in a working forced air furnace in my basement, the gas burner cycles
on and off while the blower sends the warmed up air to the house until
the thermostst is satisfied.
the main can't usually run 100 percent of the time or the plenum would
get overheated. the system is designed or adjusted at fan in and fan
out settings to deliver that range of heat to your registers.
if your furnace is not operating properly be more specific with the
symptom you have.