Is It Better to Oversize or Undersize a Furnace?

Choosing the right furnace size is one of the most important decisions you will make when installing a new heating system. Many homeowners wonder whether it is better to oversize or undersize a furnace for their home.

The answer is neither. A properly sized furnace is the key to comfort, efficiency, and longevity.

The Problems with an Oversized Furnace

An oversized furnace heats your home too quickly and then shuts off. This short cycling wastes energy, puts more wear on components, and fails to properly dehumidify your home in winter.

The Problems with an Undersized Furnace

An undersized furnace runs continuously without reaching your thermostat setpoint. This leads to higher energy bills, uneven heating, and premature wear on the system.

How Furnace Sizing Works

Professional HVAC technicians use Manual J load calculations to determine the right furnace size for your home. Factors include square footage, insulation levels, window types, local climate, and home orientation.