Yep the thick firebox thing will bite you in the butt. 1/4" is plenty thick enough to give you longevity to last as long as need be as long as your doing your part. it takes btu's longer to travel through thicker materials, hence giving them more time to exit the stack. thats an analogy but one that hopefully most can understand. my neighbor has a 3/8" firebox, and i can attest its noticeably harder on wood than other models
Between You and WillieG I have learned a lot.
there is more to things than initially meets the eye, when i first started out i assumed thicker meant better made. not true at all. i have since seen that the thicker fireboxes are being made by the smaller companies, not because its cheaper, because it shouldnt be! but because they dont fully understand the processes of how a boiler operates. i even had one manufacturer who is popular on the forum tell me that his thick steel made it more efficent cause it held heat longer. i was like wow, you really think that? why dont we just have a block of steel in the yard and circulate water through it? this game is about transferring the btu's into the water, the water is our medium of transport for the btu, if its not going into the water, its wasted.
at the same time though, you have to be thick enough to withstand the wear and tear and folks slinging wood in there and all of that. I feel with standard mild steel that 1/4" is a safe bet.. 1/2" fireboxes require over 26% more wood to put the same number of btu into the water.
stainless steel is another story. Stainless can be less corrosion resistant, but has the tendency to reflect heat. heatmaster uses 409 stainless, it can handle heat cycles really well and is more resistant to corrosion than mild steel, there fireboxes are 10 guage, or around 1/8" They arent having any issues with them either...
Hardy is an example of 304, its even more resistant to corrosion. but very expensive, about 4 bucks a pound on the market. So when hardy builds a stove, its made thin. For one reason to save money, another to allow better heat transfer as 304 tend to be more reflective than lower grades of stainless. a hardy firebox is 16 guage, or roughly 1/16" thick. Empyre has stoves available made from 304 that is around 1/8" thick, but they cost far more than there mild steel siblings.