Thermosyphon Hot Water Systems
Were popular late 1800's through early 1900's. Mr. Vanderbilt's Biltmore estate in North Carolina originally used such a system.
Working upon simple physics one, two or even three tanks are suspended from ceiling. Cold water from mains feeds into tanks. Since heated substances rise while cold sinks the latter moves to the bottom of tanks while the former rises.
Cold water goes down to the boilers where via coil inside or indirect it is heated and returns up to tanks. This endless loop works without pumps and via simple gravity and again, physics. As hot water leaves the system (taps) cold water (from mains) goes into tank or tanks, sinks and the whole process begins again.
Because the water is in constant circulation you get "instant" hot water when taps are opened. Much like then and today where buildings with steam or hot water boilers for heating use a coil in tank or indirect, thermosyphon systems provided "free" hot water. That is long as you had the boiler running anyway why not put some of that hot water/steam to use.
This system worked well with coal fired heating because the boilers are rarely out of fire. However there are problems.
First using the boiler to heat water means you have to keep the water hotter inside to cope with heating cold water "and" providing heat. Next depending upon heating power of the boiler it could take hours or even a day to replace a tank emptied of hot water. This is why places usually had more than one tank.
Next worry comes from having to keep the coal boilers fired at a decent hot rate even during merely cool or even warm weather. With coal boilers many liked to shut the things down over the summer both to save on coal and keep from heating the place up.
To solve this problem there were "Tabasco" water heaters. These smaller boilers were used to provide water or steam heating for hot water systems. They could be used in place of a larger coal boiler during periods when that thing was shut off (summer), or simply as part of a building's heating plant in its own right. Again using a separate boiler to heat water allowed taking some of the work load off the main heating boiler so you could lower the water temperature inside.
https://archive.org/stream/KewaneeW...aterHeaters/KewaneeBoilerCo.#page/n0/mode/2up
Thermosyphon also worked with "range" boiler hot water systems. That is using the kitchen or whatever range (coal or wood burning) to heat water as well. Same sort of set-up in that you have a tank for storage and a flow of water circulating basically via gravity and or physics.
You also see thermosyphon hot water systems with solar hot water heating.
Without proper safety checks thermosyphon systems could be as dangerous as any hot water system. First of all unless hot water is drained out of the system regularly sooner or later there won't be any cold water in tanks. Then you have hot water circulating around and around. Next without some sort of mixing valve you could potentially end up with scalding hot water coming out of the taps.