This page describes that system, I think:<p><a href="http:&#x2F;&#x2F;www.oldhouseweb.com&#x2F;how-to-advice&#x2F;gravity-hot-water-heating-continued.shtml" rel="nofollow">http:&#x2F;&#x2F;www.oldhouseweb.com&#x2F;how-to-advice&#x2F;gravity-hot-water-h...</a><p>(the &quot;Honeywell Heat Generator&quot;)