CSLB HVAC (C-20) Trade — All Questions

Return air for a gas furnace must not be taken from which location?

• a.A hallway serving several bedrooms
• b.A garage or space with combustion byproducts✓
• c.A conditioned bedroom with its own return grille
• d.A central living room with a high ceiling

Drawing return air from a garage, furnace room, or similar space risks pulling in carbon monoxide, fuel vapors, or other contaminants and distributing them through the home. The CMC prohibits return openings in these locations. Return air must come from conditioned, safe spaces.2022 California Mechanical Code (CMC)

When mounting an outdoor condenser on a slab, the pad should be installed how?

• a.Sloped steeply back toward the building wall
• b.Level, on a stable base off the ground✓
• c.Directly on loose, uncompacted soil
• d.Tilted so that condensate pools beneath it

The condenser pad should be level and set on a firm, stable base so the unit sits securely and drains properly, avoiding settling that stresses the line set. A level unit also ensures correct oil return and fan operation. The pad keeps the cabinet off wet ground and debris.

In the basic refrigeration cycle, which component absorbs heat from the conditioned space?

• a.The condenser
• b.The evaporator✓
• c.The compressor
• d.The metering device

The evaporator is where low-pressure liquid refrigerant boils and absorbs heat from the indoor air, cooling the space. The condenser then rejects that heat outdoors. This heat-absorption step is the cooling effect of the system.

What is the primary function of the compressor in the refrigeration cycle?

• a.To absorb heat from the conditioned space
• b.To meter refrigerant into the evaporator coil
• c.To raise vapor pressure and circulate it✓
• d.To drain condensate from the indoor coil

The compressor draws in low-pressure vapor from the evaporator and compresses it to a high pressure and temperature, then discharges it to the condenser. This pressure difference drives refrigerant flow around the loop. It is often called the heart of the system.

The condenser in an air-conditioning system performs which function?

• a.Absorbs heat and boils liquid refrigerant
• b.Rejects heat and condenses vapor to liquid✓
• c.Meters refrigerant flow into the coil
• d.Filters particles from the supply air

In the condenser, high-pressure, high-temperature vapor gives up heat to the outdoor air and condenses into a high-pressure liquid. This heat rejection is what allows the cycle to remove heat from indoors. Airflow across the condenser coil is essential for this process.

A thermostatic expansion valve (TXV) controls refrigerant flow into the evaporator to maintain what?

• a.Constant condenser pressure
• b.Constant compressor speed
• c.A relatively constant evaporator superheat✓
• d.Constant subcooling

A TXV modulates refrigerant flow to hold a stable superheat at the evaporator outlet, matching flow to the load. This keeps the coil fed efficiently and protects the compressor from liquid floodback. It responds to the temperature and pressure of the suction line.

Superheat is defined as the temperature of the refrigerant vapor above what?

• a.Its condensing temperature at high side pressure
• b.Its saturation temperature at the existing pressure✓
• c.The outdoor ambient dry-bulb temperature
• d.The compressor discharge line temperature

Superheat is the number of degrees the refrigerant vapor has been heated above its saturation temperature for the measured pressure. Adequate superheat ensures only vapor, not liquid, returns to the compressor. Technicians measure it to evaluate evaporator charge and load.

A technician measures a suction pressure whose saturation temperature is 40 F and a suction line temperature of 52 F. What is the superheat?

• a.4 degrees
• b.8 degrees
• c.12 degrees✓
• d.20 degrees

Superheat equals the suction line temperature minus the saturation temperature: 52 F minus 40 F equals 12 degrees. This value helps confirm proper evaporator feeding and charge. Very low superheat risks liquid floodback, while very high superheat suggests underfeeding.

Subcooling is measured at which point in the system?

• a.At the evaporator outlet
• b.At the condenser outlet (liquid line)✓
• c.At the compressor suction
• d.At the filter on the return duct

Subcooling is measured at the condenser outlet by comparing the liquid line temperature to the saturation temperature for the high-side pressure. It indicates how much the liquid has cooled below its condensing point. Subcooling is a primary charging check on TXV systems.

On a system with a condensing saturation temperature of 105 F and a liquid line temperature of 95 F, what is the subcooling?

• a.5 degrees
• b.8 degrees
• c.10 degrees✓
• d.15 degrees

Subcooling equals the saturation temperature minus the liquid line temperature: 105 F minus 95 F equals 10 degrees. This is a typical target range for many TXV systems. Low subcooling can indicate an undercharge, while high subcooling can indicate an overcharge or restriction.

On a fixed-orifice (piston) system, which method is generally used to check the charge?

• a.The subcooling method with a PT chart
• b.The superheat method with a target chart✓
• c.Weighing the compressor oil charge
• d.Measuring external static pressure

Fixed-orifice metering devices are charged by the superheat method, comparing measured superheat to a target based on indoor wet-bulb and outdoor dry-bulb temperatures. TXV systems, by contrast, are typically charged by subcooling. Using the correct method for the metering device is essential.

Under EPA Section 608, what must a technician do with refrigerant before opening most systems for service?

• a.Vent it slowly to the atmosphere
• b.Mix it with dry nitrogen first
• c.Recover it with certified equipment✓
• d.Burn it off with a torch flame

Section 608 prohibits knowingly venting most refrigerants and requires recovery into approved equipment before opening a system. This protects the atmosphere and conserves refrigerant. Only certified technicians using certified equipment may perform this work.EPA Section 608

EPA Section 608 certification is required for technicians who do what?

• a.Install sheet-metal ductwork and register boots
• b.Service or dispose of refrigerant equipment✓
• c.Wire thermostats and control circuits
• d.Design residential duct systems only

Section 608 requires certification for anyone who could release ozone-depleting or certain other regulated refrigerants during service, maintenance, repair, or disposal. Different certification types cover small appliances, high-pressure, and low-pressure systems. Working with refrigerant without certification is a violation.EPA Section 608

Evacuating a system with a vacuum pump primarily removes what?

• a.Refrigerant oil from the compressor sump
• b.Copper oxide scale from inside the tubing
• c.Moisture and non-condensable gases like air✓
• d.Varnish from the compressor windings

A deep vacuum boils off moisture and pulls out air and other non-condensables that would otherwise raise head pressure, form acids, or cause icing at the metering device. A micron gauge verifies the target vacuum is reached and held. Proper evacuation is essential before charging.

A micron gauge is used during evacuation to measure what?

• a.Refrigerant weight
• b.The depth of vacuum in the system✓
• c.Superheat
• d.Airflow

A micron gauge measures the absolute pressure (depth of vacuum) far more precisely than a compound gauge, confirming that the system is dry and tight. Technicians pull the system down to a target micron level and perform a decay test. Failure to hold vacuum indicates moisture or a leak.

Which instrument or method is commonly used for pinpointing small refrigerant leaks?

• a.A megohmmeter connected across the windings
• b.A manometer connected to the coil
• c.An electronic detector or bubble solution✓
• d.A pyrometer clamped to the line

Electronic leak detectors sense refrigerant escaping at joints, and bubble solutions reveal leaks visibly. Ultraviolet dye is another method for hard-to-find leaks. Locating and repairing leaks is required before recharging a system.

As a general principle, adding refrigerant charge to a properly running system tends to do what to the low-side pressure?

• a.Lower it significantly
• b.Raise it✓
• c.Have no effect
• d.Reverse compressor rotation

Adding charge generally raises system pressures, including the suction (low-side) pressure, while removing charge lowers them. Technicians interpret these trends together with superheat and subcooling. Charging must be verified against manufacturer targets, not pressure alone.

A refrigerant metering device such as a fixed orifice or TXV creates what effect on the refrigerant?

• a.Raises its pressure before the condenser
• b.Condenses it fully into a liquid
• c.Drops its pressure so it boils cold✓
• d.Superheats the vapor before the condenser

The metering device restricts flow, dropping the high-pressure liquid to a low pressure so it can boil at a low temperature and absorb heat in the evaporator. This division between high and low sides is central to the cycle. TXVs also modulate this flow to control superheat.

Which category of refrigerants is being phased down under regulations due to high global warming potential?

• a.Water-based refrigerants
• b.Many high-GWP HFC refrigerants✓
• c.Nitrogen
• d.Carbon dioxide only

Regulations at the federal and California level are phasing down high global-warming-potential HFC refrigerants in favor of lower-GWP alternatives. Technicians must stay current on approved refrigerants and handling rules. Recovery and reclamation remain required under Section 608.EPA Section 608

A compressor that is trying to compress liquid refrigerant returning from the evaporator can suffer what damage?

• a.Improved efficiency and lower head pressure
• b.Higher superheat at the compressor inlet
• c.Slugging that damages valves or bearings✓
• d.Lower amp draw at the compressor terminals

Liquid refrigerant is nearly incompressible, so liquid floodback or slugging can damage compressor valves, bearings, and rods. Maintaining adequate superheat prevents liquid from reaching the compressor. This is why superheat is carefully controlled.

When comparing the two sides of an operating system, the discharge line is normally which relative to the suction line?

• a.Colder and lower pressure
• b.Hotter and higher pressure✓
• c.The same temperature and pressure
• d.Filled only with liquid

The discharge line leaving the compressor carries high-pressure, high-temperature vapor, while the suction line carries low-pressure, cooler vapor returning to the compressor. Recognizing these normal conditions helps a technician diagnose faults. Abnormal readings point to charge or airflow problems.

A reciprocating compressor changes the refrigerant state by doing what?

• a.Condensing the high-pressure vapor into liquid
• b.Metering liquid flow into the evaporator
• c.Compressing low-pressure vapor with pistons✓
• d.Boiling liquid refrigerant in the coil

A reciprocating compressor uses pistons driven by a crankshaft to draw in and compress refrigerant vapor, raising its pressure and temperature. Scroll and rotary compressors accomplish the same result by different mechanisms. The compressor circulates refrigerant and establishes the high and low sides.

High condenser (head) pressure with normal charge is most often caused by what?

• a.Overcharged suction line insulation
• b.Poor airflow or a dirty condenser coil✓
• c.A shorted thermostat
• d.An oversized return grille

Restricted condenser airflow, from a dirty coil, failing fan, or blocked clearance, prevents adequate heat rejection and raises head pressure. Non-condensables or overcharge can also raise head pressure. Cleaning the coil and confirming fan operation are first steps.

The saturation temperature corresponding to a measured pressure is found using what?

• a.The equipment nameplate data plate
• b.A duct calculator or sizing wheel
• c.A pressure-temperature chart or app✓
• d.The unit's electrical wiring schematic

A pressure-temperature chart lists the saturation temperature for each pressure of a given refrigerant. Technicians use it to calculate superheat and subcooling from gauge readings. Each refrigerant has its own PT relationship, so the correct chart must be used.

What is the primary function of a run capacitor on a single-phase motor?

• a.To fuse and protect the motor branch circuit
• b.To provide a phase shift for smooth running✓
• c.To ground the motor frame to the cabinet
• d.To measure airflow through the blower

A run capacitor stays in the circuit during operation, creating the phase shift the motor needs for efficient torque and smoother running. A weak or failed run capacitor causes low torque, overheating, and high amp draw. It is tested with a capacitance meter against its microfarad rating.

A start capacitor differs from a run capacitor in that it is designed to do what?

• a.Stay energized continuously while running
• b.Regulate refrigerant flow to the coil
• c.Give a brief torque boost, then drop out✓
• d.Filter the supply air leaving the unit

A start capacitor provides a large capacitance boost to start a motor under load and is switched out by a relay once the motor is up to speed. It is not rated for continuous duty. It is used with hard-start kits and certain compressor and motor designs.

A contactor in an air-conditioning condenser performs what function?

• a.Meters refrigerant flow into the evaporator
• b.Switches line voltage to the compressor✓
• c.Drains condensate from the coil pan
• d.Adjusts the indoor blower motor speed

The contactor is an electrically operated switch whose coil is energized by the low-voltage control circuit, closing contacts that supply line voltage to the compressor and condenser fan. Pitted or welded contacts are a common failure. It is tested for coil continuity and contact condition.

A typical low-voltage control (thermostat) circuit in residential HVAC operates at what voltage?

• a.120 volts
• b.240 volts
• c.24 volts✓
• d.480 volts

Residential control circuits generally operate at 24 volts AC, supplied by a step-down transformer. The thermostat switches this low voltage to energize relays and contactors. Measuring 24 volts at the appropriate terminals helps confirm control-circuit operation.

If a condenser fan motor fails while the compressor keeps running, what symptom is most likely?

• a.Low head pressure and high subcooling
• b.Rising head pressure and an overload trip✓
• c.Increased subcooling with normal pressures
• d.Lower amp draw on the compressor motor

Without condenser airflow, heat rejection collapses and head pressure climbs quickly, often tripping a high-pressure switch or the compressor overload. The system may also lose capacity and overheat. Diagnosing a hot, non-spinning fan points to the motor or its capacitor.

Low airflow across an evaporator coil during cooling most commonly causes what?

• a.High superheat measured at the coil outlet
• b.Higher supply air temperature at registers
• c.A cold or frozen coil and low suction pressure✓
• d.Higher airflow across the condenser coil

Restricted evaporator airflow, from a dirty filter, blocked coil, or weak blower, reduces heat pickup so the coil runs colder and suction pressure drops, sometimes freezing the coil. Restoring airflow by cleaning filters and coils is the first corrective step. A frozen coil should be thawed before further diagnosis.

SEER (or SEER2) is a rating that describes what?

• a.The seasonal heating efficiency of a gas furnace
• b.Seasonal cooling efficiency of the equipment✓
• c.The refrigerant type used in the system
• d.The measured duct leakage rate at test

SEER, and the updated SEER2, express the seasonal cooling output divided by the electrical energy input, so a higher number means greater cooling efficiency. It is used to compare and select air conditioners and heat pumps. California energy standards set minimum SEER values.

AFUE is the efficiency rating used for what equipment?

• a.Air conditioners
• b.Cooling towers
• c.Gas and oil furnaces✓
• d.Duct systems

Annual Fuel Utilization Efficiency (AFUE) expresses the percentage of fuel energy a furnace converts to usable heat over a season. A 95 percent AFUE furnace delivers 95 percent of the fuel's energy as heat. It is the standard heating-efficiency metric for furnaces.

HSPF (or HSPF2) rates the seasonal efficiency of what?

• a.A gas furnace
• b.A heat pump in heating mode✓
• c.An evaporative cooler
• d.A duct booster fan

Heating Seasonal Performance Factor rates how efficiently a heat pump delivers heat over a heating season. A higher HSPF, or the updated HSPF2, means more heat per unit of electricity. It complements SEER, which rates the same unit's cooling efficiency.

A furnace that heats briefly then shuts off repeatedly before satisfying the thermostat is exhibiting what?

• a.Normal operation
• b.A refrigerant leak
• c.Short cycling✓
• d.High subcooling

Short cycling is repeated brief on-off operation before the space reaches setpoint, often caused by an overheating limit trip, a dirty flame sensor, oversizing, or restricted airflow. It wastes energy and stresses components. Diagnosis targets airflow, the limit switch, and flame sensing.

A dirty flame sensor on a modern furnace most commonly causes what symptom?

• a.The system runs in continuous cooling mode
• b.The burners light, then shut off in seconds✓
• c.The compressor runs at very high head pressure
• d.The evaporator coil freezes over solid

A flame sensor confirms flame is present; when it is coated with oxide it cannot conduct the flame-rectification signal, so the control shuts off the gas as a safety response. Cleaning the sensor typically restores operation. This is a very common no-heat service call.

When measuring the actual airflow of an installed system, which measurement helps diagnose a restrictive duct system?

• a.Refrigerant subcooling at the liquid line
• b.Gas manifold pressure at the valve outlet
• c.Total external static pressure at the air handler✓
• d.Condenser saturation temperature at the coil

Measuring total external static pressure and comparing it to the blower's rating reveals whether ductwork, filters, or coils are too restrictive. High static pressure indicates undersized or dirty components limiting airflow. It is a key airflow diagnostic tool.

A capacitor that reads well below its rated microfarad value should be treated how?

• a.Left in service
• b.Replaced with one of the correct rating✓
• c.Charged with refrigerant
• d.Bypassed with a jumper

A capacitor measuring significantly below its labeled microfarad rating is weak and cannot deliver the needed phase shift, causing hard starting, overheating, and high amp draw. It should be replaced with a capacitor of the same voltage and microfarad rating. Never bypass a required capacitor.

Recommended routine maintenance to preserve airflow and efficiency includes what?

• a.Adding refrigerant at every service visit
• b.Replacing the compressor once a year
• c.Changing filters and cleaning the coils✓
• d.Removing the condensate trap each spring

Routine maintenance such as changing filters, cleaning evaporator and condenser coils, and clearing condensate drains preserves airflow, capacity, and efficiency. Adding refrigerant should only follow a diagnosed leak and proper charging. Regular service prevents many common failures.

A technician measuring compressor amp draw far above the rated load amps (RLA) should suspect what?

• a.A healthy compressor under normal load
• b.High head pressure or failing windings✓
• c.An oversized air filter at the return
• d.Low control voltage at the thermostat

Amp draw well above the nameplate RLA indicates the compressor is overloaded, which can stem from high head pressure, low voltage, or degrading windings. Continued operation risks overheating and failure. The technician investigates head pressure, voltage, and mechanical condition.

The most likely cause of no cooling when the thermostat calls but the outdoor unit is completely silent is what?

• a.Low refrigerant subcooling at the condenser coil
• b.A dirty air filter at the return grille
• c.Lost power, failed contactor, or open safety✓
• d.Excess superheat in the suction line

A silent outdoor unit on a cooling call usually points to an electrical issue: tripped breaker, blown fuse, failed contactor, or an open safety such as a float switch or high-pressure switch. The technician traces control voltage and power to isolate the fault. Refrigerant issues usually still allow the unit to run.

An ECM (electronically commutated motor) blower offers what advantage over a standard PSC motor?

• a.It operates without any electricity
• b.Variable speed and better energy efficiency✓
• c.It eliminates the need for ductwork
• d.It removes refrigerant from the charge

ECM blower motors can vary speed to maintain target airflow across changing static pressure and generally use less energy than fixed-speed PSC motors. This improves comfort and efficiency and supports California energy goals. They are diagnosed differently, often through control signals and modules.

When a heat pump is stuck in cooling on a heat call, which component is a prime suspect?

• a.The condensate trap at the air handler
• b.The return air filter at the grille
• c.The reversing valve or its solenoid coil✓
• d.The supply registers in the rooms

If a heat pump blows cold air on a heating call, the reversing valve may be stuck or its solenoid coil or control signal may have failed, leaving the system in cooling mode. The technician checks the coil, the control voltage, and valve operation. A stuck valve often requires replacement.

Under Cal/OSHA, what is the purpose of a lockout/tagout (LOTO) procedure?

• a.To label refrigerant recovery cylinders
• b.To secure energy sources during service✓
• c.To measure superheat at the suction line
• d.To size the supply and return ductwork

Lockout/tagout requires de-energizing and physically locking energy-isolating devices, then tagging them, so machinery cannot be energized while a technician services it. This protects workers from electrical shock and unexpected startup. Cal/OSHA Title 8 sets the specific requirements.Cal/OSHA Title 8

Carbon monoxide is dangerous primarily because it is what?

• a.A visible green gas that has a sharp, acrid odor
• b.A refrigerant used in older sealed systems
• c.A colorless, odorless byproduct of combustion✓
• d.A vapor much heavier than refrigerant vapor

Carbon monoxide is a colorless, odorless gas produced by incomplete combustion of fuels, and it can be deadly because occupants cannot detect it. Cracked heat exchangers, poor venting, or inadequate combustion air can produce it. CO detectors and proper venting are essential safeguards.

A cracked heat exchanger in a gas furnace is a serious hazard because it can do what?

• a.Increase the cooling capacity of the indoor coil
• b.Let combustion gases mix with the supply air✓
• c.Lower the refrigerant charge in the system
• d.Improve airflow through the heat exchanger

A cracked heat exchanger can let flue gases, including carbon monoxide, leak into the circulating supply air and be distributed through the home. This is a life-safety hazard requiring the furnace to be shut down until repaired or replaced. Inspection for cracks is a standard safety check.

Cal/OSHA defines a confined space as one that has what characteristics?

• a.Any outdoor area with restricted equipment access
• b.Any interior room fitted with a door
• c.Limited entry, not for continuous occupancy✓
• d.Only spaces located below ground level

A confined space is large enough to bodily enter and perform work, has limited or restricted entry and exit, and is not designed for continuous occupancy, such as certain tanks, vaults, or attics. Permit-required confined spaces have added atmospheric or engulfment hazards. Cal/OSHA sets entry procedures and monitoring requirements.Cal/OSHA Title 8

When using a portable ladder, a safe practice is to maintain what?

• a.One hand on a tool at all times
• b.Three points of contact and proper ladder angle✓
• c.The ladder on a slippery surface
• d.The top rung as a step

Maintaining three points of contact and setting a straight ladder at roughly a 4-to-1 angle greatly reduces fall risk. Ladders must be on stable, level surfaces and inspected before use. Cal/OSHA specifies ladder safety requirements to prevent falls, a leading cause of injury.Cal/OSHA Title 8

Refrigerant can cause frostbite because liquid refrigerant does what when it contacts skin?

• a.Warms the skin as it evaporates slowly
• b.Acts as a mild lotion on the skin
• c.Evaporates fast and freezes the tissue✓
• d.Has no effect on unprotected skin

Liquid refrigerant boils at very low temperatures at atmospheric pressure, so contact with skin or eyes rapidly absorbs heat and can cause frostbite. Technicians wear gloves and eye protection when handling refrigerant. Cylinders and lines must be handled carefully to avoid sudden releases.

Refrigerant cylinders should be stored and handled how?

• a.In direct sunlight above their rated temperature
• b.Secured upright, away from heat, and never overfilled✓
• c.Laid on their side near a flame
• d.Vented periodically to relieve pressure

Refrigerant cylinders must be kept below their rated temperature, secured to prevent falling, and never overfilled, because heat raises internal pressure dangerously. Overheating or overfilling can cause a cylinder to rupture. Proper storage and handling are basic refrigerant safety practices.

The California Mechanical Code (CMC) primarily governs what?

• a.Electrical wiring and circuit overcurrent protection
• b.Plumbing fixtures and drainage piping systems
• c.Heating, ventilating, and refrigeration systems✓
• d.Structural framing, foundations, and roofs

The CMC sets the rules for installing mechanical systems including furnaces, air conditioners, ventilation, and refrigeration, covering venting, combustion air, clearances, and equipment access. Electrical work falls under the CEC and plumbing under the CPC. Contractors coordinate across these codes.2022 California Mechanical Code (CMC)