313D System Installation and Maintenance 3 — Questions and Answers
Question 1: What does a micron gauge measure during the evacuation of an air conditioning system?
- Refrigerant charge level
- Absolute pressure in the system (Correct answer)
- Voltage at the compressor
- Airflow through the evaporator coil
Correct answer: Absolute pressure in the system
A micron gauge measures absolute pressure in microns of mercury (µm Hg), allowing technicians to verify that the system has been evacuated to an acceptable vacuum level.
Question 2: A residential AC system is short-cycling on high pressure. Which maintenance action should the technician perform FIRST?
- Replace the compressor
- Check and clean the condenser coil for blockage or dirt (Correct answer)
- Add refrigerant to the system
- Replace the high-pressure switch
Correct answer: Check and clean the condenser coil for blockage or dirt
A dirty or blocked condenser coil reduces heat rejection, causing high-side pressure to rise and the system to trip on the high-pressure limit.
Question 3: When performing a seasonal startup on a residential central air system, which task should be completed BEFORE energizing the system?
- Recording supply air temperatures
- Checking the disconnect and inspecting wiring for damage (Correct answer)
- Measuring subcooling at the condenser outlet
- Adjusting the thermostat setpoint
Correct answer: Checking the disconnect and inspecting wiring for damage
Before energizing any system at startup, technicians must inspect the disconnect, electrical connections, and wiring to identify any damage that could cause a hazardous condition.
Question 4: A crankcase heater on a residential compressor is used primarily for what purpose?
- To heat the refrigerant before it enters the evaporator
- To prevent refrigerant migration and liquid slugging during the off cycle (Correct answer)
- To defrost the outdoor coil during heating mode
- To warm the suction line during low-ambient operation
Correct answer: To prevent refrigerant migration and liquid slugging during the off cycle
The crankcase heater keeps the compressor oil warm during the off cycle to prevent refrigerant from migrating into and mixing with the compressor oil, which could cause liquid slugging on startup.
Question 5: When installing ductwork for a new residential AC system, what is the purpose of using mastic sealant instead of standard duct tape?
- Mastic is easier to apply in tight spaces
- Mastic provides a durable, long-lasting airtight seal that duct tape cannot maintain over time (Correct answer)
- Mastic is required only on return ducts, not supply ducts
- Mastic improves thermal insulation of the ducts
Correct answer: Mastic provides a durable, long-lasting airtight seal that duct tape cannot maintain over time
Mastic sealant creates a permanent, flexible, airtight bond at duct joints, whereas standard duct tape degrades and loses adhesion over time, allowing air leaks.
Question 6: During routine maintenance, a technician measures the temperature difference between return air and supply air (delta T). For a properly operating residential cooling system, what is the typical acceptable delta T range?
- 5–8°F
- 10–12°F
- 16–22°F (Correct answer)
- 28–35°F
Correct answer: 16–22°F
A properly operating residential cooling system typically produces a 16–22°F temperature difference between return and supply air under normal operating conditions.
Question 7: What is the function of a filter drier in a residential air conditioning system?
- To filter combustion byproducts from the air stream
- To remove moisture and contaminants from the refrigerant circuit (Correct answer)
- To regulate refrigerant flow into the evaporator
- To separate oil from the refrigerant before the compressor
Correct answer: To remove moisture and contaminants from the refrigerant circuit
A filter drier contains desiccant material that removes moisture, acid, and particulate contaminants from the refrigerant to protect system components.
313D System Installation and Maintenance 3
What does a micron gauge measure during the evacuation of an air conditioning system?