I mentioned steam only in regards to the primary systems they're most often asked to speak about at conferences. There's a mechanical engineer that designs rather complicated hydronic systems for commercial and industrial applications that also has spec'ed many conversions on historically significant buildings. There's also an experienced mechanic that I met at an historic hotel restoration project. We were both brought in as consultants to troubleshoot some issues. He's a top expert in hydronic systems and also an experienced expert witness. Adding 12, 18 or 24 psi of pressure in a typical residential system has absolutely no significant effect on head pressure in this application. In fact it was found by closing the system, the temperature of the water could be increased. If you studied these systems and researched this topic, you'd know that the hotter the water, the faster it circulates. I've met Tom a few times and know of his plumbing and inspection experience. Do you have anything that he, or anyone else that is credible in this area, has written to back up your original statement? Well, I've only been at it for 24 years, but my experience (inspecting over 11,000 buildings built before 1900 also in a wet heat area) has never shown that in a gravity system "The natural flow of water, (cold to hot) on an open system worked/works better than the same system closed". If I'm wrong, I'd sure like to know. Please show us something from a credible source.