CSLB HVAC (C-20) Trade — All Questions

R-410A and R-454B are being compared for a residential change-out. Which statement reflects a real operating difference?

• a.R-454B operates at roughly twice the pressure of R-410A
• b.R-410A requires mineral oil while R-454B requires polyolester oil
• c.R-454B is classified A2L, so listed equipment includes mitigation features✓
• d.R-410A can be safely charged as vapor because it behaves as a single compound

R-454B carries an A2L safety classification for mild flammability, and equipment designed for it includes charge limits, sensors, or other listed mitigation. Their operating pressures are broadly similar, not double. Both use polyolester-type synthetic oils rather than mineral oil. And R-410A is a near-azeotropic blend that is still charged as liquid, not a single compound.

What does temperature glide mean for a zeotropic refrigerant blend?

• a.Saturation temperature changes across the coil at constant pressure✓
• b.The refrigerant loses capacity gradually as the system ages
• c.The blend separates permanently once it enters the evaporator
• d.The refrigerant's pressure changes while its temperature stays fixed

In a zeotropic blend the components boil at different temperatures, so saturation temperature rises through the evaporator even though pressure is nearly constant, and the technician must use the correct dew or bubble point value. Pressure changing at fixed temperature describes a different process entirely. Capacity loss over time is a maintenance issue, not a property of the blend. And the components mix again in the vapor phase rather than separating permanently.

Why must a zeotropic blend be charged into a system as liquid rather than vapor?

• a.Liquid charging is the only way to weigh the charge accurately
• b.Vapor charging removes the lighter component first and changes the blend✓
• c.Liquid charging is required to lubricate the compressor bearings
• d.Vapor charging cannot reach the pressure needed to fill the entire system

The components of a blend have different vapor pressures, so drawing vapor from the cylinder pulls off the most volatile component and leaves a different mixture behind. Cylinder pressure is adequate for vapor transfer in many conditions, so pressure is not the barrier. Oil is charged separately and does not travel with the refrigerant from the cylinder. And a scale weighs the cylinder accurately regardless of which phase leaves it.

A technician needs the saturation temperature for a measured pressure on an A2L blend. Where does that number come from?

• a.The rule that saturation temperature is 35 degrees below ambient
• b.The equipment nameplate's design pressure rating
• c.The pressure-temperature data published for that specific refrigerant✓
• d.The generic pressure-temperature scale printed on the manifold gauge face

Each refrigerant has its own pressure-temperature relationship, and blends additionally have separate bubble and dew point columns, so the specific published data must be used. A generic gauge scale covers only the few refrigerants printed on it. Nameplate design pressure is a structural limit, not an operating relationship. And there is no valid rule tying saturation temperature to a fixed offset from ambient.

What does latent heat of vaporization describe in the refrigeration cycle?

• a.The heat rejected as the vapor is desuperheated in the discharge line
• b.The heat stored in the compressor oil during normal operation
• c.The heat that raises the vapor's temperature after it leaves the coil
• d.The heat absorbed as refrigerant changes from liquid to vapor at constant temperature✓

Latent heat is the energy a refrigerant absorbs while boiling without changing temperature, and it is the mechanism that moves the bulk of the heat load. Heating vapor above its boiling point is sensible superheat, not latent heat. Desuperheating in the discharge line is also a sensible process. And oil is a lubricant, not a thermal storage medium in the cycle.

Two systems run at the same suction pressure, but one has 5 degrees of superheat and the other has 30. Which statement is accurate?

• a.The 5-degree system is at greater risk of compressor overheating
• b.Both systems must have the same evaporator capacity
• c.The 30-degree system is using more of its evaporator surface for boiling refrigerant
• d.The 30-degree system risks overheating; the 5-degree system risks liquid return✓

High superheat means the refrigerant boiled off early and the returning vapor is hot, while very low superheat means liquid may reach the compressor. High superheat wastes evaporator surface on sensible heating rather than using more of it for boiling. It is the high-superheat system, not the low one, that runs the compressor hot. And unequal superheat means unequal use of the coil, so capacity is not the same.

How does a compressor's discharge temperature respond to very high suction superheat?

• a.It stays constant, because the compression ratio governs it entirely
• b.It rises, because hotter suction gas enters the compression stroke✓
• c.It falls, because thinner vapor carries less heat
• d.It rises only if the condenser is also dirty

Return gas temperature is one of the two main inputs to discharge temperature, so hot suction vapor produces very hot discharge gas that can break down oil. Lower vapor density reduces mass flow but does not cool the discharge. Compression ratio matters but does not act alone. And the effect appears with a clean condenser as well, since it originates on the suction side.

A hermetic compressor motor is cooled by what in a typical residential system?

• a.The returning suction gas passing through the motor windings✓
• b.The outdoor fan blowing air over the compressor shell
• c.The liquid line passing back through the compressor housing wall
• d.The oil sump circulating through an external cooler

Suction-cooled hermetic compressors route return gas over the motor before compression, which is why chronically high superheat overheats the windings. Cabinet airflow provides some incidental cooling but is not the design cooling path. External oil coolers appear on large industrial machines, not residential hermetics. And the liquid line never passes through the compressor housing.

What is the primary function of a receiver in systems that have one?

• a.To separate oil from the discharge gas before the condenser
• b.To store liquid refrigerant so charge can vary with load✓
• c.To protect the compressor from returning liquid at startup
• d.To lower head pressure by adding condenser volume

A receiver holds a reservoir of liquid on the high side so the metering device always has liquid available as operating conditions change. Protecting the compressor from liquid on the suction side is the accumulator's job. Oil separation happens in an oil separator at the discharge. And a receiver does not add effective condensing surface, so it does not lower head pressure.

Head pressure control is applied to a system operating in cold outdoor conditions. Why is it needed?

• a.To prevent the compressor from overheating in low ambient
• b.To maintain enough pressure difference for the metering device to feed properly✓
• c.To keep the condenser fan running at full speed year round
• d.To prevent the suction line from freezing at the compressor

If head pressure falls too far, there is not enough pressure drop across the metering device to feed the evaporator, and the system starves. Compressor overheating is a high-superheat or high-compression-ratio problem, not a low ambient one. Head pressure control typically slows or cycles the condenser fan rather than keeping it at full speed. And the suction line does not freeze from low head pressure.

A capillary tube system reaches equilibrium during the off cycle. What does that mean for restarting?

• a.Pressures equalize, so the compressor can start against a low load✓
• b.Pressures stay split, requiring a hard start kit on every unit
• c.The metering device closes tightly to hold back the high side pressure
• d.The compressor must be started manually with a jumper

A capillary tube is an open restriction, so pressures equalize during the off cycle and the compressor faces a light starting load, which is why these systems often use lower-torque motors. Pressure does not stay split, because nothing closes the passage. Capillaries have no valve to close tightly. And no manual starting procedure is involved in normal operation.

An evaporator coil is icing over on an 80 F day with the system running continuously. What are the two most likely causes?

• a.Low airflow across the coil or a refrigerant undercharge✓
• b.High outdoor ambient or a dirty condenser coil
• c.Overcharge or a stuck-open reversing valve
• d.A failed crankcase heater or a loose thermostat anticipator

Icing means the coil is operating below freezing, which follows from too little heat reaching it or too little refrigerant boiling in it, so restricted airflow and low charge lead the list. Overcharge raises suction pressure and works against icing. A dirty condenser raises head pressure without dropping the coil below freezing. And crankcase or thermostat problems affect starting and cycling, not coil temperature.

How does an undercharged system produce a frozen evaporator?

• a.The condenser cannot reject heat, so the coil temperature drops
• b.Suction pressure falls until the saturation temperature is below freezing✓
• c.The metering device opens wider and floods the coil with cold liquid
• d.The compressor runs hotter and transfers some of that heat back to the coil

With too little refrigerant, suction pressure drops and the corresponding saturation temperature falls below 32 F, so condensate freezes on the fins. Compressor heat travels with the discharge gas toward the condenser, not backward into the evaporator. A condenser that cannot reject heat raises pressures throughout, which does not freeze a coil. And flooding the coil would raise suction pressure and coil temperature.

A technician finds a system that is overcharged. Which set of readings fits that condition on a TXV system?

• a.Low head pressure, low subcooling, and high superheat
• b.High head pressure, high subcooling, normal superheat✓
• c.High head pressure, low subcooling, and high superheat
• d.Low head pressure, high subcooling, and normal superheat

Extra refrigerant stacks in the condenser, so subcooling and head pressure both climb while the TXV keeps superheat near its setting. Low head pressure with low subcooling describes an undercharge. High head with low subcooling suggests non-condensables or a condenser problem instead. And high subcooling with low head pressure is an inconsistent combination on an operating system.

What is the purpose of a low-pressure control on a residential split system?

• a.To reverse the compressor when the coil begins to frost
• b.To stop the compressor when suction pressure indicates loss of charge✓
• c.To modulate the metering device as load changes
• d.To keep the evaporator from ever operating above its design pressure limit

A low-pressure switch opens when suction pressure falls below its setpoint, protecting the compressor from running with insufficient refrigerant. Preventing high pressure is the high-pressure control's role. Metering devices modulate on their own control inputs, not from a pressure switch. And no residential control reverses a compressor's rotation in response to frost.

A refrigerant leak is suspected in an attic coil. Which detection method is best suited to pinpointing it?

• a.Adding a can of sealant and monitoring pressures afterward
• b.Watching for an oil stain on the drain pan over the next several weeks
• c.Comparing the suction pressure before and after the coil
• d.An electronic leak detector swept slowly along the tubing and returns✓

A heated diode or infrared detector moved slowly along joints and return bends locates the leak precisely and is the standard field method. Oil staining is a useful clue but is slow and often absent on small leaks. Pressure drop across a coil reflects flow restriction, not leakage. And sealants can plug metering devices and driers and do not identify where the leak is.

A large leak has emptied a system. What must be done before the system is recharged?

• a.Install a larger metering device to restore capacity
• b.Add extra refrigerant to compensate for the loss
• c.Run the compressor briefly to circulate any oil left in the lines
• d.Repair the leak, then pressure test, evacuate, and verify✓

The leak has to be found and repaired, the repair proven with a pressure test, and the system evacuated to remove air and moisture that entered before the charge goes in. Simply adding refrigerant releases it again and is prohibited. Running a compressor with no charge damages it and does nothing useful. And changing the metering device does not address the loss of refrigerant or the entry of contaminants.

A compressor has burned out and the oil smells acidic. What service step protects the replacement compressor?

• a.Cleaning the system and installing a suction line filter drier and a new liquid line drier✓
• b.Installing a sight glass in the liquid line to monitor moisture
• c.Charging the system with 10 percent extra refrigerant
• d.Reusing the original filter drier after blowing it out with nitrogen

A burnout leaves acid and debris throughout the circuit, so the system is cleaned and both a suction line filter and a fresh liquid line drier are installed to capture what remains. Overcharging changes operating conditions and removes nothing. A moisture-indicating sight glass reports a condition but does not clean the system. And a used drier is already saturated and cannot be restored by blowing it out.

Which condition explains high suction pressure together with low head pressure on a cooling system?

• a.Worn compressor valves or an internal bypass in the compressor✓
• b.A dirty outdoor condenser coil
• c.A plugged liquid line filter drier
• d.An oversized metering device installed on an otherwise correct system

A compressor that is no longer pumping lets the two sides drift toward each other, raising suction while head pressure falls. A plugged drier starves the low side and drops suction pressure instead. A dirty condenser raises head pressure. And an oversized metering device floods the evaporator, raising suction pressure but not lowering head pressure.

How is a compressor's pumping ability verified in the field?

• a.By measuring the resistance of the run and the start windings when cold
• b.By measuring the voltage drop across the contactor
• c.By closing the suction service valve and observing how low it pulls✓
• d.By comparing the amp draw to the locked rotor amps

A closed-valve pump-down test shows whether the compressor can pull and hold a low suction pressure, which directly measures pumping capability. Winding resistance is an electrical check that says nothing about mechanical pumping. Contactor voltage drop tests the contact, not the compressor. And locked rotor amps is a starting characteristic, not a pumping measurement.

A heat pump in heating mode has the reversing valve stuck in the cooling position. What will the customer report?

• a.The system heats but never satisfies on the coldest days
• b.The outdoor unit runs but the indoor blower never starts
• c.The system heats normally but the defrost cycle never runs
• d.Cold air comes from the registers on a call for heat✓

If the valve does not shift, the indoor coil keeps acting as an evaporator and the registers deliver cold air while the thermostat calls for heat. Falling short on the coldest days is the normal behavior of a heat pump near its balance point. A blower that never starts is a control or motor fault. And a heat pump that heats normally is by definition shifting the valve correctly.

Which measurement best confirms that an outdoor condenser coil is rejecting heat properly?

• a.Static pressure across the outdoor coil
• b.Condensing temperature compared with the outdoor ambient✓
• c.Suction line temperature measured at the compressor inlet
• d.Amperage of the outdoor fan motor

A healthy air-cooled condenser condenses at a predictable margin above the entering air temperature, so comparing the two reveals fouling or airflow loss. Suction line temperature reflects evaporator and line conditions. Fan amperage indicates the motor is turning but not that the coil is clean. And static pressure is not a practical measurement across an open outdoor coil.

Why does a dirty evaporator coil reduce capacity even though the refrigerant charge is correct?

• a.It insulates the tubing, so the refrigerant leaves the coil as liquid
• b.It raises head pressure, which reduces compressor displacement
• c.It causes the metering device to overfeed and flood the compressor
• d.It restricts airflow, so less heat reaches the refrigerant and suction pressure falls✓

A blocked coil starves the refrigerant of heat, so boiling slows, suction pressure and coil temperature drop, and capacity falls. The refrigerant leaving a starved coil is heavily superheated vapor, not liquid. Head pressure actually tends to fall because less heat is being absorbed and moved. And a starved coil makes the metering device throttle back rather than overfeed.

What does the coefficient of performance describe for a heat pump?

• a.The percentage of input fuel converted to usable heat
• b.The ratio of heat output to energy input in consistent units✓
• c.The ratio of BTU output to watts input measured at rated conditions
• d.The seasonal average of heating output over a full winter

COP is a unitless ratio of energy delivered to energy consumed, and it can exceed 1.0 because a heat pump moves heat instead of creating it. Fuel conversion percentage describes combustion efficiency and cannot exceed 100 percent. A seasonal average across a heating season is HSPF, a different metric. And BTU per watt is the definition of EER, which mixes units rather than being unitless.

A service valve core is removed to speed evacuation. What is the risk if it is not reinstalled?

• a.The system will read a false high suction pressure
• b.The gauge hose cannot be attached to the port again
• c.The compressor will not build head pressure at startup
• d.The cap alone is the remaining seal, and refrigerant can leak past it✓

The valve core is the primary seal at a service port, and a cap is only a secondary seal, so an omitted core will leak the charge over time. Hoses attach to the port fitting whether or not a core is present. Suction pressure readings are unaffected by the core's presence. And nothing about a missing core prevents the compressor from building pressure.

Which practice reduces refrigerant loss every time gauges are connected and removed?

• a.Connecting only the low-side hose on every service call
• b.Leaving the hoses attached between visits to the same customer site
• c.Using low-loss hose fittings and recovering what the hoses hold✓
• d.Purging the hoses with refrigerant before connecting them

Low-loss fittings close the hose ends, and recovering the hose contents keeps the trapped charge out of the atmosphere. Using one hose does not eliminate the loss from that hose. Leaving hoses attached exposes them to weather and does not prevent loss at removal. And purging hoses with refrigerant deliberately vents it, which is exactly what the rules prohibit.

What happens to the pressure in a refrigerant cylinder as its temperature rises?

• a.It changes only if the cylinder is more than half full
• b.It falls, because the vapor expands into the free space
• c.It rises, following the refrigerant's saturation curve✓
• d.It stays constant as long as liquid remains in the cylinder

A cylinder holding both liquid and vapor sits on the saturation curve, so warming it raises pressure steadily, which is why cylinders must be protected from heat. Constant pressure would require constant temperature, not merely the presence of liquid. Vapor expanding in a sealed container raises pressure rather than lowering it. And the relationship holds at any fill level, though a fuller cylinder is more dangerous because liquid expands.

A technician wants to recover refrigerant quickly from a system with a large charge. What technique speeds it up?

• a.Warming the recovery cylinder in order to raise its internal pressure
• b.Running the system's own compressor during the recovery
• c.Recovering liquid first through both service ports with short hoses✓
• d.Connecting the recovery machine only to the suction port

Pulling liquid through both ports with short, large-diameter hoses moves the bulk of the charge fast, and the remaining vapor is recovered afterward. Warming the recovery cylinder raises the pressure the machine must push against and slows it. Running the system compressor during recovery risks damaging it and is not the intended method. And a single suction connection forces slow vapor-phase recovery of the whole charge.

During a pump-down for a component replacement, what is the goal?

• a.To equalize pressures so the compressor can restart under no load
• b.To isolate the charge in the condenser and receiver so the low side can be opened✓
• c.To lower the system to a deep vacuum before opening it
• d.To transfer the charge into the recovery cylinder for disposal

Pumping down uses the system's own compressor to store refrigerant behind a closed service valve so low-side work can be done without recovering the entire charge. A deep vacuum requires a vacuum pump and comes after the system is opened and repaired. Transferring to a cylinder is recovery, a separate procedure. And equalizing pressures is a starting consideration, not the purpose of a pump-down.

What determines the evaporator's saturation temperature during normal operation?

• a.The thermostat setpoint selected by the occupant
• b.The superheat setting adjusted at the expansion valve
• c.The suction pressure the compressor and metering device establish✓
• d.The outdoor ambient temperature at the condenser

Saturation temperature is fixed by pressure, and the balance between compressor pumping and metering device feed sets the suction pressure in the coil. The thermostat decides when the system runs, not the pressure inside it. The superheat adjustment shifts how much of the coil boils but does not set the saturation point itself. And outdoor ambient controls the high side rather than the evaporator's pressure.