it an edge over ducting the hot air.  As a man dealing with an old house with 12 inch crawl space, I would suggest avoiding the idea about using your crawlspace as a strat bell.  Like comparing apples to oranges, the two are not equals i think, and this idea might have you breathing some unhealthy air

Apologies, I meant Kirk, Dutch for place of worship, not Kirt

Miles,

If tinkering then here you go.
I would do the following:

1. Build an 8" J-tube from insulating materials, the entry opening from hard bricks.
2. Build a heat exchanger, let's say eight 2" pipes in parallel joining into a 6" pipe. I would insulate the 6" pipe VERY WELL on the portion between the bell and the building. The total cross section of the small pipes would match the cross section of the large pipe, but later on a different ratio could be tried. Also a different configuration of pipes could be used, but that would be a start of the journey.
3. Position the exchanger at the top of the bell above the firebox exit.
4. Build a small bell from insulating materials. It would be like a ceramic kiln, so most of the heat would be staying inside and heating everything to quite a high temperature, but the blowing air would be cooling it and extracting the heat.

I would experiment with blower speed so the temperature of the exhaust leaving the bell would be low enough to maintain efficiency. It could require changing the design of the exchanger - for example using higher number of small pipes, positioned horizontally and also vertically, so the cross section of the pipes would look like this when looking towards the front of the bell:

o o o o o
o o o o o
o o o o o
o o o o o

Cristobal Cristo wrote:Miles,

If tinkering then here you go.
I would do the following:

1. Build an 8" J-tube from insulating materials, the entry opening from hard bricks.
2. Build a heat exchanger, let's say eight 2" pipes in parallel joining into a 6" pipe. I would insulate the 6" pipe VERY WELL on the portion between the bell and the building. The total cross section of the small pipes would match the cross section of the large pipe, but later on a different ratio could be tried. Also a different configuration of pipes could be used, but that would be a start of the journey.
3. Position the exchanger at the top of the bell above the firebox exit.
4. Build a small bell from insulating materials. It would be like a ceramic kiln, so most of the heat would be staying inside and heating everything to quite a high temperature, but the blowing air would be cooling it and extracting the heat.

I would experiment with blower speed so the temperature of the exhaust leaving the bell would be low enough to maintain efficiency. It could require changing the design of the exchanger - for example using higher number of small pipes, positioned horizontally and also vertically, so the cross section of the pipes would look like this when looking towards the front of the bell:

o o o o o
o o o o o
o o o o o
o o o o o

Actually, the more I have been looking into it, I have been thinking of something a little different to maximize the air mixing and surface area of the air to medium contact.

Thinking of getting a couple of steel plates, having one close to the bell, but probably still behind a layer of fire brick so it doesn't pit and rust over time, or at least to delay it somewhat, anyway that one lays flat over the bell and the other one sits maybe 2cm (or about an inch) above it, with some bolts to space it out, and then keep building up on top of that, seal the whole thing inside a nice thick masonry top to the whole unit, and run the air ducting through the strat chamber, up through the plate in one corner, maybe have some baffles like a radiator but not all the way floor to ceiling, so the air always has one basic path to take along the flat plate, separate the thing in the middle leaving a gap over the bell, so the gas has to rush past the hottest area on the way through the plate, and then drain it back out and into the house through the ducting.

I was thinking on the one pipe to many idea, but I did manage to find some short threads on here of others who did have my idea, but either got talked out of it or gave up, because the threads just stop after a question or two.

One of those threads mentioned that splitting the air up will guarantee that only some of those tubes will get the majority of the airflow, while the rest get almost nothing, so getting the air into a flat plane with some fins for extra area and push it around a "U" shape made of metal plate for good heat exchange from the masonry, and back out again through the strat chamber to keep the pipe warm when it's cold outside. That can then be dug underground and insulated over to the house, and hooked up to the existing system I have under the floor after I have inspected it all and probably repaired some of the duct that is likely in some form of disrepair.

This, to me seems like the best approach, after researching heat exchanges used in hvac systems, double skinned pipes etc appear to be the order of the day, keeping the surface area as big as possible and making the air turbulent inside the plate seems like the way to go to me.

Can you think of a reason I should still go with the split pipes design though? If you think that might be better than the plate with fins idea, please let me know and I'll listen.

Cheers.

Rico Loma wrote:Yes, I think you are right about building something now.  Let us know when you are producing heat as you desire.  Perhaps that time gap could let you firmly grasp the next step, i.e., transfer of heat to the house.  If you have time, look at two tiny designs: the micro RMH at the Love Shack, and Cyclone by Kirt Mobert. If you could live with that idea it might be easier and foolproof, and give back your back yard space.

One last plea Miles go with a proven design, then use water circulation into your house or under the floor. Water's specific gravity gives it an edge over ducting the hot air.  As a man dealing with an old house with 12 inch crawl space, I would suggest avoiding the idea about using your crawlspace as a strat bell.  Like comparing apples to oranges, the two are not equals i think, and this idea might have you breathing some unhealthy air

Thanks for the advice, I think I will be going with a proven design for the rocket itself for sure, Peter of the P channel seems to have the best rocket ever made, so I will likely go with his design based on some of the cast ones people have made, but fashioned out of firebrick and clay.

As for the heat transfer, I have an idea to max out the heat transfer into air, my only worry with using water is that I then have to get that back into the house, into radiators and fanned around the rooms again, pretty much negating the ducting under the floor which I want to use if possible.

I am still planning though, water may still be on the cards depending on how this all goes, I'll see if I can draw what is in my head with some kind of accuracy to demonstrate it properly.

I may just start building then, and post pics, then test it and see how well I went when it's all done. Even if it doesn't work well as a house heater, I plan to have a pizza oven in there, so it shouldn't be a total loss

Thanks for the reply

If you must go with an outside combustion core and heat exchanger, I would second the water idea over air to get the heat into the house.

Splitting air ducting for more surface area sounds sensible, but you would have to greatly increase the total cross section of ducting to get good results. Air flowing in ducts generates friction from the duct walls giving a relatively stagnant zone next to the surface. This effectively decreases the size of the duct by an inch or so, meaning that you need much larger small ducts to allow sufficient flow. Going from 6" to several 2" ducts would probably need 2 to 4 times the nominal cross section. Friction increases faster than airspeed, so the duct with the easiest flow would speed up until flow and friction balanced between the ducts, and one would not take all of the flow, but it would not likely be evenly split. Basically, you would get better results with a large flat surface on the heat exchanger than a bunch of little ducts.

Glenn Herbert wrote:If you must go with an outside combustion core and heat exchanger, I would second the water idea over air to get the heat into the house.

Splitting air ducting for more surface area sounds sensible, but you would have to greatly increase the total cross section of ducting to get good results. Air flowing in ducts generates friction from the duct walls giving a relatively stagnant zone next to the surface. This effectively decreases the size of the duct by an inch or so, meaning that you need much larger small ducts to allow sufficient flow. Going from 6" to several 2" ducts would probably need 2 to 4 times the nominal cross section. Friction increases faster than airspeed, so the duct with the easiest flow would speed up until flow and friction balanced between the ducts, and one would not take all of the flow, but it would not likely be evenly split. Basically, you would get better results with a large flat surface on the heat exchanger than a bunch of little ducts.

My only issue with water is 1. that I then have to somehow exchange that heat again to get it into my house, and 2. that the double exchange will be more wasteful than just pumping the air through the mass above the rocket.

Wondering if you have any ideas on how to exchange the water back to the air ducting in the house? And what you think of the double plate "U" shape with fins idea.

How far from the house is this pad located? The closer the better, obviously... touching the wall would be best, especially if you could take out a section of the exterior wall and have one side of the mass be the wall.

If you really want to use ducted air as the heat transfer medium, I would make the whole top of the bell out of metal as the bottom of the duct, then insulate the whole exterior of the bell as heavily as you can manage. Every bit of heat that doesn't go to the duct is wasted, aside from pizza cooking. There is no reason to make the bell any higher than the J-tube riser requires. Making it low and wide maximizes the ceiling of the bell for heat transfer purposes. I would add a piece of heavy steel above the riser to diffuse the heat blast on the bottom of the duct.

A low wide bell could have multiple stacks of bricks inside, even up to the ceiling (helping support the heat exchange surface), as long as there is still free airflow between stacks. Bricks inside the bell store heat which cannot be lost directly to the outside, increasing the duration of useful heating to the duct.

The pizza oven would work best positioned at the top of the bell cavity, as you want that very hot. A nice mild bake oven doesn't work so well for pizza.

Miles I think you are pulled in many different directions on your project.  But doing research should involve looking at the science, and at the reliable sources of info. Cristobal and Glenn are helping you with real data and real examples, and they have decades of verifiable experience between them.  Balance that against someone who is looking for clicks on social media, then disappear if you ask a question

Why do mini split heating systems have a 200-300% efficiency in some houses? They are ductless for one thing.  Old style Hvac using ducts lose an enormous amount of heat through ducts. However Pex with hot water comes through a small hole if you want it inside, or no hole if under your floor.  If you'd try water....specific gravity 1.0..... you would be heating the things in your house or even the entire floor from underneath, not trying to heat air that then tries to heat the floor.  Hydronic floor heat is perhaps the best , most comfortable method.  Thank you for your consideration

I can vouch for the comfort of hydronic floor heat. When I built my house around 30 years ago I put in hydronic heat and a Polaris through-the-wall vented water heater. It was glorious to have warm feet all winter. But that came with the cost of propane, and when I discovered rocket mass heaters 11 years ago I built one and found that comfortable heat also, if not always the warmest feet... Wood from my land costs only some exercise and a bit of chainsaw gas.

Sometimes replying on permies is tricky. One has to find a balance between recommending a tried-out solutions and supporting new ideas that seem inefficient but sometimes could work if approached differently.

The amalgam of these two would be to build a masonry heater INSIDE of the house and at the same time play with ducting. If ducting idea fails you will still have a heater that surely will heat at least part of your house.

Not many folks have experienced mass-heated homes.
It is hard to comprehend how well it works until you do.

Mass heating does take time, to start to share its heat.
At Miles's location, he might not like a mass heater indoors, as I suspect it warms up quickly outdoors once the sun rises. (having never been down under, what do I know?)

Having a highly insulated batch outdoors and utilizing fans and the existing ductwork (perhaps brick-lined) might work well for him.
He is excited to try experimental rocket science!
We can all learn from his success or his failure.
As long as we can guide him to build his core to published dimensions, then everything beyond is playing with fire.
Something all good rocket scientists love to do!

thomas rubino wrote:Not many folks have experienced mass-heated homes.
It is hard to comprehend how well it works until you do.

Mass heating does take time, to start to share its heat.
At Miles's location, he might not like a mass heater indoors, as I suspect it warms up quickly outdoors once the sun rises. (having never been down under, what do I know?)

Having a highly insulated batch outdoors and utilizing fans and the existing ductwork (perhaps brick-lined) might work well for him.
He is excited to try experimental rocket science!
We can all learn from his success or his failure.
As long as we can guide him to build his core to published dimensions, then everything beyond is playing with fire.
Something all good rocket scientists love to do!

Thank you all for the carefully thought out replies, i will just reply once rather than make 4 posts:

My pad for the heater is about 4m from the house, at the end of the decking at my back door. So the heater itself will still be useful for winter days and nights when it's not raining to make it more comfortable on the deck, and in summer I can still use it to cook a pizza, while not heating the house if i don't turn on the fans.

I would like to use water, but at the moment that is going to be cost prohibitive, I am trying to use existing fittings in the house, water and floor heating means thousands of dollars in refitting the whole house, and I just can't do that.

Also, building in the house is not ideal, as it's a wooden house on stumps, I would have to do some major foundation work, and then insulate the walls against burning from the radiated heat, I have watched enough videos of people doing the build and then scrambling to put aluminium etc between the heater and their wood wall to know that is not the best idea unless you really insulate the area the heater will be in.

And, as someone said, it does get cold here in winter, but we don't get snow, I usually only need to heat 10 or 20c max to be comfortable day and night here, and I do still have the reverse cycle for really cold or rainy days when the rocket may not work as designed etc, though of course the goal is to not have to use it in winter at all if possible, save myself about $100 a month on my power bill for about 6 months of the year, nothing to sneeze at if I can use scrap wood from my fruit trees etc!

If this doesn't work,  I will still have a pizza oven, or I can take it apart and re-design it, at least I'll have the materials on hand to try something new if this doesn't work, but as far as I can tell, most who have asked the same questions here never actually went ahead with the build, so if I'm the first, and it fails, at least I tried and we'll know exactly why this isn't a good idea, and if it works, I'm happy to leave plans here for others to follow if they want to do something similar in milder climates.

I totally get that you guys have to balance the advice with proven theory, but still try to encourage innovation, it can't be an easy line to walk, and i appreciate you all for chiming in where you have.

I am currently working on the redesign based on all of your inputs, I will post in the next few days when I have something I'm happy enough with.

Cheers.

Give it a try, I agree with T Rubino.  Not familiar with Aussie prices but in US Pex tubing can be under 50 centavos per foot. 200 feet for 90$  If you decide on heated floor with Pex it won't be thousands of dollars, have retrofitted 70 year old  house and it wasn't hard or expensive.  Good luck with it.  

Rico Loma wrote:Give it a try, I agree with T Rubino.  Not familiar with Aussie prices but in US Pex tubing can be under 50 centavos per foot. 200 feet for 90$  If you decide on heated floor with Pex it won't be thousands of dollars, have retrofitted 70 year old  house and it wasn't hard or expensive.  Good luck with it.  

i will take a look, but I think prices in the land downunder for such things are inflated beyond belief, I have a feeling that what may cost a few hundred eagle bucks will cost me thousands in dolleroos unfortunately, but I will take a look after work tonight and see if it's viable or not. Then my biggest concern would be leaks etc.

Oh side note: I bought a stick welder yesterday, haven't welded since high school many years ago, but my other friend is a qualified welder, so if I forget anything I can just ask him, and I think I remember the basics pretty well, I will have to get some scrap steel to practice fillets etc on now, but hoping to get some welding skills up on this job too if needed.

Cheers.

https://plumbingsales.com.au/pex-gas-water/water-pex-pipe.html

hmm pex piping is about $120 for 50m, but for wood floor I think I might need quite a lot of 50m lengths, maybe more than 10... starting to look like it's just not feasible with Aussie prices, at least for me.