AdeV
Looking at your pics, I would be wondering if you even have enough heat transfer area to adequately remove the engine heat.
The numbers:
A 6/1 at full load has to dissipate nearly 18,000 BTU/HR from the cooling system to maintain it's cool. From your pics, I would be surprised if there were more than 6 or 7 loops in that tanks heat transfer coil. You said I believe that it was 22MM in diameter? well lets be generous and call it 24.5MM. I am going to work in inches from here on out. 24.5MM is 1" so that pipe will have a circumference of .79" 7 coils at approximatly 14" diameter is about 44" of pipe length per coil. 44" times 7 coils is 307" of pipe length. That times the .79" of circumference is about 243 SQ/IN, or 1.69 SQ/FT of surface area.
Copper water-water heat exchangers have a heat transfer coefficient in the area of 60-80 BTU, per SQ/foot, per degree F of temp difference, per hour. If your engine has a 195F thermostat, it is going to need say in the area of 100F returning to the engine to operate properly. That is a mean temp of around 145F. If you want domestic water temps of say around 120 F, that tank will probably range from 120 down to 80 at the bottom. As a guess, If you had 100F as a mean tank temp, that is only 45F of temp difference.
So 80 BTU times 1.69 SQ/FT times 45F difference is only 6075 BTU per hour of heat transfer capacity. That is the heat output of a 6/1 generator at about a 1KW of electrical load... If you can get more accurate dimensions, we can refine this a bit, but I don't see it being much larger based on the pics. Even if it had twice the coils, it would only double the heat dissipation capacity using these same temp numbers.
Another question, is what are you going to do with the heat? In order for the engine to survive, the BTU must leave the engine to maintain temperature. By tying the tank to the engine, the same is now true of the tank. The same ammount of BTU must leave the tank in order for it not to become heat saturated and over temp. If too few BTU are removed, the tanks temp rises. As it's temperature rises, less and less BTU are removed from the engine and it in turn overtemps.
Since the primary goal is to cool the engine adequately, I would reccomend that you put a radiator in the return line from the tank back to the engines lower coolant port. By doing it this way, The hot water tank gets first crack at gathering heat from the hotest coolant. If it cannot remove enough of the heat, the radiator can then dissipate the remaining required heat to maintain proper engine operation. A thermal switch could control the fan so it only runs when necessary to maintain temperature.
My .02
Good Luck
