Storing heat in the floor via rocket stove.

After being exposed to many different brains, with their different ideas about earth shelters, I have come up with one of my own(I think). The scenario would be a rocket mass heater that is positioned at the lowest point of the house with the exhaust running through the floor and out verses through a bench and out. The difference is that the same heater would be storing heat in the bench(say 1 ton of thermal mass) vs. the entire floor of the house(5-50 tons of thermal mass). Can anyone tell me why this would not be completely kick butt awesome?

Hi Philip,
I cannot tell you if your idea is "completely kick butt awesome" but speaking from experience a subfloor RMH presents some hidden obstacles and a bit more planning to pull it off right. Part of using a bench is to act as the battery to store the energy heat. The other benefit to the bench is that you have a comfortable and very warm spot to lounge around on while tending the fire and for hours after the fire goes out.

We have a subfloor RMH in a greenhouse. You need to insulate the entire battery, especially the sides and bottom, or risk loosing heat to the earth outside. Moisture is another issue. You will find after searching through enough posts that there is always questions of stoves not running properly while the mass is drying out. Being subfloor always seems to have a higher moisture content.

Something else to consider. Cold air settles on the ground. The heat stored in the floor is now competing with the colder air. The warm air will cool significantly before it warms you.

These are just a few things to think about while your searching.

I did consider the moisture issue, and it would definitely need insulated, unless this was built in a PAHS type building. If I didn't miss something when reading PAHS then I believe that the umbrella over the structure drys out the area around the house and underneath to some extent, right? Though I did wonder about wicking action. In your greenhouse you don't want the soil to dry out because it grows stuff but in a house the opposite is true.

Call me a permies anarchist, but I dont think PAHS has been proven effective yet. As to the extra mass being a solution I see it being a problem. Thermal mass can sometimes be a hindrance. Like when you want to quickly turn down or turn up the heat. The bigger problem is where all that heat is going and how long can you keep it around between burns.

Brian,

I would by no means consider myself a PAHS veteran but I do like the idea of it very much. I would love to hear your arguments against it though. I have a few concerns as well.

Reluctant to change the subject of your thread Philip because I do think the idea has merit but I suppose the problems are related and since you seem to be considering one of these structures and asked.. To me it comes down to control. When you put insulation right up against the building envelope, you can control how much heat is lost. By putting Thermal Mass TM in between the envelope and insulation you lose control.

The idea of PAHS seems to be that the temperature of the earth/TM will eventually stabilize to the human comfort range of around 70 degrees. This is a big unknown and there is no control of when or if this will actually happen. Insulation against the envelope works very well. I just dont see the need to add more thermal mass which ultimately makes it more difficult to control the indoor air temperature.

The amount of BTUs needed to heat so much TM will be enormous and it will be an unknown amount of time for "x" soil temperature to reach 70 degrees. If this is even possible, it then becomes difficult to stop the inertia or reverse it. Surely its possible with the right details and monitoring equipment. At this point however, it seems like a pretty expensive and/or time consuming experiment when there are more proven techniques available.

Then there is the moisture concern. Below grade construction is risky. In our local soils, bulk water flows from all directions. Water vapor below grade is a strong and never ending force that doesnt care how good the above umbrella is detailed. I think PAHS construction can be built for water resistance but many of the ideas Ive seen presented are worrisome and I would feel safer sticking to more conventional yet proven details.

For a contrast in high performance building, look at the passivehaus movement. They pretty much shun below grade construction and TM altogether. Their results however are impressive. They reduce heating and cooling costs by 90%, mainly through air-sealing and insulation. There are tens of thousands of successful examples and they have created modeling software that is very accurate, predictable and repeatable.

I dont necessarily completely buy in to passivehaus as I think TM, PV vs superinsulation and below grade construction can be appropriate in the right situations. I also commend those that are innovating and trying new techniques like PAHS. Iam hoping that my concerns will be disproven and that people will develop alternative paths to high performance homes through PAHS techniques. At this point however, I hope people fully understand that the physics dont seem favorable and the moisture/soil gas concerns need more attention to detail.

During my research b4 I built mine I saw a project a guy was doing that amazed me. He had a large RMH and was running flue pipe through the entire footprint of his 2000 sq. ft. home b4 building the home. I didnt follow the project so I dont know exactly what he did after. But basically he leveled the footprint area then dug 10x10 trenches every 2 feet then poured 2 inches of gravel in trenches laid the flue pipes and covered with gravel. He had the plumbing in also that made me think he was pouring a slab over the network of flue pipe and building home on slab. I remember thinking wow this guy must know what he is doing investing that much $ in flue pipe LOL. Partly encouraged me to build mine.

Those are good observations Brian. While I may not be sold on PAHS as a whole I definitely buy into the PAHS principle; dry dirt is nice to have around your house. Even a porch could act as an umbrella and help with regulating the temperature of a conventional poured basement. As far as your concern about storing too much heat; I think that is unfounded. If your house is too hot from excess heat in the surrounding thermal mass simply open a window or door for 15 minutes.

Check this!

https://permies.com/t/33247/rocket-stoves/Milk-mass

Interesting thread unfolding there max, thanks.

Thanks Philip, my views are obviously not endorsed by anyone other than myself. So what happens when its more warm/humid outside than inside? Opening windows would make it worse.

Philip I like the floor as heat storage mass concept. It seems central to Ondol systems; Korean underfloor flues: en.wikipedia.org/wiki/Ondol
I wonder if there's an ideal amount of thermal mass. I don't know the tonnage. With 7 tons, maybe one could burn fires once a week, but wouldn't one have to spend half the day fueling fire after fire. Err, I guess one could have a bigger fire.
Here at home, the steam heat's too much, but the adjustment's broken, so I've thrown a rug over the radiator. I understand that in Ondol systems, quilts would be kept on the floor, to put ones legs beneath, while also keeping the house from getting too hot if one's burned too much wood too early.

Philip Nafziger a couple of things, This Topic is now linked to the Rocket Mass Heater/Rocket stoves forum where it will be seen by more people. And In-Ground

and Bench style RMHs are extensively covered in Rocket MassHeaters in Greenhouses! It is one of the larger Forum Threads -if you have not already been to it !

For the good of the craft ! Big AL

What do you-all think about this RMH-Ondol concept?
Tile floor set into damp clay that seals horizontal stones sitting on vertical/on edge stones, to create under-floor flue. Smoke-burning bell of firebrick/fireclay below. Vermiculite layer between vertical stones to insulate fire path.