NATE Gas Heating Practice Exam

For every 1000 feet above sea level, a natural gas furnace's capacity is de-rated by what percentage?

1. 1%

2. 2%

3. 3%

4. 4%

The correct answer is: 4%

As elevation increases, the density of the air decreases, which affects the combustion process in a natural gas furnace. Specifically, for every 1000 feet above sea level, a natural gas furnace's capacity is typically de-rated by 4%. This is because the lower air density means that there is less oxygen available for combustion, which reduces the furnace's ability to produce heat effectively. In practical terms, when a furnace is installed at higher elevations, adjustments to the system are necessary to ensure it operates safely and efficiently. The 4% de-rating allows for proper combustion and helps prevent issues such as incomplete combustion, which can lead to increased emissions of carbon monoxide and a decrease in operational efficiency. This knowledge is critical for technicians and HVAC professionals to ensure that heating appliances are correctly sized and adjusted according to their altitude, providing safety and comfort to the occupants.