FC Energy Efficiency & System Optimization 2 — Questions and Answers
Question 1: What does AFUE stand for in the context of furnace efficiency ratings?
- Annual Fuel Utilization Efficiency (Correct answer)
- Adjusted Furnace Unit Energy
- Air Flow Usage Evaluation
- Average Fuel Use Estimate
Correct answer: Annual Fuel Utilization Efficiency
AFUE (Annual Fuel Utilization Efficiency) measures how efficiently a furnace converts fuel to heat over a heating season.
Question 2: A two-stage furnace improves efficiency over a single-stage furnace primarily by:
- Using two heat exchangers simultaneously
- Operating at a lower capacity during mild weather to reduce short-cycling (Correct answer)
- Burning two types of fuel alternately
- Running the blower at two different speeds only
Correct answer: Operating at a lower capacity during mild weather to reduce short-cycling
Two-stage furnaces run at reduced capacity (e.g., 65%) during mild conditions, reducing short-cycling and improving overall seasonal efficiency.
Question 3: Which of the following best describes a variable-speed blower motor's advantage for system efficiency?
- It consumes more electricity to move air faster
- It adjusts airflow to match demand, using significantly less electricity than PSC motors (Correct answer)
- It eliminates the need for an air filter
- It operates only during the heating cycle
Correct answer: It adjusts airflow to match demand, using significantly less electricity than PSC motors
ECM variable-speed motors can use up to 75% less electricity than standard PSC motors by ramping speed to match actual load requirements.
Question 4: The minimum AFUE required by the US Department of Energy for gas furnaces in northern states as of current federal standards is:
- 78%
- 80% (Correct answer)
- 90%
- 96%
Correct answer: 80%
Federal minimum AFUE for gas furnaces is 80% nationwide, though northern states have regional standards requiring 90% AFUE for non-weatherized furnaces.
Question 5: Condensing furnaces achieve higher AFUE ratings than non-condensing furnaces because they:
- Use larger burners to heat faster
- Extract latent heat from flue gases by condensing water vapor (Correct answer)
- Recirculate exhaust gases back into the combustion chamber
- Operate at higher static pressure
Correct answer: Extract latent heat from flue gases by condensing water vapor
Condensing furnaces use a secondary heat exchanger to extract latent heat from water vapor in the exhaust, recovering energy that would otherwise be lost.
Question 6: What is the purpose of a modulating gas valve in a high-efficiency furnace?
- To shut off gas supply during power outages
- To continuously vary gas flow rate to precisely match heating demand (Correct answer)
- To switch between natural gas and propane
- To increase gas pressure during cold weather
Correct answer: To continuously vary gas flow rate to precisely match heating demand
A modulating gas valve adjusts gas input anywhere from about 35% to 100% of capacity, allowing the furnace to precisely match heat output to the building's needs.
Question 7: Proper sizing of a furnace using Manual J load calculation is critical because an oversized furnace will:
- Use more electricity but less gas
- Short-cycle, reducing comfort and efficiency while increasing wear (Correct answer)
- Always operate at peak efficiency
- Require a larger flue pipe
Correct answer: Short-cycle, reducing comfort and efficiency while increasing wear
An oversized furnace reaches setpoint quickly and shuts off, causing frequent short cycles that reduce efficiency, create temperature swings, and accelerate component wear.
FC Energy Efficiency & System Optimization 2
What does AFUE stand for in the context of furnace efficiency ratings?