I use a coil in the tank for my setup, and I'll start to get idling as the tank gets above 160 or so. In my never-so-humble opinion, this issue is one of the strongest arguments for stratification. I think the ideal storage tank would be pressurized and highly stratified - 180 at the top and 100 at the bottom. If you could reduce turbulence and mixing so as to maintain a sharp boundary (thermocline) between the hot water at the top and the colder water below, then you could deliver consistently cool water to the boiler while pumping hot water into the tank. As you do this, the thermocline would move downwards, and you wouldn't see increased boiler inlet temps until the entire tank was full of hot water.