HVAC Heating Systems 4 ā Questions and Answers
Question 1: What is the purpose of the expansion tank in a closed-loop hydronic heating system?
- To store reserve fuel for the boiler
- To absorb the increase in water volume as it heats up, preventing excessive pressure buildup (Correct answer)
- To separate air bubbles from the circulating water
- To regulate the flow rate between supply and return lines
Correct answer: To absorb the increase in water volume as it heats up, preventing excessive pressure buildup
Water expands when heated; the expansion tank (with a pre-charged air bladder) accommodates this volume increase without overpressurizing the system.
Question 2: Which venting category applies to a non-condensing gas furnace with a flue gas temperature above 300Ā°F and negative vent pressure?
- Category I (Correct answer)
- Category II
- Category III
- Category IV
Correct answer: Category I
Category I appliances have non-positive vent pressure and flue temperatures high enough to avoid condensation, typically vented with Type B double-wall vent pipe.
Question 3: A technician measures 24V AC at the thermostat R and W terminals but the furnace does not fire. What component should be checked next?
- The thermostat itself
- The transformer primary winding
- The control board and its safety lockout inputs (Correct answer)
- The gas meter pressure
Correct answer: The control board and its safety lockout inputs
With 24V confirmed at the thermostat, the signal is reaching the furnace; the control board's safety circuit (pressure switches, limit switches) is the next likely failure point.
Question 4: What is the minimum clearance from combustibles typically required for a Category I B-vent connector section on a residential gas furnace?
- 1 inch (Correct answer)
- 6 inches
- 12 inches
- 18 inches
Correct answer: 1 inch
Listed Type B vent and single-wall connector sections have specific clearance requirements; single-wall connectors typically require 6 inches, but B-vent connectors are often listed for 1-inch clearance.
Question 5: In an electric resistance heating system, what happens to the heating output when supply voltage drops by 10%?
- Output increases by 10% due to higher current draw
- Output decreases by approximately 19% because power varies with the square of voltage (Correct answer)
- Output is unaffected because resistance elements self-regulate
- Output drops by exactly 10% proportionally
Correct answer: Output decreases by approximately 19% because power varies with the square of voltage
Electric heating power = VĀ²/R; a 10% voltage drop means (0.9)Ā² = 0.81, so output falls roughly 19%, not just 10%.
Question 6: A two-pipe direct-return hydronic system has uneven heating across zones. What is the most likely cause?
- Boiler water temperature set too low
- Hydraulic imbalance ā zones closest to the pump receive more flow than distant zones (Correct answer)
- Defective aquastat failing to open remote zone valves
- Incorrect pipe insulation type on the supply header
Correct answer: Hydraulic imbalance ā zones closest to the pump receive more flow than distant zones
In a direct-return system, the first zone has the shortest circuit and least resistance, causing it to receive disproportionate flow; balancing valves or a reverse-return layout corrects this.
Question 7: What is the primary safety function of the pressure switch on an induced-draft gas furnace?
- It verifies gas pressure is within range before opening the main valve
- It confirms the draft inducer is operating and creating sufficient negative pressure before allowing ignition (Correct answer)
- It monitors supply air static pressure to prevent overheating
- It detects heat exchanger cracks by measuring CO in the airstream
Correct answer: It confirms the draft inducer is operating and creating sufficient negative pressure before allowing ignition
The pressure switch senses the negative pressure (draft) created by the inducer motor and will not allow ignition if draft is insufficient, preventing combustion gas spillage.
HVAC Heating Systems 4
What is the purpose of the expansion tank in a closed-loop hydronic heating system?