NATE Gas Heating 2

Question 1

What is the AFUE rating and what does it measure for a gas furnace?

Accepted Answer

Annual Fuel Utilization Efficiency; measures what percentage of fuel energy is converted to heat

Answer

AFUE expresses as a percentage how much of the fuel's energy is converted to usable heat; a 96% AFUE furnace wastes only 4% up the flue.

Question 2

What type of venting is required for a 96% AFUE condensing gas furnace?

Accepted Answer

PVC plastic pipe for both flue and combustion air

Answer

High-efficiency condensing furnaces produce cool, moist flue gases that must be exhausted through PVC pipe; Category IV designation.

Question 3

What does the flame sensor do in a modern electronic ignition furnace?

Accepted Answer

Passes a small DC current through the flame to prove ignition and keep the gas valve open

Answer

The flame sensor uses flame rectification—the ionized flame allows DC current to flow—proving the burner is lit and preventing gas buildup.

Question 4

What is a condensate drain trap used for on a high-efficiency furnace?

Accepted Answer

Prevent flue gases from backing up through the condensate drain while allowing water to drain freely

Answer

The condensate trap maintains a water seal that blocks furnace gases from escaping through the drain while permitting condensate to flow out.

Question 5

What color should the burner flame be on a properly adjusted natural gas furnace?

Accepted Answer

Mostly blue with slight yellow tips

Answer

A properly burning natural gas flame should be predominantly blue, indicating complete combustion, with minor yellow tips acceptable.

NATE - North American Technician Excellence Practice Test