Since heating and air conditioning systems typically last about 20 years, homeowners may need some updated information when it comes time to replace their old heating system.

Much has changed recently in the world of furnace efficiency. However, the basic concept, design and operation of residential heating equipment have remained the same. The efficiency rating of older furnaces (defined as 15 to 20 years old ) were about 60% to 70% AFUE (Annual Fuel Utilization Efficiency). Federal Law now mandates that the basic minimum furnace efficiency must be at least 80% AFUE. Higher-efficiency models may possibly reach as much as 96.6% AFUE.

It is not difficult in our moderate climate to heat most homes. However, the capacity needed to heat each home should be verified. This is determined by a computer calculation known as a Manual J Load Calculation. Without this information, the capacity needed for your home is just guesswork. Most reputable contractors will do this calculation for you, at no cost, and provide you with the results of the report.

All modern furnaces use some type of electronic ignition. They may use a hot surface ignitor or a spark ignitor, but standing gas pilots are a thing of the past. These new furnaces also use some type of inducer motor to help pull the heat generated by the burning gas through the firebox (heat exchanger).

The term “cracked firebox” refers to the section of your furnace where the gas flame burns. Under normal operating conditions, the high temperatures generated by the gas flame can, over time, cause the firebox to “crack” or rupture. Under certain conditions, this crack can allow products of combustion (i.e., burned natural gas) to enter your home. This is an unsafe condition and must be addressed. When this condition occurs, the firebox, or the furnace must be replaced.

The good news is that temperatures generated by new furnaces are lower than their predecessors and cracked fireboxes should be a thing of the past. In addition, newer furnaces have a variety of safety devices to shut them off if a potential problem arises.

Jim Berry was a heating and air conditioning service technician prior to becoming a Sales Consultant for Kahn Air Conditioning. Kahn hosts free monthly workshops to help homeowners learn more about heating and air conditioning systems. For any questions regarding heating or air conditioning, send an e-mail to Jim at

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