Two zones? With one steam boiler? Do tell! I have only seen it once in the local church where I was brainwashed--er, I mean spent a lot of time. There was a motorized vale for each zone that was controlled by a low-voltage thermostat.But it has been decades since I have seen the mechanicals.
In my Cape-Cod style house of 1.5 stories (storys => UK) I added a loop of hydronic (hot-water) baseboard heating to the basement. This is possible because the bottom of the boiler and the floor level of the heated space was the same. The upper story-and-a-half remained steam heat.
The circulator (small pump) needs to pull water out of the bottom of the boiler, run it through the "radiators" (actually baseboard-style convectors) and return it about one foot (30cm) up. The circulator was designed for an "open-loop" system. Meaning it was designed to be used with oxygenated water. In a closed system (your typical hydronic heating systems) the oxygen is eventually depleted and bled out of the system. A one-pipe steam system is actually an open-to-air system that remains with oxygenated water. [a circulator/pump designed for an oxygen-free closed system, and used in oxygenated water, will deteriorate, rust and die prematurely).
Normally, in a "closed" (i.e. airtight) hydronic system the returning water is pumped back into the boiler by the circulator. The hottest water is taken from the top and pumped apound the heating (radiator) loop(s).
