HVAC Electrical Controls and Safety 2

Question 1

What color is the common wire (C-wire) in a standard HVAC thermostat wiring scheme?

Accepted Answer

Blue or Black

Answer

The C-wire (common wire) is typically blue or black and provides the return path for the 24V control circuit, allowing smart thermostats to receive continuous power.

Question 2

In a standard thermostat wiring system, what does the 'Y' terminal control?

Accepted Answer

The compressor/cooling call

Answer

The Y terminal sends the cooling signal to the outdoor unit contactor coil, energizing the compressor and condenser fan when the thermostat calls for cooling.

Question 3

What type of motor is most commonly used for HVAC variable-speed blowers in high-efficiency systems?

Accepted Answer

ECM (Electronically Commutated Motor)

Answer

ECM (Electronically Commutated Motor) blowers are brushless DC motors that can vary speed precisely and are 60-70% more efficient than standard PSC motors.

Question 4

What is the purpose of a lockout/tagout (LOTO) procedure in HVAC service?

Accepted Answer

To isolate and de-energize equipment to prevent accidental startup during maintenance

Answer

LOTO procedures ensure that hazardous energy (electrical, mechanical, refrigerant pressure) is isolated and locked before service personnel work on equipment, preventing serious injury.

Question 5

What does a sequencer do in an electric furnace?

Accepted Answer

Staggers the energization of multiple heating elements to prevent large current spikes

Answer

A sequencer uses a bimetal element to introduce a time delay between energizing each heating strip, preventing all strips from turning on simultaneously and causing large current inrush.

HVAC - Heating, Ventilation, and Air Conditioning Practice Test

HVAC - Heating, Ventilation, and Air Conditioning Practice Test

HVAC Electrical Controls and Safety 2