My own take on this is that pressurized storage is troublesome as the volume increases. My thoughts on pros and cons: Advantages of pressurized storage: - one less heat exchanger and/or pump - no 'delta T' loss - you get the hottest possible water Disadvantages of pressurized storage: - heavier / more expensive vessel - more leak possibilities - need BIG expansion tank - more dissolved oxygen to eat away at steel components Disadvantages of unpressurized total system: - dramatically increased corrosion of steel components - need water treatment - harder to flush air bubbles out of zones There are clearly many designs that work, and work well. My current thinking is that the ideal is something like this: - Pressurized / sealed system for boiler and zones - Large (1000++ gallons) unpressurized storage - Finned stainless HX coil(s) in storage tank, or alternately a bronze pump and plate HX Because of my background in aerospace engineering, I like systems where any component 'fails safe' and can be isolated from the rest of the system quickly and without disabling the rest of the system. That's one of the reasons that I don't like connecting wood and oil boilers in series. By the same token, I don't want a leak in a large pressurized underground tank to prevent my heating system from working. I'd like to keep the boiler's vital working fluid in as small and simple a set of plumbing as possible. Just my thinking today. May be different tomorrow.