My rented flat (in the UK) has 2 expansion tanks behind the boiler, one red, one blue.
From googling around I can find lots of references to hot water expansion vessels; the purpose seems to be to allow for expansion of the water in the closed system of the heating circuits as it gets hotter.
What's the purpose of the cold water vessel? Is it just to prevent water hammer?
Update: here's a description of the lines and where they go, as requested in a comment. Please correct my terminology, I don't know what it should be but I'm keen to learn:
- cold water seems to arrive from outside - there's a stopcock.
- When I close the stopcock, I have no cold water anywhere in the flat.
- Immediately after the stopcock, is what I think is a pressure reducing valve (Caleffi - 3.5 bar is printed on it).
- After that, there are a few branches off this pipe:
- one leads to the blue expansion tank.
- one leads to a cylindrical tank below ('Keston Spa'). Where it joins, it's marked "MAINS CWS" (Cold Water Supply?)
- one leads to a hose with closed valves that I believe can be temporarily opened to 'top up' the closed heating system: beyond the closed valves, I can see that it connects to the return port on the boiler (according to the manual), as well as the red expansion tank.
- finally it seems to go off around the flat.