What is a characteristic of high-efficiency condensing furnaces?

High-efficiency condensing furnaces possess the characteristic of utilizing outdoor combustion air. This feature is significant because it helps to improve the overall energy efficiency of the furnace. By using outdoor air, these furnaces can achieve better combustion and subsequently extract more heat from the combustion process. This is particularly advantageous as it reduces reliance on indoor air and can contribute to improved indoor air quality, as it minimizes the recirculation of stale air.

Additionally, outdoor combustion air helps to keep combustion and ventilation processes separate, which is vital in maintaining safety and efficiency. Unlike traditional furnaces that may draw combustion air from within the home, high-efficiency models often incorporate direct venting systems that draw air from outside, allowing for more effective heat transfer and reducing energy waste.

The other options do not reflect the typical characteristics of high-efficiency condensing furnaces. For example, they are designed to minimize exhaust gases rather than produce more, they can often function effectively with existing ductwork and do not generally operate without a thermostat. Thus, the use of outdoor combustion air stands out as a defining feature of these systems.