A WIDE RANGE OF KNOWLEDGE
A heating system problem might have nothing to do with the furnace itself. The repair and maintenance of modern heating systems have expertise across plumbing, electricity, ductwork, thermostats, electronic digital controls, building codes, environmental regulations, and more.
The person you trust to maintain and repair your home's heating system should have knowledge across a spectrum of skills, technologies, and even legalities. After all, you don't want someone recommending a new furnace when your current one is simply suffering from a malfunctioning thermostat or some poorly configured ductwork.