HVAC - Heating, Ventilation, and Air Conditioning Heating Systems 1 — Questions and Answers
Question 1: What does the Annual Fuel Utilization Efficiency (AFUE) rating measure in a gas furnace?
- The ratio of heat output to electrical energy consumed
- The percentage of fuel energy converted to usable heat over a heating season (Correct answer)
- The maximum BTU output the furnace can produce
- The rate at which the furnace cycles on and off per hour
Correct answer: The percentage of fuel energy converted to usable heat over a heating season
AFUE expresses what percentage of the fuel's energy content is converted into useful heat for the home over an entire heating season. A 95% AFUE furnace converts 95% of the gas energy to heat, wasting only 5% through exhaust.
Question 2: Which type of heating system uses water heated by a boiler and distributed through pipes to radiators or radiant floor tubing?
- Forced-air system
- Hydronic system (Correct answer)
- Heat pump system
- Gravity furnace system
Correct answer: Hydronic system
Hydronic heating systems use hot water as the heat transfer medium. A boiler heats the water, which is then pumped through a closed-loop system to baseboard radiators, panel radiators, or radiant floor tubing to deliver heat.
Question 3: What is the primary purpose of a heat exchanger in a gas furnace?
- To regulate the gas pressure entering the burner
- To separate combustion gases from the air distributed to the living space (Correct answer)
- To ignite the gas burner during startup
- To control the speed of the blower motor
Correct answer: To separate combustion gases from the air distributed to the living space
The heat exchanger is a metal barrier that allows heat from combustion to transfer to the supply air without mixing combustion byproducts (like carbon monoxide) with the circulated air. A cracked heat exchanger is a serious safety hazard.
Question 4: In a hot water boiler system, what is the function of an expansion tank?
- To store extra fuel for the burner during peak demand
- To accommodate the volume increase of water as it heats and expands (Correct answer)
- To increase water pressure when demand is high
- To filter sediment from the return water before it re-enters the boiler
Correct answer: To accommodate the volume increase of water as it heats and expands
Water expands as it heats up. The expansion tank (also called a compression tank) provides a cushion of air or a bladder to absorb this volume increase, preventing dangerously high pressure from building up in the closed-loop system.
Question 5: What type of ignition system has largely replaced standing pilot lights in modern gas furnaces?
- Piezoelectric ignition
- Electronic spark or hot surface ignition (Correct answer)
- Thermocouple ignition
- Glow plug ignition
Correct answer: Electronic spark or hot surface ignition
Modern furnaces use electronic ignition — either an intermittent spark igniter or a hot surface igniter (a silicon carbide or silicon nitride element that glows red-hot). These systems only activate at call for heat, eliminating the energy waste of a continuously burning pilot flame.
Question 6: What is the purpose of a modulating gas valve in a high-efficiency furnace?
- To shut off gas flow immediately if a flame failure is detected
- To vary the gas flow rate so the furnace can operate at partial capacity to match heating demand (Correct answer)
- To regulate gas pressure from the utility supply to burner pressure
- To allow the furnace to switch between natural gas and propane fuels
Correct answer: To vary the gas flow rate so the furnace can operate at partial capacity to match heating demand
A modulating valve allows the furnace to fire at varying percentages of its rated capacity (e.g., 40–100%) rather than only full-on or full-off. This closely matches heat output to actual demand, improving comfort, efficiency, and reducing temperature swings.
HVAC - Heating, Ventilation, and Air Conditioning Heating Systems 1
What does the Annual Fuel Utilization Efficiency (AFUE) rating measure in a gas furnace?