HVAC Heating Systems 4

Question 1

What is the purpose of the expansion tank in a closed-loop hydronic heating system?

Accepted Answer

To absorb the increase in water volume as it heats up, preventing excessive pressure buildup

Answer

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 300°F and negative vent pressure?

Accepted Answer

Category I

Answer

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?

Accepted Answer

The control board and its safety lockout inputs

Answer

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?

Accepted Answer

1 inch

Answer

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%?

Accepted Answer

Output decreases by approximately 19% because power varies with the square of voltage

Answer

Electric heating power = V²/R; a 10% voltage drop means (0.9)² = 0.81, so output falls roughly 19%, not just 10%.

HVAC Practice Test

HVAC Practice Test

HVAC Heating Systems 4