Oh, well I can save you some time... stacks will not produce the heat neccesary to give you water at a temp a health dept is gonna want to pass. If they do you are gonna spend more in winds than just going for the firebox heat.
You need what I built on my Perseverance long ago. I had maybe 4-5 black pipe sleeves welded through the firebox to act as protection for the copper inserts going through it. A pump to re-circulate it. I have seen some attached to the sides of fireboxes with steel sheets protecting them and no heat exchanger. The system wasn't "closed" as you just filled a
reservoir and it recycles the water. Water to the tap is gravity fed.
I think maybe I had a 10 gallon reservoir but could have uses more.
I did a test once on the Funkotorium to see how many windings (I tossed some rolled copper tube in the firebox with my on demand pump rounded to it) to see if I could meet the demands of my shower, bath, kitchen and everything from only a cold water supply (this means that unlike the example above the water would not recirulate until it got hot.
It suprisingly was extremely hot even with the pit at only 250 Chamber degrees (remember it was from the firebox I had the coils connected to)
I could get hot water just as fast as with the orginal heater (which came with the camper and was burned out). I addition, is was hot enough to HEAT the camper as well, but I could not cofirm before I sold it whether it woudl do both.
Finally, I also connected a 12 recirculating pump to the same coils and ran them through a BOAT hot water heater (some Motor Campers have these too) and was able to heat water by running it through the heat transfer routing. I did need a blow valve for that one but antifreeze helped. Motor campers and boats get heat from the engine to make hot water while underway.
Really any temp would work as long as I know the chamber temps also so I can get a basis for how the 2 numbers (or 3 if I also get the stack steel temp also) work together. I'm looking to build a water heater MOD and the stack seems like the best place to get a temp safe for the water. I dont want to generate steam. I'm figuring if I build a manifold and wrap about 4 - 1/4" copper pipes around the stack it would give me ample hot water for a prep-station. I'm looking at how to use copper wrapper around the steel stack and not suffer from the corrosion caused by electrolisis. I'm also trying to decide if I should go with a storage tank (6-15 gallons) and a small rv water pump (1.5 gal per min) and make it also with a stepdown pressure regulator like in an RV so I could also plug in a water source. Just thoughts I've had in this build.
Any thoughts are welcome and I already know this is going way overboard when getting something like a coleman on demand hot water is available and simple but then that isnt fun!