HVAC Heating Systems 2 — Questions and Answers
Question 1: What is the primary purpose of a heat exchanger in a forced-air furnace?
- To filter airborne particulates
- To transfer heat from combustion gases to supply air without mixing them (Correct answer)
- To regulate gas pressure entering the burner
- To circulate refrigerant through the system
Correct answer: To transfer heat from combustion gases to supply air without mixing them
The heat exchanger separates combustion gases from supply air while allowing heat transfer, preventing carbon monoxide from entering living spaces.
Question 2: A radiant floor heating system loses efficiency because the floor surface temperature exceeds 85°F. What is the most likely cause?
- Undersized circulation pump
- Supply water temperature set too high (Correct answer)
- Incorrect thermostat calibration
- Air trapped in the return line
Correct answer: Supply water temperature set too high
Radiant floor systems are designed for low supply water temperatures (typically 85–120°F); excessively high supply temps cause floor surface temps to overshoot comfort limits.
Question 3: Which combustion efficiency rating measures only the steady-state efficiency of a furnace while it is actively firing?
- AFUE
- SEER
- COP
- Thermal efficiency (Correct answer)
Correct answer: Thermal efficiency
Thermal (combustion) efficiency measures real-time burner performance, while AFUE accounts for cycling losses over an entire heating season.
Question 4: In a hot-water boiler system, what device prevents system pressure from exceeding safe limits?
- Backflow preventer
- Pressure-reducing valve
- Relief valve (Correct answer)
- Expansion tank
Correct answer: Relief valve
The pressure relief valve (PRV) opens automatically to discharge water if system pressure exceeds the rated set point, preventing vessel failure.
Question 5: A condensing furnace produces water drainage as a byproduct. What causes this condensation?
- Oversized air filter restricting airflow
- Combustion gases cooling below their dew point in the secondary heat exchanger (Correct answer)
- Refrigerant migration from an adjacent cooling coil
- High humidity supply air saturating the heat exchanger
Correct answer: Combustion gases cooling below their dew point in the secondary heat exchanger
Condensing furnaces extract so much heat that flue gases cool below their dew point (~130°F), causing water vapor in the exhaust to condense.
Question 6: What is the function of a draft inducer motor in a gas furnace?
- It pre-heats combustion air entering the burner
- It draws combustion gases through the heat exchanger and vents them safely outdoors (Correct answer)
- It speeds up supply air circulation across the heat exchanger
- It modulates gas valve pressure based on outdoor temperature
Correct answer: It draws combustion gases through the heat exchanger and vents them safely outdoors
The draft inducer creates negative pressure to pull flue gases through the heat exchanger and exhaust them, enabling safe and efficient venting.
Question 7: Which type of heating system uses a series of zones with individual thermostats and zone valves controlled by a central boiler?
- Variable air volume (VAV) system
- Hydronic zoned heating system (Correct answer)
- Multi-stage forced-air system
- Ductless mini-split heating system
Correct answer: Hydronic zoned heating system
Hydronic zoned systems pipe hot water from a boiler through multiple loops, each controlled by a zone valve that opens when the zone thermostat calls for heat.
HVAC Heating Systems 2
What is the primary purpose of a heat exchanger in a forced-air furnace?