Can you tell me what this is?

[gtblum]

This is on the 2nd floor of a single family home in what was once an upper apartment

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[Chad Fabry]

An expansion tank for an open to atmosphere boiler

[Neal Lewis]

That is the expansion tank for a gravity hot water heating system. The glass gauge indicates the water level in the system. There may have been an overflow pipe for the tank going out the roof. The system could have been converted into a forced circulated type with the installation of a pump. If that's the case, the tank would no longer be needed; an expansion tank would be installed at the boiler.

You can probably find some articles about gravity hot water systems at heatinghelp.com

[gtblum]

Tanks guys.