There are two basic types of hot water boiler expansion tanks. There is the older style large horizontal steel tank and the newer style smaller diaphragm tank (highlighted in the diagram above).
In older boiler systems the steel expansion tank may be found in the attic or suspended between joists in the basement. In newer systems the diaphragm expansion tank may be attached to the boiler piping near the boiler.
The expansion tank’s purpose is to provide an air cushion for water expansion as it gets hot. If this air expansion cushion did not exist, internal boiler pressure would increase beyond the release setting of the Pressure Release Valve (PRV). That would allow the PRV to open and allow water to escape the system and causing a problem with low system pressure. Read More
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