Furnace technology has advanced significantly in recent years. Modern furnaces are designed to provide more even and efficient heating than past furnaces, which can impact both how your system operates,sounds and what you notice about your system.
To better regulate temperatures and airflow, modern furnaces move more air over the heat exchanger than older furnaces. The air that comes out of your furnace registers may not seem as warm as the air from your old furnace, but overall airflow is improved. Better airflow means higher comfort.
Also, new furnaces are designed to integrate with high-efficiency air conditioners, so furnace blowers are more powerful to accommodate add-on cooling. Since cold air is much heavier than warm air, your system needs an extra boost from the blower to deliver cool air throughout your home. If you have an older home, this performance boost could produce unfamiliar sounds because air duct systems were originally designed for heating only. To minimize sound levels, choose a variable speed product which automatically changes speeds to meet the airflow needs of both heating and cooling cycles.