The Second Law of Thermodynamics implies it’s not wise to convert high-grade energy, such as the chemical energy in a fuel, into lower-grade thermal energy until the latter is needed.
Most of the pellet boilers being installed in North America are retrofit into existing hydronic heating systems that have one or more fossil-fuel boilers.
Electrically powered heat pumps, in many varieties, are destined to play an increasingly important role in heating and cooling buildings around the world.