Hi all, interesting thread. Just wanted to mention that it strikes me that all of the submerged heat exchangers (and your flatplate, Eric) seem quite undersized. For example, we would specify THREE of the 180' STSS coils to be used with a 60kW boiler like Eric's EKO or our Solo Plus 60. Remember, you are sizing the heat exchanger to the boiler output, not the heating demand of the house. I have not personally done the calcs, but I know the sizing is based on the difficulty of heat exchange into a tank when boiler and tank temps are close to one another (low Delta T). Coils should be plumbed in parallel, not series, by the way.
As you can see by the cost of these coils, there is good reason to consider having one set of coils act as both charging coils and discharging coils as shown in our drawings ("Square Tank", for example). A bit more complicated on the controls side, but saves the cost of a second set of coils and allows you to send heat straight to the zones if tank temps are down. In other words, the heat from the wood boiler doesn't have to go 'through' the tank to get to the zones, so you don't have to wait for tank temp to come up before you can get usable heat to the zones.
hope this is helpful