When a water heater is connected to a closed system, an expansion tank must be installed to prevent excessive pressure caused by thermal expansion. By hominspector September 26, 2017 at 11:10AM ...

Many water heaters feature an expansion tank to protect the system. Here's what a water heater expansion tank does and how you know if your system needs one.

That is an expansion tank installed on the water supply pipe to the water heater. Its purpose is to deal with thermal expansion of water as it heats up in the water heater […] Skip to content.

The larger the boiler, the larger the expansion tank. In your case, the technician probably looked at the chart, and found that based on the BTU capacity of the boiler, that the tank was undersized.