Hello Everyone! Very excited to begin commisserating with such a creative and talented people!
What I have come to share with you, and perhaps gain perspective on, is the continuation of a project which has just taken an interesting turn.
Some time back, I began to build a Domestic SolarWater Heater to offset some of the electricity my family of five has been using. To do this, I dutifully studied the information available at Builditsolar.com, a treasure trove of information if you don't already know. My inspiration came from the idea that I could re-purpose a natural gas hot water heater to be used as a heat exchanger in a drainback solar thermal hot water system. As envisioned, my system would cap off the inner cavity of the water tank, and tap both the top and bottom with fittings so the water from the Solar Thermal Collector could flow through it, pre-heating the 50 gallons of water in the domestic water part of the tank. The water would then move, on demand, to the conventional electric hot water heater. Ideally, this setup would only require the conventional hot water heater to keep the water up to temp.
I soon realized that this system was sorely limited by the size of the hot water storage so I devised a way to daisy chain a series of 4 poly drums to hold pre-heated water in case there was so little demand on the system that I could "bank" some of that heat.
I was quite happy with this setup, but, being winter, I had not yet built the solar thermal collector.
I have a basement fireplace with a broken flue damper. It occurred to me that this might very well be a nice spot to build a Rocket Mass Heater and bench/daybed, using the chimney as the vent. In my mind I skipped ahead. What if it gets too hot to sit on???
So I devised a plan to embed pex tubing in loops within the cob mass. The Thermal Differential Controller from the Solar Hot Water system can handle extra inputs, so I figured the addition of another temperature sensor in the cobb, near the part of the cobb that comes in contact with a person, could be used to control when water would be routed through the pex and back into the hot water storage. In this way, the temperature of the cob bench can be controlled, and I can optimize my solar collector angle for mid-summer, since the RMH will be doing the heavy lifting for water heating in the colder months. I know some people are wary of combining RMH with water heating, but I don't think it would be an issue in this application.
I do not expect these temperatures to be high. Certainly not enough to make steam. I'm not really certain how hot a cob bench gets.
As an aside. If someone would offer any sage advice on the above ideas, I would love to hear it. Also, I am somewhat uncertain as to how I should insulate under the firebox. I assume, pink foam board would be fine for under and in back of the bench, but what do we use under the J-tube area to keep from spalling the concrete floor, or loosing our heat due to heatsink? Would the higher temperature resistance of Polyisocyanurate insulation board be a better choice for the area just after the burn area? Anyone used it in a RMH construction?
If anyone would like any clarification of these ideas, I'd be more than happy to do that.
I would suggest the you use copper tubing in the cob to collect heat. Pex does not transfer heat very well. Add a T fitting at the drain of the tank and take cold water from the to send to the cob and splice the hot water on top. You wont even need a pump if your close enough. A thermos siphon will circulate the water. If the tank is far insulate the pipes to and from the tank. You could even sit the tank on the bench close to the stove.
If you can afford it, you can use insulating brick on the floor and build your stove on top of that. Very effective, but expensive. Aircrete block as a base would also work very well. I wouldn't trust pink foam to insulate under the stove. Or anything than could melt for that matter. I've seen some where built on "pilons" of brick that let a lot of air flow throw under the stove.
There are surly others that have different solutions.