All good points, but I'll stand by the issue of intervening heat exchangers. Anyone who has attempted to 'top off' an unpressurized tank (yours truly, for one) can attest to the problem of the last 10 degrees.
The heat transfer rate of a heat exchanger depends on the difference in heat between the two fluids. When storage is cold it's no problem to dump the full output of the boiler through the heat exchanger into storage. As storage heats up, the maximum possible heat transfer rate drops. At some point it's less than the boiler's output and the boiler will start to idle. As the storage gets closer to the boiler outlet temp, it becomes increasingly hard to transfer much at all.
If you have an immersed tubing HX (like mine) the hot water near the top of storage becomes less able to remove heat long before the top of storage reaches boiler outlet temp. As this happens, hotter water gets farther down the coil and starts heating the middle and then the bottom of the tank. This diminishes stratification. More feet of coil and/or lower flow rates reduce this effect.
A properly designed pressurized system on the other hand can maintain a sharp thermocline between the hot (boiler outlet temp) water at the top and the cold water at the bottom. The boiler sees cold water for almost the entire volume of storage, and all of storage is brought right to boiler outlet temp.
The heat transfer rate of a heat exchanger depends on the difference in heat between the two fluids. When storage is cold it's no problem to dump the full output of the boiler through the heat exchanger into storage. As storage heats up, the maximum possible heat transfer rate drops. At some point it's less than the boiler's output and the boiler will start to idle. As the storage gets closer to the boiler outlet temp, it becomes increasingly hard to transfer much at all.
If you have an immersed tubing HX (like mine) the hot water near the top of storage becomes less able to remove heat long before the top of storage reaches boiler outlet temp. As this happens, hotter water gets farther down the coil and starts heating the middle and then the bottom of the tank. This diminishes stratification. More feet of coil and/or lower flow rates reduce this effect.
A properly designed pressurized system on the other hand can maintain a sharp thermocline between the hot (boiler outlet temp) water at the top and the cold water at the bottom. The boiler sees cold water for almost the entire volume of storage, and all of storage is brought right to boiler outlet temp.