RMH: Second Barrel Water Heater

Tried sev times to attach a sketch but (on my phone) does not serm to work.
So here the more cumbersome verbal picture.
Take the standard drawing/ design of a basic RMH. 2  - 3 feet along the flow duct/ bench, away from the burn site, picture a second barrel, 30 gal size?, sitting directly (with supports so it does not crush the duct(s)) atop the ducts and being heated by radiating heay from them. Has to be sealed well all round of course. Then the ducts proceed through the bench as is stsndard and exit bwo chimney.

Reason I came to this design is reading in this forum how tricky, high tech (by my definition), and potentially dangerous it is to install a bot water devise closer to or around the riser/barrel.
Logically, the air would have cooled a few feet away from the combustion chamber and, second barrel sitting on top (though insulated), would receive considerably less heat so that a float level control and pressure relief valve may be all the safety prep it needs. Barrel would have water in and outlets, and likely a pump to trandport it to its places of use.
I'll try uploading the sketch again if/when I get to a PC. Perhaps what's presented here is vlear enough for a few constructive, critical comments.
Thanks much.

I think it's a good idea.   May perhaps work best as a second bell also.  Similarly I think an external copper coil, located around the exhaust duct , or on the lower outside of the first bell, would work also.  

I rely on propane for my hot water, lay year I used propane for heat and hot water, but this year I've been using my match box rocket mass heater so I'm not spending money on propane as much.  I have free dead hard wood and 10 acres to collect wood from, if I could make hot water and heat BOTH from wood, I would definitely convert away from relying on purchased fuels.  A hot water tank like my 40 gallon unit could serve as a storage tank during winter and a heater in the summer.  And could also be retained as propane fired, in case of emergency or summer or simply hotter water.  

I've also recently discovered drain heat exchangers, which wrap around your shower drain, and instead of bringing cold water into the tank, it coils around the warm drain first slightly increasing the temperature of the water tank input.   I always thought pumping fresh cold water into a huge hot water tank was silly, why not coil the cold well water around a heat source, or chimney, or drain, and then run that warmer fresh water into the hot water storage tank.   90% of hot water tanks I've ever seen have cold water inputs and burn a ton of energy trying to warm that cold fresh input water, something I never could understand when the average chimney sends 200-300 degree waste outside.   Even a fresh air heat exchanger could warm the water with the warm air that leaks out of your house while fresh air comes in via displacement.   There is no reason to pump cold water into a hot water tank and burn energy trying to keep it constantly warm when it could be warm or hot water being introduced into the tank reducing energy consumption by a substantial amount.  

The dangers people refer to are boom squish, basically water boiling in the line causing vapour flash explosions, which could easily be avoided by using a cooler spot for the copper cool, as you mentioned.  Placing the coil externally can also help tremendously in Avoiding boiling and excess heat.  

Hi John, thanks for your response. I feel the same way about standard wster heaters and have long had an unsightly array of cooking anf water heating pots on my wood king. No shortage of firewood here either but getting it home and cut IS work and time and one should capitalize on that.
Are you aware of a post or two that give approximate temps for short distance from the heat riser unit? I looked at the posts using copper coil, overheating was/seemed an issue but further away would certainly be reduced as well.