Heating Systems Flashcards
7 cards from real HVAC practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Heating Systems flashcards as text
What is the purpose of the expansion tank in a closed-loop hydronic heating system?
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.
Which venting category applies to a non-condensing gas furnace with a flue gas temperature above 300°F and negative vent pressure?
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.
A technician measures 24V AC at the thermostat R and W terminals but the furnace does not fire. What component should be checked next?
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.
What is the minimum clearance from combustibles typically required for a Category I B-vent connector section on a residential gas furnace?
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.
In an electric resistance heating system, what happens to the heating output when supply voltage drops by 10%?
Answer: Output decreases by approximately 19% because power varies with the square of voltage
Electric heating power = V²/R; a 10% voltage drop means (0.9)² = 0.81, so output falls roughly 19%, not just 10%.
A two-pipe direct-return hydronic system has uneven heating across zones. What is the most likely cause?
Answer: Hydraulic imbalance — 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.
What is the primary safety function of the pressure switch on an induced-draft gas furnace?
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.