HVAC Heating Systems 3 — Questions and Answers
Question 1: What does the AFUE rating of 96% mean for a residential gas furnace?
- 96% of fuel energy is converted to usable heat over the heating season (Correct answer)
- The furnace operates at 96% of rated capacity during peak demand
- 96% of supply air is recirculated rather than exhausted
- The furnace loses no more than 4% of heat through duct leakage
Correct answer: 96% of fuel energy is converted to usable heat over the heating season
AFUE (Annual Fuel Utilization Efficiency) measures what percentage of fuel input is converted to useful heat annually; 96% means only 4% is lost.
Question 2: When sizing a residential heating system using Manual J, which factor has the greatest impact on the calculated heating load?
- Number of occupants in the home
- Envelope insulation levels and window U-values (Correct answer)
- Orientation of the roof pitch
- Color of exterior wall surfaces
Correct answer: Envelope insulation levels and window U-values
Manual J calculations are most sensitive to the building envelope — insulation R-values, window U-values, and infiltration rates drive the majority of heat loss.
Question 3: A gas burner with a yellow-orange flame instead of a blue flame indicates what condition?
- Correct air-fuel ratio for efficient combustion
- Incomplete combustion due to insufficient combustion air (Correct answer)
- Overpressure in the gas supply line
- High static pressure in the heat exchanger
Correct answer: Incomplete combustion due to insufficient combustion air
A yellow or orange flame indicates incomplete combustion, often caused by restricted air supply, which can produce carbon monoxide.
Question 4: What is the role of an aquastat in a hydronic heating system?
- It measures flow rate through each zone circuit
- It controls boiler water temperature by cycling the burner on and off (Correct answer)
- It balances water pressure between supply and return headers
- It monitors dissolved oxygen levels in boiler water
Correct answer: It controls boiler water temperature by cycling the burner on and off
The aquastat is a temperature controller that energizes and de-energizes the burner to maintain boiler water within a set temperature range.
Question 5: Which type of gas valve is considered the safest for residential furnaces because it requires two independent failures to allow uncontrolled gas flow?
- Single-seat valve
- Manual shut-off valve
- Redundant dual-seat combination gas valve (Correct answer)
- Pressure-regulating valve
Correct answer: Redundant dual-seat combination gas valve
Redundant (dual-seat) combination gas valves contain two independent operators in series; both must fail simultaneously for gas to flow uncontrolled.
Question 6: In a forced-air system, what is 'heat rise' and why does it matter?
- The rate at which outdoor temperature drops during a cold snap
- The temperature difference between supply air and return air across the heat exchanger (Correct answer)
- The increase in static pressure as filters become dirty
- The thermal expansion rate of the heat exchanger metal
Correct answer: The temperature difference between supply air and return air across the heat exchanger
Heat rise is the supply air temperature minus the return air temperature; it must stay within the furnace manufacturer's specified range to prevent overheating and damage.
Question 7: A homeowner notices the furnace short-cycles (turns on and off rapidly). What is the most common cause?
- Gas pressure set too low at the meter
- Oversized furnace heating the space too quickly and triggering the high-limit switch (Correct answer)
- Cracked flue pipe allowing cold air infiltration
- Faulty thermocouple sending incorrect signals
Correct answer: Oversized furnace heating the space too quickly and triggering the high-limit switch
An oversized furnace heats the space rapidly, causing the supply air temperature to exceed the high-limit set point, which shuts the burner off before the thermostat is satisfied.
HVAC Heating Systems 3
What does the AFUE rating of 96% mean for a residential gas furnace?