Basics of BTUs: What They Are

The BTU is the standard measurement of heat. It stands for British Thermal Unit and has a lot to do with how comfortable your home is in both winter and summer. Furnaces and air conditioners both deal in BTUs, but in different ways: your furnace generates BTUs of heat to warm the house and your A/C extracts BTUs to keep it cool. The raw number of BTUs a furnace or air conditioner can generate or extract, respectively, is referred to as the unit’s “capacity.” The more heating capacity the furnace has, the warmer it can keep your home. Higher A/C capacity keeps the house cooler.

The BTU figure of a furnace or air conditioner is also one of the elements utilized to calculate its efficiency rating. This important rating is a numeral printed on the yellow EnergyGuide sticker that accompanies every unit. It’s also prominently mentioned in the manufacturer’s specifications.

• Furnace efficiency is rated by AFUE (Annual Fuel Utilization Efficiency). AFUE expresses the percentage of BTUs of heat energy generated by the unit that actually contribute heat to the home, versus that amount which is lost in the combustion process — usually in the form of hot gases exhausted up the furnace vent. Today, a standard efficiency furnace is required to have an AFUE rating of 80 percent, while high-efficiency furnaces that recapture some of those lost BTUs have AFUE ratings above 90 percent. Higher AFUE ratings generally mean lower operating costs over the long term.
• Air conditioner efficiency is expressed by a unit’s SEER (Seasonal Energy Efficiency Ratio). This figure calculates the amount of BTUs removed from an interior space during a given time frame in ratio to the amount of electricity consumed during that same time. The higher the SEER rating, the more energy-efficient the unit. Standard efficiency air conditioners have SEER ratings of 14 while more expensive high-efficiency A/C units use less electricity and deliver SEER performance over 20.

Ask the experts at Rinaldi’s Energy Solutions for more how the BTU capacity of your furnace or air conditioner impacts home efficiency and comfort.