HVAC Heating Systems 5 — Questions and Answers
Question 1: Which heating system type is best suited for a building where individual room temperature control and zoning flexibility are the top priorities?
- Single-zone forced-air system with one large furnace
- Hydronic baseboard system with individual zone valves per room
- Electric resistance baseboard heaters with individual thermostats per room (Correct answer)
- Single-stage oil furnace with a central thermostat
Correct answer: Electric resistance baseboard heaters with individual thermostats per room
Electric resistance baseboards with individual thermostats offer the most granular room-by-room control with no shared ductwork or piping constraints.
Question 2: A gas furnace's inducer motor runs but the igniter does not glow. The control board has power. What is the most likely cause?
- Tripped high-limit switch or failed pressure switch preventing the board from advancing the sequence (Correct answer)
- Defective gas valve preventing gas flow to the pilot
- Undersized circuit breaker limiting current to the igniter
- Cracked heat exchanger allowing exhaust gases to interfere with ignition
Correct answer: Tripped high-limit switch or failed pressure switch preventing the board from advancing the sequence
After the inducer runs, the control board waits for pressure switch confirmation before energizing the igniter; a stuck or failed pressure switch halts the sequence at that step.
Question 3: What is the purpose of a modulating gas valve in a variable-capacity furnace?
- To switch between natural gas and propane fuel sources automatically
- To continuously vary the gas firing rate between minimum and maximum to match heating demand (Correct answer)
- To maintain constant flue gas temperature regardless of outdoor conditions
- To mix combustion air and gas before delivery to the burner
Correct answer: To continuously vary the gas firing rate between minimum and maximum to match heating demand
A modulating gas valve adjusts firing rate continuously (e.g., 40–100% of capacity), improving comfort and efficiency by matching output closely to the actual heating load.
Question 4: When retrofitting a new high-efficiency condensing furnace to an existing chimney-vented system, what must the technician do with the chimney?
- Line the chimney with stainless steel liner and connect the PVC vent to it
- Abandon the chimney for flue venting and run PVC vent pipes directly through a wall or roof (Correct answer)
- Install a barometric draft damper at the chimney base
- Increase chimney height by 2 feet to improve natural draft for the new unit
Correct answer: Abandon the chimney for flue venting and run PVC vent pipes directly through a wall or roof
Condensing furnaces produce cool, moist flue gases unsuitable for masonry chimneys; they require PVC or CPVC direct vent piping routed through an exterior wall or roof.
Question 5: In a heat pump heating mode, at what outdoor temperature does the system's COP typically drop to approximately 1.0, making it no more efficient than electric resistance heat?
- 32°F (0°C)
- 17°F (-8°C) to 0°F (-18°C), depending on the system (Correct answer)
- 45°F (7°C)
- 55°F (13°C)
Correct answer: 17°F (-8°C) to 0°F (-18°C), depending on the system
Most standard air-source heat pumps reach a COP near 1.0 between 0°F and 17°F, below which auxiliary electric strip heat becomes the primary heat source.
Question 6: What type of pipe is required for the condensate drain on a 96% AFUE condensing gas furnace?
- Galvanized steel pipe rated for high-temperature drainage
- Schedule 40 PVC or CPVC rated for acidic condensate (Correct answer)
- Copper tubing, as used in standard plumbing drain applications
- Black iron pipe to resist the corrosive acidic condensate
Correct answer: Schedule 40 PVC or CPVC rated for acidic condensate
Condensate from high-efficiency furnaces is mildly acidic (pH 3–4) and should be drained with PVC or CPVC; metal pipes corrode and must be avoided.
Question 7: A technician performs a combustion analysis and finds the CO2 reading in the flue is 12% for a natural gas furnace. What does this indicate?
- Dangerously incomplete combustion requiring immediate shutdown
- A near-stoichiometric air-fuel ratio indicating efficient combustion (Correct answer)
- Excessive dilution air entering the flue reducing efficiency
- Refrigerant contamination in the combustion air supply
Correct answer: A near-stoichiometric air-fuel ratio indicating efficient combustion
For natural gas, peak theoretical CO2 is about 12%; readings of 10–12% CO2 combined with low CO indicate good combustion efficiency.
HVAC Heating Systems 5
Which heating system type is best suited for a building where individual room temperature control and zoning flexibility are the top priorities?