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HVAC Excellence Master LC-AC practice questions

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Question 1Air Conditioning

A technician finds a split-type air conditioning system operating with a suction line that is sweating but a suction pressure well below the manufacturer's chart for the measured indoor wet-bulb. Which condition best explains this combination?

  1. An overcharge of refrigerant raising head pressure above design
  2. A failed condenser fan motor reducing heat rejection at the outdoor coil
  3. A restricted liquid line filter drier creating a pressure drop across the drier
  4. An undercharge of refrigerant causing low evaporator pressure while the line still condenses moisture Correct answer.

Explanation

Low suction pressure with a cold, sweating suction line points to insufficient refrigerant mass flow through the evaporator, so the coil runs colder than normal while pressure drops. An undercharge reduces both saturated suction temperature and capacity, and the line can still sweat if it is below dew point.

Question 2Air Conditioning

When charging a system by the superheat method, which measurement pair is required at the evaporator outlet?

  1. Compressor current draw and supply voltage
  2. Liquid line temperature and discharge pressure converted to saturation temperature
  3. Outdoor ambient dry-bulb and indoor return air dry-bulb
  4. Suction line temperature and suction line pressure converted to saturation temperature Correct answer.

Explanation

Superheat is the difference between the actual suction line temperature and the saturated evaporating temperature derived from suction pressure. Both the line temperature and the pressure-to-temperature conversion must be taken at the same point, normally the evaporator outlet.

Question 3Air Conditioning

A fixed-orifice residential split system shows a suction superheat of 2°F at the evaporator outlet with a normal indoor load. What is the most likely consequence if left unchanged?

  1. The metering orifice will become oversized over time
  2. The evaporator will starve and lose capacity
  3. The condenser will run colder than ambient
  4. Liquid refrigerant may flood back and wash oil from the compressor Correct answer.

Explanation

Very low superheat means refrigerant is not fully vaporized before leaving the evaporator, so liquid can reach the compressor. Floodback dilutes lubricating oil, reduces lubrication, and can damage valves and bearings.

Question 4Air Conditioning

Which statement correctly describes subcooling as applied to an air conditioning condenser?

  1. It is the temperature rise of air across the condenser coil
  2. It is the difference between the saturated condensing temperature and the liquid line temperature Correct answer.
  3. It is the difference between the suction line temperature and the evaporating temperature
  4. It is the pressure drop measured between the compressor discharge and the condenser inlet

Explanation

Subcooling measures how far the liquid refrigerant has been cooled below its condensing temperature before leaving the condenser. It is calculated from condensing pressure converted to saturation temperature minus the measured liquid line temperature.

Question 5Air Conditioning

A technician measures 18°F of subcooling on a TXV system with a clean condenser and correct airflow. Which condition does this most strongly suggest?

  1. A failed compressor valve
  2. Refrigerant overcharge or a restriction backing liquid up in the condenser Correct answer.
  3. Low indoor airflow across the evaporator
  4. Refrigerant undercharge

Explanation

Subcooling rises when excess liquid refrigerant accumulates in the bottom of the condenser, which happens with overcharge or a restriction downstream that limits liquid flow. High subcooling with normal airflow and a clean coil is a classic overcharge signature.

Question 6Air Conditioning

In a capillary tube system, what is the primary function of the capillary tube?

  1. To remove moisture from the refrigerant circuit
  2. To control condenser fan speed
  3. To meter refrigerant flow and maintain a pressure difference between the high and low sides Correct answer.
  4. To store excess refrigerant during low-load operation

Explanation

The capillary tube is a fixed restriction that drops refrigerant pressure from condensing to evaporating pressure and regulates flow. Its bore and length are selected for the design charge and load of the system.

Question 7Air Conditioning

A thermostatic expansion valve with an external equalizer is installed on a distributor-fed evaporator. Why is the equalizer line necessary?

  1. It vents refrigerant to the atmosphere to limit maximum pressure
  2. It supplies additional refrigerant to the bulb for faster response
  3. It allows the valve to act as a check valve during the off cycle
  4. It compensates the valve opening for pressure drop through the evaporator and distributor Correct answer.

Explanation

An externally equalized TXV senses evaporator outlet pressure through the equalizer line so the pressure drop across the coil and distributor does not bias the valve toward overfeeding. It keeps the superheat setting accurate on larger or distributor-fed coils.

Question 8Air Conditioning

Which component in a residential split system is responsible for removing moisture and particulate contamination from the refrigerant before it reaches the metering device?

  1. The suction accumulator
  2. The muffler
  3. The reversing valve
  4. The liquid line filter drier Correct answer.

Explanation

The filter drier is installed in the liquid line and contains a desiccant plus a filter screen to trap moisture and debris. Keeping the circuit clean and dry protects the metering device and compressor.

Question 9Air Conditioning

During a cooling call, the compressor contactor coil is energized but the compressor does not start and the overload trips repeatedly. The capacitor capacitance measures within tolerance. Which check is most appropriate next?

  1. Add refrigerant to raise suction pressure
  2. Increase the indoor blower speed
  3. Replace the thermostat to rule out a control fault
  4. Measure voltage at the compressor terminals and verify the start components and winding resistances Correct answer.

Explanation

With the contactor pulled in, the technician must confirm that full voltage reaches the compressor and that start components and windings are intact. Repeated overload trips usually indicate a start circuit, capacitor, or winding problem rather than a refrigerant issue.

Question 10Air Conditioning

A run capacitor on a permanent split capacitor compressor reads 0 microfarads on a meter, but the compressor still occasionally starts. What is the most likely effect on the motor if the capacitor is not replaced?

  1. Lower current draw and cooler operation
  2. Increased winding current and overheating leading to eventual failure Correct answer.
  3. Higher suction pressure with normal current
  4. No effect because the capacitor only suppresses electrical noise

Explanation

The run capacitor shifts current in the auxiliary winding to create starting and running torque; without it the motor draws higher current to produce the same torque. Sustained overcurrent overheats the windings and shortens compressor life.

Question 11Air Conditioning

What is the purpose of the accumulator in a heat pump or air conditioner using a fixed metering device?

  1. To increase the subcooling at the condenser outlet
  2. To store liquid refrigerant for the condenser during high load
  3. To bypass the metering device during defrost
  4. To prevent liquid refrigerant from entering the compressor during low-load or off-cycle conditions Correct answer.

Explanation

The accumulator is a suction-line vessel that allows any liquid to boil off and oil to return at a controlled rate before vapor reaches the compressor. It protects the compressor from slugging when evaporator load is low or refrigerant migrates during the off cycle.

Question 12Air Conditioning

An R-410A system has a suction pressure of 118 psig. Using a pressure-temperature chart where 118 psig corresponds to 40°F saturation, and a measured suction line temperature of 52°F, what is the superheat?

  1. 12°F Correct answer.
  2. 64°F
  3. 52°F
  4. 40°F

Explanation

Superheat equals measured suction line temperature minus the saturated suction temperature from the chart. Here 52°F minus 40°F gives 12°F of superheat.

Question 13Air Conditioning

A technician measures a liquid line temperature of 95°F and a condensing saturation temperature of 115°F. What is the subcooling?

  1. 95°F
  2. 210°F
  3. 20°F Correct answer.
  4. 10°F

Explanation

Subcooling is the condensing saturation temperature minus the liquid line temperature. Subtracting 95°F from 115°F yields 20°F of subcooling.

Question 14Air Conditioning

A customer reports that the indoor coil ices over after about 30 minutes of operation, then thaws when the system is off. Which set of conditions should the technician verify first?

  1. Thermostat programming schedule and setpoint
  2. Condensate drain slope and trap depth
  3. Outdoor fan blade pitch and condenser pad level
  4. Indoor airflow, filter condition, and refrigerant charge Correct answer.

Explanation

Evaporator frosting results from coil temperature dropping below freezing, usually caused by low airflow or low refrigerant charge. Checking the filter, blower, ductwork, and charge addresses the most common causes first.

Question 15Air Conditioning

A system has correct charge and clean coils, but the evaporator is starved because the TXV is stuck nearly closed. Which symptom pattern is expected?

  1. High superheat, low suction pressure, and low evaporator capacity Correct answer.
  2. Normal superheat with high head pressure
  3. Low superheat, high suction pressure, and liquid floodback
  4. High subcooling with normal suction pressure

Explanation

A TXV stuck closed restricts refrigerant into the evaporator, so the coil is starved and superheat climbs while suction pressure falls. Capacity drops because less refrigerant is evaporating in the coil.

Question 16Air Conditioning

Why must a refrigerant line set be evacuated to a deep vacuum before charging a new split system?

  1. To increase the system charge capacity
  2. To calibrate the manifold gauge set
  3. To remove moisture and non-condensable gases that would damage the system Correct answer.
  4. To test the compressor's running current

Explanation

Evacuation boils off moisture and removes air that would otherwise raise head pressure and form acids with refrigerant and oil. A deep vacuum verified with a micron gauge confirms the circuit is dry and leak-free.

Question 17Air Conditioning

A technician pulls a vacuum on a system and the micron gauge rises steadily after the pump is isolated. Which conclusion is most reasonable?

  1. The system is fully dehydrated and ready to charge
  2. The micron gauge is reading in the wrong units
  3. The vacuum pump is too large for the system
  4. There is a leak or residual moisture continuing to boil off in the circuit Correct answer.

Explanation

A standing vacuum that rises after isolation indicates vapor is entering or being generated inside the system, typically from a leak or moisture still evaporating. A tight, dry system holds its vacuum level.

Question 18Air Conditioning

Which practice is correct when using the superheat method to charge a system with a fixed orifice?

  1. Charge until the liquid line is warmer than ambient
  2. Charge to the highest suction pressure the compressor will tolerate
  3. Charge until the sight glass is completely clear of bubbles at all times
  4. Charge to the target superheat from the manufacturer's chart using indoor wet-bulb and outdoor dry-bulb Correct answer.

Explanation

Fixed-orifice systems require the target superheat chart because the correct value changes with indoor wet-bulb and outdoor dry-bulb conditions. The technician adjusts charge until measured superheat matches the chart value.

Question 19Air Conditioning

A heat pump in cooling mode has a reversing valve solenoid that is not energized. What is the expected result?

  1. The compressor will not run at all
  2. The indoor blower will stop
  3. The outdoor fan will run in reverse
  4. The system will operate in heating mode instead of cooling Correct answer.

Explanation

In most heat pump designs the reversing valve is energized for cooling, so a failed solenoid or control signal leaves the valve in the heating position. The compressor may still run, but the system delivers heat rather than cooling.

Question 20Air Conditioning

What is the function of the defrost control in an air-source heat pump?

  1. To limit compressor current during startup
  2. To switch the system between occupied and unoccupied schedules
  3. To maintain a constant indoor humidity setpoint
  4. To periodically reverse the cycle or stop the outdoor fan so frost melts from the outdoor coil Correct answer.

Explanation

In heating mode the outdoor coil runs below ambient and can frost, so the defrost control initiates a temporary reversal to heating the outdoor coil or otherwise melts frost. It terminates defrost based on temperature, pressure, or time.

Question 21Air Conditioning

A technician is checking airflow on a system with a PSC blower motor and measures excessive external static pressure. Which condition is the most likely cause?

  1. A refrigerant undercharge
  2. A restrictive filter or undersized ductwork Correct answer.
  3. A failed run capacitor on the condenser fan
  4. A miswired thermostat

Explanation

External static pressure reflects resistance in the return and supply duct systems plus the filter and coil. High static pressure with a PSC motor usually means restricted airflow from a dirty filter, tight ductwork, or excessive fittings.

Question 22Air Conditioning

Why is a compressor crankcase heater used on a residential air conditioning system?

  1. To preheat the indoor coil before a cooling call
  2. To raise suction pressure at startup
  3. To reduce the condenser approach temperature
  4. To keep oil warm so refrigerant does not migrate and dilute it during the off cycle Correct answer.

Explanation

During the off cycle refrigerant can condense in the compressor crankcase and mix with oil, reducing lubrication at startup. A crankcase heater keeps the oil warm enough to discourage migration and drive off refrigerant.

Question 23Air Conditioning

A technician must verify that a condenser coil is clean and properly rejecting heat. Which measurement set best supports this conclusion?

  1. Condenser entering and leaving air temperature difference plus condensing saturation temperature compared with ambient Correct answer.
  2. Suction superheat and indoor wet-bulb only
  3. Liquid line pressure drop across the filter drier only
  4. Compressor current and supply voltage only

Explanation

Condenser performance is confirmed by comparing condensing saturation temperature to outdoor ambient and by checking the air temperature rise across the coil. A large approach or low air rise suggests a dirty coil, low airflow, or recirculation.

Question 24Air Conditioning

A technician finds a system with a blocked condensate drain that has overflowed into the return duct. Which immediate action is most appropriate?

  1. Shut down the system, clear the drain, and verify the safety switch and trap Correct answer.
  2. Increase blower speed to dry the duct
  3. Add refrigerant to compensate for lost capacity
  4. Reset the compressor overload and restart

Explanation

Standing water in a duct creates biological growth and equipment damage, so the system should be stopped and the drain cleared. The float or overflow safety switch and trap should then be verified to prevent recurrence.

Question 25Air Conditioning

A technician suspects a liquid line restriction downstream of the condenser. Which pair of observations best supports that diagnosis?

  1. Equal temperatures on both sides of the restriction with normal subcooling
  2. A temperature rise across the restriction with low subcooling
  3. A temperature drop across the restriction with high subcooling upstream and low superheat downstream Correct answer.
  4. A pressure increase across the restriction in the direction of flow

Explanation

A restriction produces a pressure and temperature drop across it, so the liquid ahead of the restriction backs up and subcooling rises. Downstream, less liquid reaches the evaporator, which can show unusual superheat depending on the metering device.

Question 26Air Conditioning

On a split air conditioning system, what does a reading of 20°F of evaporator superheat with a fixed orifice metering device indicate about the refrigerant charge?

  1. The charge is likely correct because any positive superheat is acceptable
  2. The charge is likely low because the evaporator is not being fully flooded Correct answer.
  3. The charge cannot be evaluated without first measuring subcooling
  4. The charge is likely high because superheat above 12°F signals overcharge

Explanation

With a fixed orifice, superheat is the primary charging indicator, and target values at typical indoor conditions generally run in the 8°F to 12°F range. A superheat of 20°F means the refrigerant is fully vaporized well before the evaporator outlet, so the coil is starved and charge is likely low. Subcooling is a secondary check on fixed-orifice systems.

Question 27Air Conditioning

Which condition is the normal, intended state of refrigerant entering the compressor of a properly operating air conditioning system?

  1. A superheated vapor with enough superheat to protect the compressor Correct answer.
  2. A two-phase mixture with a quality of about 50 percent
  3. A saturated liquid-vapor mixture at evaporator pressure
  4. A subcooled liquid at condensing pressure

Explanation

Compressors are designed to pump vapor only, and a small amount of superheat at the suction ensures no liquid droplets reach the valves or scroll set. Liquid slugging dilutes oil and can cause mechanical damage. The metering device and evaporator are sized so the refrigerant is fully vaporized before leaving the coil.

Question 28Air Conditioning

A technician measures 12°F of subcooling on an R-410A system with a TXV and a nameplate target of 10°F. What does this reading most directly indicate?

  1. The liquid line contains flash gas from a restriction
  2. The condenser is rejecting heat adequately and the charge is close to target Correct answer.
  3. The suction line is picking up excess heat before the compressor
  4. The evaporator is starved and the TXV is stuck closed

Explanation

Subcooling measures how far the liquid line is below condensing temperature, which reflects how much liquid is stacked in the condenser. On a TXV system, subcooling is the primary charge indicator, and 12°F against a 10°F target is within normal tolerance. Flash gas or a starved evaporator would produce very different symptoms.

Question 29Air Conditioning

What is the function of the suction line accumulator in a system using a fixed orifice metering device?

  1. It meters refrigerant flow into the evaporator during low load
  2. It stores liquid refrigerant to supplement the charge during high load
  3. It separates oil from refrigerant so oil can be drained to the condenser
  4. It traps and slowly returns any liquid refrigerant that reaches the suction line Correct answer.

Explanation

An accumulator is a vessel on the suction line that allows liquid refrigerant to boil off and be returned as vapor, protecting the compressor during off-cycle migration or flooding. It is standard on fixed-orifice systems where superheat control is less precise. Metering and oil management are handled by other components.

Question 30Air Conditioning

A technician is preparing to braze refrigerant lines on a new split system. Which practice best protects the system's internal surfaces during the process?

  1. Wet the fittings with refrigerant oil before applying heat
  2. Flow dry nitrogen through the tubing while brazing and cap open ends Correct answer.
  3. Purge with compressed air from a shop compressor during brazing
  4. Use a high-silver rod without flux and rely on the alloy for protection

Explanation

Brazing in air forms copper oxide scale on the inside of the tubing, and that scale later migrates to metering devices, valves, and compressor bearings. A slow dry nitrogen purge displaces oxygen and prevents scale formation. Open ends must also be capped to keep out moisture and debris.

Question 31Air Conditioning

Which instrument is required to determine evaporator superheat on a system in operation?

  1. A vacuum gauge plus a micron-level leak detector
  2. A manifold gauge set plus a thermometer on the suction line at the evaporator outlet Correct answer.
  3. A hygrometer plus an anemometer at the return grille
  4. A voltmeter plus a clamp-on ammeter at the compressor

Explanation

Superheat is the difference between the measured suction line temperature and the saturation temperature corresponding to suction pressure. That requires both a pressure reading and a temperature reading taken at the same location, normally the evaporator outlet. Electrical and airflow instruments do not provide these values.

Question 32Air Conditioning

In a residential split system, which condition most commonly causes the indoor coil to ice during a cooling call?

  1. Restricted airflow across the evaporator combined with low refrigerant charge Correct answer.
  2. A condenser fan motor drawing slightly high current
  3. A dirty outdoor coil raising condensing temperature
  4. An oversized return duct reducing static pressure

Explanation

Evaporator icing occurs when coil temperature falls below freezing, which happens when airflow is too low to carry heat to the coil or when low charge reduces the boiling temperature. Reduced airflow from a clogged filter, dirty coil, or failing blower is a frequent cause. Condenser-side problems raise head pressure instead.

Question 33Air Conditioning

What is the primary purpose of a crankcase heater on a residential air conditioning compressor?

  1. To keep refrigerant from condensing and diluting oil in the crankcase during the off cycle Correct answer.
  2. To warm the suction line and increase superheat at the compressor
  3. To raise suction pressure at startup so the compressor loads faster
  4. To reduce head pressure during low ambient cooling operation

Explanation

During the off cycle, refrigerant can migrate to the coldest part of the system, which is often the compressor crankcase, where it condenses and dilutes the oil. A crankcase heater keeps the oil warm enough to prevent that migration and to boil off any liquid present before startup. It does not control suction or head pressure.

Question 34Air Conditioning

A technician measures a suction pressure of 68 psig on an R-22 system, which corresponds to a saturation temperature of 40°F. The suction line temperature at the evaporator outlet is 52°F. What is the superheat?

  1. 12°F Correct answer.
  2. 52°F
  3. 28°F
  4. 40°F

Explanation

Superheat is the measured suction line temperature minus the saturation temperature for the measured suction pressure. Here 52°F minus 40°F equals 12°F. The 68 psig and the 40°F are the same physical condition expressed in different units and must not be added or compared directly.

Question 35Air Conditioning

A technician reads a liquid line temperature of 100°F and a condensing saturation temperature of 120°F. What is the subcooling, and what does it suggest on a TXV system?

  1. 20°F, which is above typical target and suggests an overcharge or liquid line restriction Correct answer.
  2. 20°F, which is below typical target and suggests an undercharge
  3. 120°F, which indicates the condenser is operating normally
  4. 80°F, which suggests the condenser is not rejecting heat

Explanation

Subcooling equals condensing saturation temperature minus liquid line temperature, so 120°F minus 100°F is 20°F. Typical targets run near 10°F to 15°F, so 20°F suggests excess liquid backing up in the condenser from overcharge or a restriction downstream. High subcooling with normal head pressure often points to a restriction.

Question 36Air Conditioning

A technician finds a system with normal suction pressure, normal head pressure, and correct superheat, but the space never reaches setpoint. Which additional measurement set is most appropriate to investigate?

  1. Compressor winding resistance and capacitor microfarads
  2. Refrigerant sight glass moisture indicator color
  3. Temperature drop across the evaporator coil and total external static pressure Correct answer.
  4. Condenser fan blade pitch and motor horsepower

Explanation

When refrigerant-side readings are normal, the remaining likely cause of poor capacity is an air-side problem such as low airflow, duct leakage, or a bypassing coil. Measuring the air temperature drop across the coil and the external static pressure evaluates airflow and coil performance. Electrical checks would be indicated only if the compressor were not running properly.

Question 37Air Conditioning

A technician is evacuating a new split system and the micron gauge holds at 480 microns after the pump is isolated for 20 minutes. What is the correct interpretation?

  1. The system is adequately evacuated and dry enough for charging Correct answer.
  2. The micron gauge is faulty because it did not rise at all
  3. The vacuum pump is undersized and the process must be restarted
  4. The system still contains a large amount of moisture that must be boiled off

Explanation

A standing vacuum test that holds below about 500 microns with no significant rise indicates the system is dry and leak-free. A steady rise would indicate a leak or continued moisture vaporization. Holding steady confirms the evacuation was successful.

Question 38Air Conditioning

A technician measures a 30°F temperature rise across the condenser coil on an R-410A system. Which conclusion is most consistent with this reading?

  1. The compressor is not running and only the fan is operating
  2. The condenser is rejecting heat within a reasonable range for typical conditions Correct answer.
  3. The condenser is flooded with liquid and the system is overcharged
  4. The condenser coil is bypassing air and the system is undercharged

Explanation

Air temperature rise across an air-cooled condenser typically falls in the range of roughly 15°F to 30°F depending on ambient and design. A 30°F rise is at the upper end but still plausible with a warm ambient or dirty coil. Refrigerant-side readings would be needed to confirm charge, so the reading alone does not prove overcharge or undercharge.

Question 39Air Conditioning

A technician is diagnosing a system with a suction pressure well below normal, high superheat at the evaporator outlet, and a frost pattern beginning at the metering device. Which cause fits these observations?

  1. A restriction at the inlet screen of the metering device Correct answer.
  2. A compressor with worn valves
  3. An overcharge of refrigerant in the system
  4. A failed condenser fan motor

Explanation

A restriction at the metering device inlet starves the evaporator, lowering suction pressure and raising superheat, and the pressure drop across the restriction produces localized frosting. Overcharge and condenser problems raise head pressure and subcooling instead. Worn compressor valves typically reduce capacity while raising suction pressure.

Question 40Air Conditioning

A technician must verify that the condenser coil on an operating unit is clean and rejecting heat properly. Which measurement set best supports that conclusion?

  1. Return air temperature, supply air temperature, and filter pressure drop
  2. Condensing saturation temperature, outdoor ambient temperature, and liquid line temperature Correct answer.
  3. Compressor current, supply voltage, and capacitor microfarads
  4. Suction pressure, suction line temperature, and indoor wet bulb temperature

Explanation

Condenser performance is judged by comparing condensing saturation temperature to outdoor ambient and by checking subcooling from the liquid line temperature. A condensing temperature more than about 20°F to 30°F above ambient suggests poor heat rejection. Suction-side and electrical readings describe different parts of the system.

Question 41Air Conditioning

A technician suspects that a TXV is not modulating properly because superheat swings widely between 2°F and 25°F during steady operation. Which condition best explains this behavior?

  1. A hunting valve with a sensing bulb that is poorly clamped or improperly located Correct answer.
  2. A condenser coil that is partially plugged
  3. A compressor that is short cycling on the overload
  4. A completely blocked liquid line filter drier

Explanation

TXV hunting is oscillation of superheat caused by unstable bulb sensing, such as poor thermal contact, incorrect bulb orientation, or a bulb exposed to the wrong temperature. A blocked drier would produce consistently high superheat rather than swings. Condenser and compressor issues would show different primary symptoms.

Question 42Air Conditioning

A heat pump is operating in the cooling mode and the reversing valve solenoid is de-energized. What is the expected result on a valve that is de-energized in cooling?

  1. The outdoor fan stops and the indoor blower continues
  2. The system continues to cool because the valve is in its normal cooling position Correct answer.
  3. The compressor locks out on high pressure immediately
  4. The system reverses to heating because the solenoid must be energized to cool

Explanation

Most residential reversing valves are configured so the de-energized position is cooling, and the solenoid is energized to switch to heating. Therefore a de-energized solenoid during a cooling call does not by itself cause a mode reversal. The valve position must match the manufacturer's design, which is why the normal position is confirmed against the wiring diagram.

Question 43Air Conditioning

A technician is checking airflow on a system with a PSC blower and measures an external static pressure of 0.9 in. w.c. against a rated 0.5 in. w.c. Which condition is most likely present?

  1. A refrigerant overcharge increasing evaporator load
  2. An oversized blower wheel delivering excess airflow
  3. A failed run capacitor on the condenser fan motor
  4. A restrictive duct system or clogged filter reducing airflow Correct answer.

Explanation

External static pressure above the rated value indicates the blower is working against excessive resistance, typically from undersized ducts, a dirty filter, or a restrictive coil. High static pressure reduces airflow, which in turn reduces capacity and can freeze the coil. Refrigerant charge and condenser fan issues do not raise indoor static pressure.

Question 44Air Conditioning

A technician finds that the condensate drain is blocked and water has overflowed into the return duct. Which immediate action is most appropriate?

  1. Increase blower speed to dry the duct faster
  2. Add refrigerant to compensate for reduced capacity
  3. Shut down the system, clear the drain, and address the water damage and safety hazards Correct answer.
  4. Reset the high-pressure switch and restart the unit

Explanation

Standing water in a return duct creates electrical, biological, and structural hazards, and continued operation spreads moisture through the building. The correct response is to stop the equipment, clear the blockage, verify the drain trap and slope, and remediate the water damage. Adjusting charge or speed does not address the cause.

Question 45Air Conditioning

A technician calculates a target superheat of 10°F but measures 4°F at the evaporator outlet on a fixed-orifice system. Which action is most appropriate?

  1. Recover refrigerant to reduce the charge and recheck superheat Correct answer.
  2. Replace the metering piston with a larger size
  3. Leave the charge as is because low superheat is always safe
  4. Add refrigerant until superheat reaches the target

Explanation

Low superheat means liquid refrigerant is reaching the evaporator outlet and possibly the compressor, which risks slugging and oil dilution. On a fixed-orifice system the correction is to remove refrigerant and recheck. Adding charge would worsen the condition, and changing the piston alters the design match.

Question 46Air Conditioning

A technician measures 0 volts at the compressor contactor coil while the thermostat is calling for cooling and the indoor blower is running. Which diagnostic step is most appropriate next?

  1. Replace the thermostat without further testing
  2. Add refrigerant and recheck the contactor
  3. Replace the compressor contactor immediately
  4. Check the control circuit for an open safety, float switch, or low-voltage wiring fault Correct answer.

Explanation

No voltage at the contactor coil during a call means the control circuit is open somewhere upstream, such as a tripped high-pressure switch, an open condensate float switch, a low-pressure lockout, or broken low-voltage wiring. Tracing the circuit locates the open device. Replacing parts without diagnosis risks replacing good components.

Question 47Air Conditioning

A technician must determine whether a system is operating with the correct airflow using only temperature measurements. Which pair of readings provides the most useful information?

  1. Liquid line temperature and suction line temperature
  2. Outdoor ambient dry bulb and condenser leaving air dry bulb
  3. Return air wet bulb and supply air wet bulb for enthalpy difference Correct answer.
  4. Compressor discharge temperature and crankcase temperature

Explanation

The difference in wet bulb temperature across the evaporator reflects the enthalpy removed from the air, which combined with airflow yields capacity. Dry bulb difference alone ignores latent removal. Refrigerant line temperatures describe the refrigerant circuit, not air-side performance.

Question 48Air Conditioning

A system with a TXV shows low suction pressure, high superheat, normal subcooling, and no temperature drop across the liquid line drier. Which component is the most likely cause?

  1. The compressor valves are worn and bypassing
  2. The condenser coil is restricted with debris
  3. The system is overcharged with refrigerant
  4. The TXV is stuck partially closed or its sensing bulb has lost charge Correct answer.

Explanation

Normal subcooling and no drier temperature drop rule out a liquid line restriction, while low suction pressure with high superheat points to the evaporator being starved at the metering device. A stuck-closed valve or a bulb that has lost its charge will not open the valve properly. Overcharge and condenser restrictions would raise head pressure and subcooling.

Question 49Air Conditioning

A technician compares a system's measured capacity to its rated capacity and finds it delivering about 70 percent of rating with correct charge and clean coils. Which combination of factors best explains the shortfall?

  1. A correctly sized liquid line with normal pressure drop
  2. Low indoor airflow and a high entering wet bulb reducing latent removal Correct answer.
  3. A compressor drawing current within nameplate rating
  4. Slightly high condenser subcooling and a warm outdoor ambient

Explanation

Capacity depends on both refrigerant-side and air-side performance, and reduced airflow lowers the heat exchanged at the coil. A high entering wet bulb shifts the balance toward latent removal, which can reduce sensible capacity and total delivered capacity. The other options describe normal or unrelated conditions.

Question 50Air Conditioning

A technician is evaluating a system where the suction line is cold and sweating at the evaporator but warm and dry at the compressor. Which condition best explains this pattern?

  1. A failed condenser fan motor reducing heat rejection
  2. A refrigerant overcharge raising mass flow through the evaporator
  3. A restricted liquid line drier causing flash gas
  4. Excessive pressure drop or heat gain along the suction line between coil and compressor Correct answer.

Explanation

A large temperature change along the suction line indicates either an excessive pressure drop from undersized or restricted line, or heat gain from an uninsulated line in a hot space. Either condition reduces density at the compressor and lowers capacity while raising superheat at the compressor. Charge and condenser issues affect both ends of the line similarly.