FC Energy Efficiency & System Optimization 3 — Questions and Answers
Question 1: Which duct system characteristic most significantly reduces overall HVAC system efficiency?
- Ducts made of sheet metal instead of flex duct
- Leaky ducts located in unconditioned spaces like attics or crawlspaces (Correct answer)
- Ducts with 90-degree elbows instead of sweeping turns
- Round ducts instead of rectangular ducts
Correct answer: Leaky ducts located in unconditioned spaces like attics or crawlspaces
Duct leakage in unconditioned spaces can reduce system efficiency by 20–30% because conditioned air is lost to areas outside the thermal envelope.
Question 2: A programmable thermostat contributes to energy savings primarily by:
- Measuring outdoor temperature to adjust setpoint automatically
- Reducing heating demand when occupants are away or asleep (Correct answer)
- Increasing furnace AFUE during peak hours
- Controlling duct dampers to balance airflow
Correct answer: Reducing heating demand when occupants are away or asleep
Programmable thermostats save energy by automatically lowering setpoints during unoccupied or sleeping hours, reducing heat loss over those periods.
Question 3: Zoning systems improve energy efficiency in a multi-story home by:
- Increasing total furnace capacity to heat all zones simultaneously
- Delivering conditioned air only to occupied zones, reducing overall runtime (Correct answer)
- Eliminating the need for return air ducts
- Allowing each zone to have its own fuel source
Correct answer: Delivering conditioned air only to occupied zones, reducing overall runtime
Zoning with motorized dampers directs heating only to occupied areas, preventing energy waste in unused rooms and reducing total system runtime.
Question 4: What effect does a dirty air filter have on furnace efficiency and performance?
- Improves filtration efficiency, reducing energy use
- Restricts airflow, increasing heat exchanger temperature and reducing efficiency (Correct answer)
- Has no impact on efficiency, only on air quality
- Decreases static pressure, allowing the blower to work less
Correct answer: Restricts airflow, increasing heat exchanger temperature and reducing efficiency
A clogged filter restricts airflow, causing the heat exchanger to overheat, the high-limit switch to trip, and overall system efficiency to drop significantly.
Question 5: The sensible heat ratio (SHR) is most relevant when evaluating a furnace's:
- Gas combustion efficiency
- Interaction with a cooling coil in a combined HVAC system (Correct answer)
- Flue gas temperature at the vent terminal
- Electrical consumption of the igniter
Correct answer: Interaction with a cooling coil in a combined HVAC system
SHR describes the proportion of sensible to total heat, which affects how a combined heating/cooling system handles both temperature and humidity.
Question 6: Indoor air quality accessories such as UV lights and whole-house humidifiers affect system efficiency by:
- Always reducing efficiency by adding electrical load and static pressure
- Potentially improving efficiency if they reduce the need to ventilate with unconditioned outdoor air (Correct answer)
- Increasing furnace AFUE by improving combustion air quality
- Having no measurable effect on efficiency
Correct answer: Potentially improving efficiency if they reduce the need to ventilate with unconditioned outdoor air
By controlling indoor contaminants and humidity without excessive outdoor air exchange, whole-house accessories can reduce the energy cost of treating outside air.
Question 7: A technician notices a furnace is short-cycling on the high-limit switch. The most likely root cause related to efficiency is:
- Faulty igniter drawing too much current
- Insufficient airflow caused by a dirty filter, blocked registers, or undersized ducts (Correct answer)
- Gas pressure set too low at the manifold
- A cracked heat exchanger leaking combustion gases
Correct answer: Insufficient airflow caused by a dirty filter, blocked registers, or undersized ducts
Insufficient airflow causes heat to build up in the heat exchanger, triggering the high-limit switch as a safety measure, which also dramatically reduces operating efficiency.
FC Energy Efficiency & System Optimization 3
Which duct system characteristic most significantly reduces overall HVAC system efficiency?