We are looking at a similar project.
I too have been researching the
rocket mass heater idea for a few weeks, to find a more efficient way to heat with wood instead of the old stove we find our selves mostly using, since our house is surrounded by woods.
We ordered "the book" and think we have a generally good grasp of the concepts. However, we already have a house with propane powered in floor radiant heat water filled pex, so building yet another mass storage device wouldn't really take advantage of the resources we have available.
We are trying to find those with
experience using water as the heat storage with wood and are finding the examples to be a bit thin.
I found a discussion on this forum about
Geoff Lawton's design, but that is an outdoor unit and not really suited for indoor use. I have seen nothing mentioned of the videos by Van Powell which heat a coil of water on top of the rocket barrel and use thermo siphoning to move the water, preventing pressure problems. His experimentation and observations seem rather compelling, though I am not sure I would feel comfortable leaving a fire open to a room to burn unattended as he states is a goal in the last rocket fireplace video.
https://www.youtube.com/watch?v=EVpseNZI9vY
Having lived with a water heated heating system for years, it doesn't seem it would be that difficult to figure out how much water at a time a particular
rocket stove could heat with even a full load and make sure you have that much available at all times. Either a copper coil with thermo siphoning as he has in the video, or a larger tank on top that heats a larger amount of water at a time to be sent to the exchanger in larger quantities. Since we have many sqft of floor to store the water in as well, there surely is a way to figure out how to get it to work. We want the water to go at least as hot as 140 occasionally, but no more than 180 on a regular basis. It would seem a simple matter of math, then add a bit more water (mass) for a safety's sake.
I have read a few cautions about Legionella bacteria and from inadvertent experience (open systems are no longer best practice) it does seem that as long as the water is heated up to over 140 right before it is released into a tap, the bacteria is killed. In 15 years, no one has ever gotten sick in spite of the fact that the water sits stagnant all summer and eventually gets cycled back when the floor heat is turned on for the winter.
However, now that I know there is a possible concern, I am inclined to separate the heating water from the potable water. I would like to find the best way to do that heating it with wood inside the house.
Having tried many heating methods, steam radiators,
wood stove, in floor radiant, forced air furnace and heat pump, we prefer them in the order I listed them in. We were disappointed to find we do not like the slow constant release of heat with a whole house radiant floor as the sole heat source. It isn't able to respond fast
enough to the varying temperature changes a typical day holds here in Virginia. This seems to be an issue noted with masonry heaters as well. This how we ended up using mostly our wood stove and leaving the in floor system off in all but a few distant rooms. We find we prefer having colder bedrooms, medium working areas and warm lounging areas, rather than one uniform temperature throughout the house. This was hard to achieve even with separate zones. The temperature does vary throughout the day, but it is so slow to respond it often can't cool/heat fast enough.
The rocket stove mass heater seems like it offers both the immediate heat factor from the barrel and the storage factor as well. If we can only figure a way to attach it to the mass storage we already have I think we would have a winner. We still plan to leave some zones off altogether.
We are adding a
solar water heating component that we picked up from someone removing it from their house. (Does make you wonder if it worked so well, why they were removing it.) The previous owner said it did well at pre-heating the water, but I am sure we will need to augment it. (Separate post in solar forum) After talking to someone with this system we are now leaning towards hooking it up to the potable water heater instead.
So any suggestions on how we can best utilize what we have most efficiently and comfortably?
While the idea of just "trying" a tried and true design sounds like a good start, I know from the many experimental building ideas already in the house, a thing that takes as much time and energy to build as a well designed rocket mass heater would not be something easily "tweaked" once it is in place. After hearing it takes up to a year for the
cob to really dry, one can expect to be looking at even the experiment as being a long term project. I want to be reasonably sure we are pretty close to what we want.
Adding a cooking component, as is seen in the masonry heaters, appeals to me as well, so for now I am thinking of creating something like the Van Powells rocket fire place with an oven between the top of the barrel and the water coil. The oven would also absorb some of the heat before it got to the water coils which should reduce the worry of overheating the water further.
I am envisioning building the oven out of cob on the outside with metal inner walls. The heat would flow through an empty channel between the walls up and exit out of a top hole to heat the water coils on top of that. Each piece could be "modular" and removed if not used or some part needed replacement. Looking like the oven in this video but with copper coils on top to heat water like the Van Powell design. Also with no hole in the top of the drum, so no smoke in the house.
Since we already have the pex in the floor, I figure we need to insulate underneath and/ or use mostly insulating cob, with a thin structural finish coat until we get to the oven bit, to reduce the heat on the floor.
What do those of you with experience think of combining those ideas?