I just want to throw this up for the sake of discussion here....
My firebox is lined with 2.5" thick fire brick, all sides, my only direct contact heat transfer is a 3" tall by 36" wide secondary flue pass. About 40" long. Now, when I do my efficiency calculations by weighing the wood I put in and recording the temperature increase in the water over a given amount of time,I actually have a higher rate of heat transfer with the brick than without. Which supports a theory that a firebox made with 1" plate should be more efficient than one of 1/4" plate. The reason here is that even though the heat travels to the water more slowly, it is still being absorbed and eventually transferred through. But, the greatest advantage lies in the fact that I can make a much smaller, hotter, more efficient burn, with less wood, and still make as much or more heat. Because the surface temperatures in the burn chamber are much closer to actual combustion temperature. Now, if it takes an hour for the heat to travel through the bricks, through the metal, to the water, big deal. It takes four hours of burning to store enough heat for twelve hours of use. So, after the 400 gallons of water I have has started to cool, which is probably around the 4-6 hour mark, those bricks are still adding heat. Holding the temperature at a reasonable level for quite some time thereafter. Consider comparing two 9 volt batteries paired as opposed to a deep cycle automotive battery, the two 9v make more voltage combined but the auto battery will run your lil lightbulb for days. Mass storage is key to any regenerative system, be it heat or electricity or whatever. The more storage you have the less of a change there is from start to finish of a cycle and the longer the system lasts as there is less direct load on it. The simplest laws of chemistry and physics are at work here. If, say I build a burner with a firebox that has 8 inch thick walls, the heat will still travel through to the metal at almost the same rate as water, which with steel, is almost instantaneously, the steel will continue to heat until it has reached the phase change temperature of the water, 210 degrees, at with point there will be a gradient established. The steel will continue to heat hotter than the water, potentially all the way to the melting point. But the surface of the metal that is in direct contact with the water will never achieve higher than 210 degrees because the water absorbs the heat. As heat is taken from the water, it is then taken from the metal, the metal slowly cools and continues to do so long after the fire has gone out. Because it was able to absorb more heat than the water, heat that would have been wasted if the wall was only 1/4 thick. But the cool part is that the water temperature could potential remain at exactly 210 degrees for many hours long after there is no fire.
Just a small blurb of late night physics to ponder on. I'm not sure how clear it came across as its far past my bed time.