Chapter 18 Quiz: How Heating Systems Work

Part A: Multiple Choice

Circle the best answer for each question.

1. A heat exchanger in a gas furnace serves which primary purpose?

A) It stores heat energy to allow the blower to start immediately B) It transfers heat from combustion gases to household air while keeping the two streams physically separated C) It measures the temperature of the combustion gases for safety shutoff D) It condenses water from combustion gases to improve efficiency

2. A cracked heat exchanger is dangerous primarily because:

A) It allows cold air to enter the combustion chamber, causing inefficiency B) It can cause the furnace to overheat and start a fire C) It allows combustion gases, including carbon monoxide, to enter the household airstream D) It reduces system efficiency and causes higher gas bills

3. Which of the following is NOT a warning sign of a potential heat exchanger problem?

A) Yellow or orange burner flame on a gas furnace B) The blower running for one to two minutes after the burner shuts off C) Visible soot marks around the burner compartment D) CO detector activating when the furnace is running

4. A condensing furnace differs from a standard furnace in that:

A) It uses a larger heat exchanger made of stainless steel instead of aluminized steel B) It captures additional heat from exhaust gases until water vapor condenses, achieving 90–98% AFUE C) It condenses refrigerant to provide cooling as well as heating D) It requires a larger gas supply line than standard furnaces

5. In a hot water boiler system, what is the purpose of the expansion tank?

A) It provides additional water storage for pressure-washing applications B) It contains extra hot water for periods of peak demand C) It absorbs the volume increase of water when heated, preventing dangerous pressure buildup D) It equalizes pressure between different zone loops

6. The "balance point" of a heat pump refers to:

A) The temperature at which the refrigerant charge is considered correct B) The outdoor temperature below which the heat pump alone cannot meet the heating load and auxiliary heat is needed C) The point at which heating and cooling capacity are equal D) The thermostat setting that minimizes energy consumption

7. Dave Kowalski's rural home is heated with propane. One critical safety difference between propane and natural gas is:

A) Propane is non-flammable under normal conditions, while natural gas is highly flammable B) Propane is heavier than air and will accumulate in low spaces, while natural gas is lighter and rises C) Propane requires a larger combustion chamber than natural gas D) Propane does not have an added odorant, making leaks undetectable by smell

8. Why is an aggressive thermostat setback (large overnight temperature drop) generally NOT recommended for heat pump systems?

A) Heat pumps cannot reduce temperature; they can only maintain or increase setpoints B) Recovering from a large setback may require the expensive auxiliary heat to run for an extended period C) Heat pump controls are not compatible with programmable thermostats D) Large temperature swings damage the reversing valve in the heat pump

9. Electric baseboard heating is described as 100% efficient. This means:

A) It heats the room faster than any other system B) Every unit of electrical energy consumed is converted directly to heat — no combustion losses C) It costs less to operate than a 95% AFUE gas furnace D) It qualifies for federal energy efficiency tax credits

10. The hot surface igniter in a modern gas furnace:

A) Burns continuously as a standing pilot to ignite the main burner when called B) Is a ceramic element that glows hot when powered, replacing the standing pilot light C) Generates a high-voltage spark to ignite the main burner each heating cycle D) Monitors flame status and shuts off gas if ignition fails

Part B: Short Answer

Answer each question in 3–5 sentences.

11. A homeowner has a 1988 gas furnace with an AFUE of 70%. They are debating whether to repair it again (cost: $800) or replace it with a new 96% AFUE unit (cost: $4,000). Their annual gas heating bill is $1,600. What factors should guide their decision, and what does the math suggest?

12. Explain in plain language how a heat pump extracts heat from cold outdoor air in winter. Why is a heat pump's performance measured differently from a furnace's efficiency?

13. You arrive home to find your CO detector alarming. You suspect the gas furnace is the source. What do you do, and why?

14. Describe the difference between conduction, convection, and radiation as they apply to residential heating. Give one example of each from a real heating system.

15. What is the difference between a heat pump defrost cycle and a malfunction? How can a homeowner tell the difference?

Answer Key

Part A: 1. B 2. C 3. B 4. B 5. C 6. B 7. B 8. B 9. B 10. B

Part B — Model Answers:

11. The math: savings from upgrading = ($1,600 × (96−70)/96) = approximately $433/year. At $4,000 minus $800 = $3,200 net investment, payback is roughly 7.4 years. However, a 36-year-old furnace is likely near the end of its safe service life. Investing $800 into equipment that may fail within a few years is poor economics, and the risk of a heat exchanger failure — which is essentially guaranteed at this age — makes continued operation a safety gamble. Replacement is the right answer.

12. A heat pump doesn't generate heat; it uses refrigerant to absorb heat energy from outdoor air (which still contains thermal energy even at cold temperatures) and deliver it inside at higher temperature through the compression process. Because it's moving heat rather than generating it, a heat pump can deliver 2–4 units of heat per unit of electricity consumed — performance measured as COP (coefficient of performance) rather than efficiency percentage. The maximum efficiency of a furnace is 100%; a heat pump can appear to be 200–400% efficient because it's harvesting ambient energy.

13. Evacuate everyone (including pets) immediately. Do not stop to open windows or turn switches on or off — even a light switch can arc and create a spark, though CO is not flammable. Call 911 and the gas company from outside the home. Do not re-enter until emergency responders clear the building. CO is potentially lethal; evacuation is not optional and should not wait for confirmation.

14. Conduction is heat moving through physical contact between materials — the cast-iron heat exchanger wall conducting heat from combustion gases on one side to household air on the other. Convection is heat moved by a fluid — the furnace blower (forced convection) pushes air over the hot heat exchanger and carries heat into the house through ducts. Radiation is heat transferred as electromagnetic waves without any medium — a radiant electric baseboard heats objects and people in the room directly by infrared radiation rather than warming the air first.

15. A defrost cycle occurs when frost accumulates on the outdoor coil at temperatures between about 25°F and 45°F in humid conditions. The heat pump briefly reverses to cooling mode to melt the frost; you'll see steam from the outdoor unit and may notice a brief cool air discharge inside. This is normal and lasts only a few minutes. A malfunction looks like: the outdoor unit completely encased in ice that never melts, frost that builds continuously regardless of weather, or ice that extends down the refrigerant lines to the indoor unit. Malfunction requires professional attention; defrost cycles do not.