What You Need to Know About BTUs for Your HVAC System

October 6, 2016

The BTU (British thermal unit) is the standard by which all HVAC capacity is measured. It's commonly applied to heating systems, and while not directly used to describe air conditioning, forms the basis for their sizing. Technically, the term refers to the amount of energy required to change the temperature of a specific amount of water. BTUs refer to the size of furnaces and boilers, as well as cooling systems, although it's far more common to see BTUs as tons in this context. Each ton of cooling equates to 12,000 BTUs. When applied to heating systems, the BTU indicates capacity but not heat output. By their nature, heating systems that use gas or oil aren't 100 percent efficient. While a furnace may have 80,000 BTUs, it may only produce 64,000 BTUs of heat if it's only 80 percent efficient.

How Heating Efficiency is Measured

All combustion heating systems have AFUE (annual fuel utilization efficiency) ratings that tell you how much of the fuel the system uses generates heat for your home versus what's wasted. The minimum standard for new furnaces stands at 80, which means that it wastes 20 percent of the fuel it uses. The most efficient systems waste just two percent of the fuel they use, giving them an AFUE of 98.

Selecting a New Heating System

BTUs and AFUE ratings play a prominent role when you're choosing a new HVAC system. A small system with high efficiency will produce just as much conditioned air as larger equipment with low efficiency. Although more efficient systems cost more initially, they pay for themselves in lower energy costs throughout their lifetimes. Finding the right size based on BTUs and efficiency is essential for comfort, low energy bills and system durability. The BTU capacity of HVAC systems indicates their size, but it's only part of the selection process.