There certainly seem to be a lot of opinions regarding this topic. I have an Attack DP35 Profi - 145KBTUH. I am adding an 800 gallon storage tank, located about 40 feet from the boiler. I am trying to determine how large a flat plate exchanger to get and also how large the lines to/from the tank need to be. None of the HX sizing tables I've seen properly address the delta T's you see in a storage loop, so they don't help a lot. So far the general concensus appears to be bigger is better. I am considering a 4" x 10" 70 plate model and 1" pex. Problem is, when you consider the ID of the fittings, 1" pex has less ID than 3/4" copper. I have a couple of Taco 007 circulators left over from my recent conversion to zone valves, so I plan to use one on each side of the tank, allowing me to run the circuit in iether direction, depending on whether I'm charging or discharing the tank. Not sure how much flow I can expect from the 007 with 1" pex. The lines will be nearly perfectly straight with only gentle bends, with the exception of the entrance into the tank where a 90 elbow will be required.