I can see you are like me and others I know.We like to figure out a way to take a design and keep improving it to make it work.BUT At what point do you have to say why are we reinventing the wheel
I think you will be wasting btu's out the block walls,burn chamber roof?,flue pipe..You could easily capture all those btu's in a stove with a 2 or 3 pass exhaust in the water jacket.
Jackel440, you and WillieG are exactly right again. I was fixated on the free water heaters and blinded to spending just a little money to get a much more efficient system with a water jacket and heat exchangers.
Following this good advice, I will now build my own water jacket from 200 feet of 1" galvanized pipe that will sit inside the firebox and also function as a grate. I will abandon the thin steel of an oil tank as a firebox and make it concrete and firebrick.
I will zigzag the exhaust flue so that it radiates heat into the concrete shed and install 120,000 BTU/hr of air to water heat exchangers at the top of that space for heat absorption after the water passes through the jacket. I think the water jacket and heat exchangers provide enough reserve capacity for a safety margin if I don't overload the firebox. At 350,000 BTU's per cubic foot of wood, I should fill the firebox with no more than ten cubic feet and set the airflow to achieve a 12 hour burn, so the fire would generate about 300,000 BTU's per hour times my conversion efficiency (66%?) will give me perhaps 200,000 BTU's per hour. Perhaps capturing heat off the water jacket and also the 120,000 air to water heat exchanger is overkill but good for safety and only costs $200. Although 200' of 1" galvanized pipe as a water jacket with a 15 gpm flow in the firebox may already collect 350,000 BTUs per hour (see next paragraph) so the heat exchanger at the top of the shed may be redundant.
If the O.D. of 1" pipe is 1.31" and that times pi times 12" per foot is 50 square inches times 200' of pipe is 10,000 square inches, or 70 square feet. With water on one side the heat absorption is about 2 BTUs per square foot per hour per degree F difference. If incoming water is 80F and the firebox is 580F, then I am getting a heat transfer of 140 BTUs per square foot (2 BTUs times 70 square feet) times 500 degree differnce is 9,800 BTU's of gain per pass. If my 30 gpm pump effectively flows half that after pressure loss to the heat exchangers and pipe walls then 15 gpm on a total volume of 25 gallons (500' of 1" pipe at 5 gallons per 100'), then the water will make 36 passes per hour (15 gpm times 60 min per hour divided by 25 gallon volume). So my heat gain should be about 352,000 BTUs per hour just on the water jacket without counting the air to water heat exchanger at the top of the room.
Inside the house I will have three heat exchangers totalling 350,000 BTUs per hour and 300' of pex tubing. I can add extra radiators or pex tso that the indoor heat exchange is greater than the outdoor heat collection for safety. Now my head hurts from all the math. What am I missing?