It is about time that I follow up on this thread! We hope to move into oue new home in a few weeks. The sidearm works fantastic as constructed. Today I fired the boiler to fill and test my radiant wall panel loops. Boiler temps were running around 155 as I was filling the radiators and introducing a lot of cold water to the system. During the first two hours, water in the primary loops (and sidearm) took 80 gallons of DHW up to 150 degrees. Once I was done adding cold water, boiler temps rose to 175 and the DHW was around 165 when I left. I had 3 zones drawing water off the loop prior to the sidearm. What amazes me about this primary/secondary configuration is how hot the primary water stays for all of the zones. Supply drop after those three zones (40,000 btus worth of wall panel radiators) is typically only 3-6 degrees. What also suprised me was how quickly the thermosyphon allows the DHW tank to aproach nearly the same temperature as the supply water.