What does "low mass" mean?

Low mass refers to the lower water levels inside the boiler itself. By having to heat a smaller amount of water each time it fires, a low mass boiler is far more efficient than a traditional boiler. One risk with certain low mass boilers is that drops in water flow (such as when a second heat zone turns on) can cause a risk that the burner can damage the material in the boiler. Ray is made from rugged cast iron, and is engineered to work with today's variable flow systems.