I know on my water to water heat exchanger They told me to mount it as close to the water tank as possible top side and vertical side no more than six inches away. The closer the better they flow from the difference in water temp. The problem is sediment in the water wants to plug up the pipe inside the exchanger. I didn't have a water softener till last week either though. Mine always plugged up with lime. If your water tank isn't really close to your stove it may not self circulate, and you may have problems with minerals in your stainless water heater on the wood box too. My water is all separate form the domestic water, so I can put a corrosion prohibitor in the water. Plus what minerals are in my system wont be added to by daily use of water. They wanted my system to be brass, and plastic. They don't recommend using copper or black pipe. That might just be because of the chemicals they sold me to keep my stove from rusting on the inside, but then again they put it in their stainless stoves too. I already had copper running through the whole house for base board heat, so I said screw it and ran with it. I'm on my fourth year and haven't had a problem yet other than the heat exchanger and water heater plugging up. I think it's something to do with the slow flow that it makes the lime stick to the plumbing? You may be ok with really clean water? My exchanger had to be cleaned once a year. I would hate to think of the whole system.
