Chapter 21 Quiz
Multiple Choice
1. The "deadband" on a thermostat refers to:
a) The area of the home that cannot be reached by HVAC airflow b) A temperature range around the set point within which the system stays off c) The time delay before the system starts after a call for heat or cool d) The minimum temperature drop before emergency heat activates
Answer: b — The deadband prevents constant on/off cycling by allowing temperature to drift slightly before the system responds.
2. The C wire in thermostat wiring stands for "Common" and its primary function is:
a) Cooling — it carries the signal to the compressor b) Providing a power return path so the thermostat can run its own electronics c) Connecting the thermostat to a central home automation hub d) Carrying the 120V power supply to the furnace
Answer: b — The C wire completes the 24V circuit, allowing smart thermostats to power their display, Wi-Fi, and scheduling features.
3. According to research and energy monitoring data, smart thermostats save money primarily through:
a) Learning algorithms that optimize system runtime automatically b) Wi-Fi connectivity that allows remote adjustment c) Temperature setback scheduling — running the system less when comfort isn't needed d) Advanced sensors that measure humidity and adjust set points accordingly
Answer: c — Setback scheduling is the mechanism behind virtually all thermostat energy savings. Learning and remote access are just more convenient ways to achieve the same setback.
4. On a standard thermostat wiring diagram, the G terminal controls:
a) The gas valve in the furnace b) The ground connection for electrical safety c) The blower fan, independently of heating or cooling d) The garage door opener integration
Answer: c — G energizes the air handler blower independently, allowing fan-only operation for ventilation.
5. The O terminal on a heat pump thermostat:
a) Energizes the outdoor unit compressor b) Is used only for oil furnace systems c) Energizes the reversing valve, typically putting the system in cooling mode d) Controls the auxiliary heating strips
Answer: c — O energizes the reversing valve for cooling in most brands (Carrier convention). B does the opposite (Rheem/Ruud convention).
6. A bypass damper in a zoning system is necessary because:
a) It allows conditioned air to bypass the filter, improving airflow b) Closing zone dampers increases duct static pressure, which the bypass damper relieves c) Heat pump systems require a separate airflow path for defrost operation d) It balances the temperature between floors in multi-story homes
Answer: b — When zone dampers close, the supply blower would build excessive pressure. The bypass damper opens to prevent this from damaging the blower and ductwork.
7. The most common reason a room is persistently cold in winter — even in a home with a functional HVAC system — is:
a) An insufficient number of zoning dampers b) The thermostat is programmed incorrectly c) Duct sizing, balance, or building envelope issues rather than the lack of zoning d) The furnace is not powerful enough for the home's square footage
Answer: c — Most room comfort complaints are duct, insulation, or air sealing problems. Adding a zone damper to an inadequately served room doesn't fix the underlying deficiency.
8. During heat pump defrost mode, the thermostat or control board activates the auxiliary heat strips because:
a) The outdoor temperature dropped below the balance point b) The compressor needs extra electrical power during defrost c) The system temporarily switches to cooling mode, which would blow cold air indoors d) The reversing valve requires higher voltage to switch direction
Answer: c — Defrost works by briefly running the system in reverse (cooling mode), which would send cold air through the ducts. Auxiliary strips maintain indoor air temperature during the brief defrost cycle.
9. Which of the following is a genuine red flag when hiring an HVAC contractor to install a new thermostat?
a) The contractor recommends a specific brand of thermostat b) The contractor asks about your current wiring before quoting c) The contractor offers to do the work without a permit to save you money d) The contractor charges $75–$150 for labor on a thermostat installation
Answer: c — Unpermitted work creates liability, bypasses inspection, and is a warning sign of unlicensed or uninsured work. The other options are normal professional behavior.
10. Ecobee's room sensor feature addresses a real limitation of single-thermostat systems by:
a) Controlling individual room dampers based on each sensor's reading b) Averaging temperature readings from multiple locations or prioritizing specific rooms c) Providing occupancy data that automatically opens and closes vents d) Sending real-time temperature alerts to your smartphone
Answer: b — Room sensors feed temperature data to the thermostat, which can average readings from multiple locations or weight the temperature of occupied rooms more heavily — addressing the core problem that a single thermostat can't know what's happening in the rest of the house.
Short Answer
11. You buy a new smart thermostat and discover there is no C wire at your thermostat location. Describe three different solutions to this problem, including their relative cost and complexity.
Model answer: (1) Check at the furnace — there may be an unused conductor in the existing cable already connected to C at the equipment end; connect it at the thermostat end for free. (2) Install an add-a-wire kit ($25), which encodes signals over existing wires, freeing up a conductor for C. (3) Use the manufacturer's power adapter kit ($20–$40), which steals power from the G circuit. (4) Run new 18/5 thermostat wire from furnace to thermostat ($0.15–$0.25/foot plus labor). Solutions 1 and 2 are DIY-appropriate; solution 4 may require professional help if walls are inaccessible.
12. Explain why a homeowner who installs a zoning system to solve cold bedroom complaints might be disappointed, and what they should have investigated first.
Model answer: Zoning systems control which zones receive conditioned air, but they can't fix inadequate airflow to a zone. If a bedroom is cold because its supply duct is undersized, poorly connected, or leaking before reaching the room, adding a zone damper elsewhere doesn't increase airflow to that room. The homeowner should first investigate: duct sizing and airflow balance (a professional can measure CFM at each register), duct leakage (a blower door test with duct pressurization), insulation levels in the exterior walls and ceiling of the cold room, and window condition. Fixing these root causes typically costs less and delivers better results than a zoning system.