mass heat exchanger

I was thinking, if you had a small mass say 12" x 12" x 20' with your 8" duct running through it and also run piping along the duct surround with sand or gravel or something would this efficiently draw off heat from the flue gas to yeld the low temp exhaust you normally get from a RMH then draw the heat off by circulating water in the piping to store in a large vented water tank and use the stored heat in an existing radiant floor heating system. you only need to generate about 120-140 degrees F. for radiant heat.

Water is a variable mass (it gets cold and hot spots) this makes it a great mass to use but in my experience limits the amount of mass you want in the system after you heat the water. In other words the water can cool the exhaust so low the stove cannot function if your exhaust ducting is run through more mass. you get what we call a cold block. you can heat water with an RMH and use it for radiant floors you just need to be careful you dont flash to steam in the heat exchanger. if you put your heat exchanger in the first 5 feet of ducting (build a heavily insulated box around it so you can get into it for repairs) you will get temps that wont flash the water to steam and you can put iit into a water tank. I didnt do this to our house because i needed to get the stove in fast so we would have heat for the winter. I will probably do this for a boat system. it works just remember that it takes power to run the pump to circulate the water in the floor. the power goes out and t your heat is going to be pretty darn close to the stove if you didn't build it to heat the house.