Just what is low mass?

bruce pirger

Quick question guys from the newbie:

I can't seem to find a "definition" of "low mass" boiler. I am looking at oil fired.

I assume it is a question of how much water is stored in the boiler.

Is <5 gallons considered low mass?
Greater than 15 gallons high mass?
10-15 "medium"?

Clearly with the gas fired tankless...that's low mass.

Crown's boiler is called low mass...with 4 gallons...and 380 pounds of cast iron.

Just trying to understand....Thanks.