Uses an electric fan to build up pressure in the oven to improve heat exchange with the copper coil. Gives gas the opportunity to fully combust at the top of the oven before it can start cooling as it passes the copper coil downwards. Uses thermal siphon to circulate water in reservoir. Is this a good design?
Yes, at the point shown, you would be likely to still have temperatures around 1000F, at the least, which would flash the water in the coil to steam and possibly explode the copper tubing. It would also break down the concrete fairly soon. Copper melts at closer to 2000F, but that doesn't matter when the coil is destroyed.
People have built safe water heating setups, mostly with a non-pressurized water tank in the exhaust stream, and a pressurized coil submerged in the tank. Pressurized water boils at a higher temperature than atmospheric, so even if the tank boils, the coil won't.
Another design has coils surrounding the barrel of a standard RMH, with atmospheric pressure so any steam generated will just spit and not cause an explosion.
What is the purpose of the hot water? For heat or for domestic use?
The thermal hot water siphon system I'm familiar with worked remarkably well, but there was a substantial difference in elevation to give the hot water a place to rise to. Three stories above the basement in which the hot water was heated. It rose and was distributed to all three levels before the cooled water was returned to the basement to be heated again. Hot water would rise., cold water would sink. Once the loop started, it purred along nicely.
Your design places the reservoir below the peak of the heat....below the point at which the water is being heated. Seems to me it would mostly sit there until it turned to steam and then expanded in all directions. Forward and backward. Once the part exposed to the high heat went from liquid to steam, any subsequent liquid returned to the heat source would flash off with a potential violent expansion, i.e., could explode.
The hot water system I'm thinking of would place a 50 gallon drum horizontal on top of the horizontal bed part of a RMH, and becomes part of the thermal mass downstream of the high temp part. Hot water from that could then rise to another elevated drum or reservoir, with a return to the lowest part of the heated drum.
To keep the whole thing flowing, you need yet a third smaller drum above the top one to act as an overflow reservoir (to keep the siphon going), and as an expansion tank. Lastly, above that you also need a pressure relief valve. A brass plumb bob siting in the end of a copper pipe might work. Same principle as the jiggler on top of a pressure cooker.
Until a person has seen the affects of steam, you cannot believe how much force is at work. Clanking pipes in a steam heat system? That is water flashing off as steam and clanking noise is the sound of the explosion as the live steam goes shooting off down the steam supply lines with the force of a gunshot. The difference between a gunshot and a grenade is the amount of powder generating the explosion. If you are not careful, you can turn a hot water system in to a grenade.