HVAC Heating Systems — Questions and Answers
Question 1: What is the Annual Fuel Utilization Efficiency (AFUE) rating used for?
- Rating the efficiency of air conditioning systems
- Rating the seasonal efficiency of fossil-fuel heating equipment (Correct answer)
- Measuring airflow in forced-air systems
- Calculating heat loss through building walls
Correct answer: Rating the seasonal efficiency of fossil-fuel heating equipment
AFUE measures how efficiently a furnace or boiler converts fuel into heat over a heating season, expressed as a percentage.
Question 2: What is the minimum AFUE required for a gas furnace to qualify as high-efficiency in the US?
- 80%
- 90% (Correct answer)
- 95%
- 78%
Correct answer: 90%
Gas furnaces with an AFUE of 90% or higher are considered high-efficiency (condensing) furnaces, while standard efficiency is around 80% AFUE.
Question 3: In a gas furnace, what is the purpose of the heat exchanger?
- To mix combustion gases with supply air
- To separate combustion gases from the conditioned air while transferring heat (Correct answer)
- To control the flow of gas to the burners
- To recover heat from the flue gases in the chimney
Correct answer: To separate combustion gases from the conditioned air while transferring heat
The heat exchanger transfers heat from combustion gases to the circulating air supply while keeping the two airstreams completely separated.
Question 4: What type of venting is typically required for a 90%+ AFUE condensing gas furnace?
- Category I metal flue venting
- PVC plastic pipe venting (Correct answer)
- Type B double-wall vent
- Masonry chimney
Correct answer: PVC plastic pipe venting
Condensing furnaces produce cool, acidic flue gases that require PVC plastic pipe for safe venting rather than traditional metal flue pipes.
Question 5: What is the ignition system called that replaced the standing pilot light in modern gas furnaces?
- Thermocouple ignition
- Hot surface ignitor (HSI) (Correct answer)
- Glow plug ignition
- Piezo ignition
Correct answer: Hot surface ignitor (HSI)
Hot surface ignitors (HSI) replaced standing pilots in modern furnaces, using an electrically heated silicon nitride or silicon carbide element to ignite the burner.
Question 6: What safety device shuts off a gas furnace if the heat exchanger area overheats?
- Pressure switch
- High-limit switch (Correct answer)
- Flame sensor
- Rollout switch
Correct answer: High-limit switch
The high-limit switch senses supply air or heat exchanger temperature and shuts down the burner if the temperature exceeds a safe threshold.
HVAC Heating Systems
What is the Annual Fuel Utilization Efficiency (AFUE) rating used for?