HVAC Electrical Controls 5 — Questions and Answers
Question 1: A thermostat wired with an 'R', 'C', 'Y', 'G', and 'W' terminal set: which terminal, when energized alone, activates indoor fan only?
- Y
- W
- G (Correct answer)
- C
Correct answer: G
The G terminal energizes the indoor (air handler) fan relay, running the fan independently of heating or cooling calls.
Question 2: What is the purpose of a Time-Delay Relay (TDR) in a multi-stage commercial HVAC startup sequence?
- Limit voltage surges from the utility transformer
- Stage the starting of multiple loads to reduce peak demand current (Correct answer)
- Monitor phase loss on three-phase systems
- Automatically adjust setpoints based on outdoor temperature
Correct answer: Stage the starting of multiple loads to reduce peak demand current
TDRs introduce a delay between starting multiple large loads (compressors, fans) so their inrush currents don't overlap, reducing peak demand charges and stress on wiring.
Question 3: An electronic expansion valve (EEV) is controlled by which type of signal from the controller?
- Analog 4–20 mA only
- Pulse-width modulated (PWM) or stepper motor pulses (Correct answer)
- On/off relay contact
- 120V AC direct
Correct answer: Pulse-width modulated (PWM) or stepper motor pulses
EEVs use a stepper motor driven by PWM pulses from the controller, allowing precise modulation of refrigerant flow in small incremental steps.
Question 4: Which regulation requires HVAC electrical work to follow specific wiring methods, box fill calculations, and grounding rules?
- ASHRAE 90.1
- NFPA 70 (National Electrical Code) (Correct answer)
- EPA Section 608
- UL 1995
Correct answer: NFPA 70 (National Electrical Code)
NFPA 70 (NEC) governs all electrical installations including HVAC equipment wiring, disconnect sizing, conductor ampacity, and grounding requirements.
Question 5: A technician measures 26V AC at the transformer secondary but only 18V AC at the contactor coil. The most likely cause is:
- Overloaded control circuit drawing excessive current through wiring resistance (Correct answer)
- Transformer primary voltage is too high
- Contactor coil is open
- Low refrigerant charge
Correct answer: Overloaded control circuit drawing excessive current through wiring resistance
Excessive current draw in the control circuit (from added loads or a partially-shorted coil) creates voltage drop across the wiring, lowering voltage at the load.
Question 6: In a demand-controlled ventilation (DCV) system, CO2 sensors are used to:
- Detect combustion gas leaks in the heat exchanger
- Modulate outdoor air dampers based on occupancy levels (Correct answer)
- Control the speed of exhaust fans to maintain static pressure
- Monitor refrigerant concentration in case of a leak
Correct answer: Modulate outdoor air dampers based on occupancy levels
DCV uses CO2 as a proxy for occupancy; as CO2 rises above setpoint (typically 1100 ppm), the controller opens outdoor air dampers to dilute contaminants and provide fresh air.
Question 7: A normally-closed (NC) low-pressure switch in a refrigeration system opens when suction pressure drops too low. This action typically:
- Energizes a warning light only
- De-energizes the compressor contactor coil, stopping the compressor (Correct answer)
- Switches the system to heating mode
- Opens the liquid line solenoid to add refrigerant
Correct answer: De-energizes the compressor contactor coil, stopping the compressor
When the NC low-pressure switch opens due to low suction pressure, it breaks the control circuit to the contactor coil, de-energizing the contactor and stopping the compressor.
HVAC Electrical Controls 5
A thermostat wired with an 'R', 'C', 'Y', 'G', and 'W' terminal set: which terminal, when energized alone, activates indoor fan only?