RMH integration into new construction with decoupled heat battery

Hi everyone.

I'm looking for a half dozen acres or so acres for a small scale tree farm.  I plan to have a house built on this property, and in my spare time I started thinking about an efficient way to turn the wood on the land (sustainably), and the excess of what I bring in, into heat energy.  I wanted to go beyond putting inserts with waterjackets into the fireplaces and a wood/pellet boiler in the basement, and I started researching heat batteries to store the heat, and distribute it around the house, and be controlled by some software I plan to write.

I'm still in the very preliminary phases, but within an hour I discovered RMHs, and as a result I've needed to recalibrate my thinking.  For a greenhouse and/or the garage and/or perhaps a large open basement (and  maybe not attic space), these seem like mostly a no brainer over traditional stoves.

For the living room, I'm not so sure about the wife approval factor.  She'll be concerned about the aesthetics and novelty to her is not a bonus.  To be fair, she wouldn't want a regular stove in the living room either, and, knowing her as I do, I'm not gonna bother broaching the subject.  Also, I don't see a good solution for quickly cooling the thing if it gets uncomfortably hot as of now (I guess the simplest solution would be to throw an insulated blanket on the mass and run some outside air through the system after putting out the fire).  I could write several posts just about all the things I know I don't know at this point, but I wanted to focus on just how to integrate the thing into the home, and store the heat to be extracted by a separate liquid loop as needed.

For the first part of that, I think I have a good solution, though I'm not sure about the efficiency implications.  I can draw this out if it helps, although I believe it's simple enough, and I'll probably end up drawing something suggested by you guys instead.  The basic concept is to bore out a space either in the back of your existing chimney's firebox for batch fed or at the bottom for vertically fed.  I like this solution because you could still make a regular fire, if you're into that sort of thing.  In either case the exhaust will have to drawn out of the back of the house into the little chimney will feed into the second combustion chamber, and then back into the existing chimney, up and out.  Air would have to be drawn in to the first combustion chamber from the outside or from the house.

Before any reader violently facepalms his or herself at this point, let me say now that the point is not to heat the outside.  Instead, this brings me to the second part of that question:  after the chimney is sufficiently hot, I'm wondering if it would be possible to have the flue gasses travel a larger distance than is typically seen, perhaps even downward for a stretch, to charge a large heat battery that may be located underneath the basement.  It could possibly something like a 3 M x 3 M sand and stone heatsink.  Would the relatively large distance and the brief downward trajectory traveled by the flue gasses greatly reduce efficiency, if the pipe is well insulated?  I assume it will make it harder to get the exhaust drawn in the correct direction, but a small fan should be able to assist in getting it started.

If anyone can address that, I'd appreciate it, and I'd also appreciate any unrelated thoughts, criticisms, suggestions, etc.  For instance, I'm thinking about an attached shed of some sort, or maybe even a large masonry outdoor grill with a space to the wall with the chimney, wherein I can hide the guts of the stove and perhaps make it more efficient by not exposing it to the outside.  My concern about hiding it behind an outdoor grill is only that if both the grill and stove are used at the same time, how hot will it get inside that space?  Maybe that's actually a good thing, since I'd draw the air in from there pre-heated.  Maybe there's a more creative solution.

Finally, here are a couple of factors other than the integration I'm currently considering as justification, though I could be wrong.  First, the house is probably going to be a bit larger than average, but it probably won't be large enough to have one of those Great Rooms or a large foyer area that the rest of the house wraps around.  If this changes, then perhaps it might make sense to have the option to also heat a stone in the vicinity of a firebox (or perhaps this can be a second routing option for a regular living room setup).  That said, it may be large enough that I could foresee the living room being uncomfortably warm, while the bedrooms on the 2nd (3rd?) floor(s) are cold.  Rather than adding a couple of extra degrees to the area of the stove, it may be more efficient overall to move that heat elsewhere.  Also, a large general purpose stone heatsink can be charged in the summer months with excess electricity generated from solar, and probably a few other methods that I have not considered yet.  I like the idea of having multiple inputs and outputs, and would be concerned about building all of this directly into a cob structure in my living room.

Thanks for reading.    

Hi Brian; Welcome to Permies!
Well I would say you have caught the RMH bug... there's no saving you now!
As far as your living room , aesthetics and your wonderful wife!    Look around here at permies.  There are beautiful brick bell batch box rmh's that look outstanding in a living room AND will keep you toasty warm with hardly any wood at all. Rarely would one get SO HOT you wanted to cool it off.  
Do you own a copy of the Rocket Mass Builder's Guide ?  Highly recommended for all new rocket scientists. In it you will learn all the proper terminology and safe building practices. Readily  available  on Amazon.
Keep posting with your plans and progress and any pictures , Our friendly crew of volunteer's  are here to help.

Hi, and thanks for the response.

I definitely plan on getting the book.  The only reason I haven't yet is because I'm currently living in a city, and don't have any appropriate space to tinker with anything.  That would be frustrating.

I'll show wifey some pictures, but I already know what she is going to say.

Also, many of the videos I've seen with RMHs show them installed in open style homes in the 1800 ft² range (about 170 m², for those who just insist on using units of measure that make sense).

As I mentioned above, in a more compartmentalized house that may be double that size or more, is it really more efficient to have a source of radiant heat, even if it is plentiful and in a central location, if you still need a separate boiler for a room that is perhaps not locared near the radiator?  It seems more effective to have the living room be a couple of degrees cooler, and that bedroom on the other side of the house be a bit warmer, in order to not use public gas or oil.

I haven't given much thought to the layout of the house, but it could be that a radiant heat source in the LR does make sense.  I'm thinking of a layout where other room wrap around a central room with a high ceiling (obviously not great for efficiency). Even still, what about an option to reroute the heat when we are not in the LR?

That said, I'm not sure how feasible a decoupled heat battery is, and I presume there are many challenges that I am not able to see yet.

Hi Brian;   I realize this is all in the planning stages and your just kicking around idea's. Also that your wife has strong opinions about her soon to be new home.  
Your worry's about getting to hot.  At times any wood stove (rmh or standard) can try to run you out of the house.  Provided it is still cold out side this is quickly cured by opening windows/doors  …  nothing better than sitting next to a hot wood burner with your window wide open letting 20 degree fresh air into your 80 degree home.

Personally I prefer a cooler bedroom for sleeping. Some folks who have large homes , who like warm bedrooms add a second small brick bell to heat the sleeping area's.
Having a heat sink in the basement is not a bad idea but trying to plumb a rmh thru might be. Asking one to travel extra distance even when hot (particulally downward) is very problematical, and not recomended.
Using solar to generate any significant electric heat would require a very large solar aray... Though solar panels are rather low cost theses days.(compaired to 30 years ago)

You haven't mentioned (or i missed it) if your hoping to be off grid or on ?

The problem with an outside wood burner is feeding it.  Most folks will not want to troop outside to add wood to the stove.

Location?   How cold are you thinking? Backup heat ? propane ?  Are you thinking of wind power ? Hydro power ?  I know you don't even have your land yet but ...

thomas rubino wrote:Hi Brian;   I realize this is all in the planning stages and your just kicking around idea's. Also that your wife has strong opinions about her soon to be new home.  
Your worry's about getting to hot.  At times any wood stove (rmh or standard) can try to run you out of the house.  Provided it is still cold out side this is quickly cured by opening windows/doors  …  nothing better than sitting next to a hot wood burner with your window wide open letting 20 degree fresh air into your 80 degree home.

Personally I prefer a cooler bedroom for sleeping. Some folks who have large homes , who like warm bedrooms add a second small brick bell to heat the sleeping area's.
Having a heat sink in the basement is not a bad idea but trying to plumb a rmh thru might be. Asking one to travel extra distance even when hot (particulally downward) is very problematical, and not recomended.
Using solar to generate any significant electric heat would require a very large solar aray... Though solar panels are rather low cost theses days.(compaired to 30 years ago)

All good points.  The ideas I've been kicking around have changed somewhat.  

With respect to integrating an RMH into a living room fireplace:  I showed my wife some RMH videos, and got a better reaction than I was expecting.  That said, I was expecting an eye-roll at best.  I think a cob bench in the LR might still be a bridge too far (the garage or basement is fine though!), but perhaps something can be done with the masonry in front and around tbe fireplace (e.g. the hearth, mantle, and/or buildout).  The problems with this configuration that I already see include:  1)  a vertical surface is not as good as horizontal, and 2) the bell is still outside, and given that we are no longer trying to move the heat to a battery in the basement, some care needs to be taken to prevent it from entirely going outside.  Thinking outside the box:  perhaps the bell can be built into some sort of decorative turret in a fashion that makes it still serviceable.  The turret would have to be close to flush with the bell, so a layer of insulation can be placed between the bell and the brick.  It might be pretty cool if this insulation can be removed as well, if the space around it is something like an outdoor patio that could use wasteful radiant heat, similar in concept to those outdoor gas stoves that bars sometimes have in cold areas  Alternatively, perhaps something functional for cooking outside can be incorporated on that half, with it's own firebox, so that in it's entirety is an indoor/outdoor RMH fireplace and grill.  Just spitballing...

With respect to flue gas and draft, and decoupling the heat battery:  turning a fireplace into an RMH based central heating unit was probably always going to be problematic.  However, if you had a separate RMH in the basement, and your heat battery were close by beneath it, perhaps it could be made to work safely.  I'm thinking about some sort of fan/venturi setup to ensure gasses go the right way, if needed.

This brings me to your next question:

thomas rubino wrote:
You haven't mentioned (or i missed it) if your hoping to be off grid or on ?

I'm going to be on the grid, but I'd like to be able to at least supplement with on-site generation.  I'll likely install small scale panels or a bit of solar roof, mostly for the associated infrastructure, like the inverter and battery.  I'd like to be able to get some credits from the power co, and store some excess.  Why would I have extra power?  This brings us back to the heat battery and another thread I've started.  I don't want to expand the scope here, but the one sentence summary is that there are a couple of options available for heat engines to convert woody biomass into electricity, hot air, and hot water; and it might not be totally crazy anymore to consider this feasible on the scale of 1 - 10 KW (one "commercial lite" engine has an optional rocket style wood stove for an input, and of course an optional water jacket).

I expect that I will be able to make more hot water than I will ever be able to use in any case, and in the summer I will obviously make more heat than I will need.  There may be times that, like at night in the spring and fall, where I make more of both than I need.  That's why I'm interested in decoupling the stove from the battery.  That said, a heat battery actually starts to become fairly complicated if you consider that you may want inputs and outputs of different temperature, but that's a subject for another thread.  For this thread, I'm more concerned with the heat's journey than it's destination.

thomas rubino wrote:
The problem with an outside wood burner is feeding it.  Most folks will not want to troop outside to add wood to the stove.

I can see the novelty wearing off pretty quick.  That's one reason I like the idea of putting one in a basement or chimney.

thomas rubino wrote:
Location?   How cold are you thinking? Backup heat ? propane ?  Are you thinking of wind power ? Hydro power ?  I know you don't even have your land yet but ...

Winters get down to 0 F, and summers can get to 100 F in the area I'm looking in the NE US (-18 to 38 C, Zone 6 most likely, but could be 5 or 7), but usually not.  I'm thinking about wind and hydro, but mostly as small scale hobby projects.  If the elevation lends itself to a small tower for outdoor WiFi, I could see myself putting a small turbine on there for fun, but not to gain any large savings.  I doubt there's enough wind, and that the devices are efficient enough to make financial sense around here.  And funny you should mention hydro, I was thinking about what I would do if there were a pond or large enough creek on the land, and my son were old enough to participate as some kind of educational project.  I wouldn't want to needlessly hurt any fish or other aquatic life, though.

Thanks again for the response!

Hey Brian;
Just a quick piece of information.   A horizontal mass does not have to be cob.  Mine is all red clay brick... yes I do have cob inside the brick but you see very little of it.  Mine does have a 55 gal barrel covered in cob … but you do not have to use a barrel, can be built with brick instead.

thomas rubino wrote:Hey Brian;
Just a quick piece of information.   A horizontal mass does not have to be cob.  Mine is all red clay brick... yes I do have cob inside the brick but you see very little of it.  Mine does have a 55 gal barrel covered in cob … but you do not have to use a barrel, can be built with brick instead.

I'm tumbling ideas around in my head about this.  The hearth can be made larger.  Some decorative yet functional structure  (as shelving, for instance) might work.  A clay fountain statue of a cherub peeing into a pond can double as a humidifier (just kidding).

I'm hoping the architect may have some thoughts on the matter.

Thanks for the feedback.

Also, a vertical surface for the mass is not inferior to a horizontal surface; they would have different utility (hard to lounge on a wall), but in terms of heating, they are pretty much equivalent. A tall mass may throw more radiant heat to far corners of the room than a low bench mass.

Thanks Glenn.  That's something I read somewhere, and just kind of took it at face value, but, obviously, radiation travels downward just fine.

OTOH, I can't recall where I read it, but as you approach extremes, it makes sense.  If you have a 15 foot (4.5 m) ceiling, and ia part or all of the wall radiated heat, then the highest 1/3rd (1/3rd ), may not be as effective as if it were on the ground.

That wouldn't be a concern for me, though.

Something not yet mentioned is that Rocket Mass Heaters are basically a more cost-conscious version of a masonry heater, one that uses inexpensive, locally-sourced materials and is generally built by the home owner.  

If your wife is concerned about aesthetics, a masonry heater looks like a beautiful fireplace, although it usually quite a bit more expensive to build.

Some people have also built their mass as a floor, so the mass itself could be totally hidden. You can also replace the barrel with something else, either something that looks more like a generic wood stove, or something more decorative that looks more like an art sculpture, so long as it can house the burn chamber and heat riser.