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HVAC Excellence Master AC practice questions

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Question 1HVACR Fundamentals and Refrigeration Theory

In the refrigeration cycle, what is the primary function of the compressor?

  1. It absorbs heat from the conditioned space and vaporizes the refrigerant
  2. It lowers refrigerant pressure to create a cold, low-pressure liquid-vapor mixture
  3. It removes moisture and acid from the refrigerant before it reaches the metering device
  4. It raises the pressure and temperature of the refrigerant vapor so it can reject heat Correct answer.

Explanation

The compressor is the heart of the cycle: it takes low-pressure vapor from the evaporator and compresses it to a higher pressure, which also raises its saturation temperature above ambient so the condenser can reject heat. It moves refrigerant mass and creates the pressure differential that drives the whole circuit.

Question 2HVACR Fundamentals and Refrigeration Theory

A refrigerant enters the evaporator as roughly 25% vapor and 75% liquid by mass. What is this mixture quality?

  1. 0.50
  2. 0.25 Correct answer.
  3. 0.75
  4. 1.25

Explanation

Quality is defined as the mass fraction of vapor in a liquid-vapor mixture, so 25% vapor by mass equals a quality of 0.25. The remaining 0.75 is the liquid fraction, which is why quality plus liquid fraction always equals one.

Question 3HVACR Fundamentals and Refrigeration Theory

Which statement correctly describes the pressure-enthalpy diagram of a basic vapor-compression cycle?

  1. The evaporator appears as a horizontal line at constant pressure while enthalpy rises Correct answer.
  2. The compressor is drawn as a horizontal line showing constant enthalpy compression
  3. The condenser is drawn as a vertical line because it is an isothermal process at constant pressure
  4. The expansion device appears as a curve that increases both pressure and enthalpy

Explanation

On a P-h diagram the evaporator is a horizontal segment at evaporating pressure, during which enthalpy increases as the refrigerant absorbs heat and vaporizes. Constant-pressure phase change plots as a horizontal line because pressure does not change while latent heat is added.

Question 4HVACR Fundamentals and Refrigeration Theory

What is the purpose of superheat at the evaporator outlet?

  1. To increase the condensing temperature so the condenser can reject more heat
  2. To reduce the compressor discharge temperature below ambient
  3. To subcool the liquid line and improve metering device capacity
  4. To guarantee that only vapor, with no liquid droplets, returns to the compressor Correct answer.

Explanation

A small amount of superheat ensures the refrigerant is fully vaporized before it leaves the evaporator, protecting the compressor from liquid slugging. Liquid refrigerant entering a compressor can wash out oil and cause mechanical damage.

Question 5HVACR Fundamentals and Refrigeration Theory

A technician measures a condenser approach temperature that is higher than the equipment design value. What does this most likely indicate?

  1. The metering device is overfeeding and flooding the evaporator
  2. The evaporator is starved of refrigerant and has excessive superheat
  3. The condenser is fouled or has poor airflow, reducing heat rejection Correct answer.
  4. The compressor is short-cycling on the low-pressure control

Explanation

Condenser approach is the difference between condensing temperature and the leaving cooling-medium temperature; a high approach means the condenser is not rejecting heat efficiently. Dirty coils, scaled tubes, or restricted airflow are the usual causes.

Question 6HVACR Fundamentals and Refrigeration Theory

Which refrigerant safety classification, per ASHRAE Standard 34, indicates lower toxicity and no flame propagation?

  1. B2
  2. B1
  3. A1 Correct answer.
  4. A3

Explanation

ASHRAE 34 uses a letter for toxicity (A = lower toxicity, B = higher toxicity) and a number for flammability (1 = no flame propagation). A1 therefore means lower toxicity and non-flammable, the classification of refrigerants such as R-134a and R-410A.

Question 7HVACR Fundamentals and Refrigeration Theory

What is the primary reason a thermostatic expansion valve uses an external equalizer line?

  1. To limit the maximum operating pressure of the valve
  2. To sense evaporator outlet temperature instead of bulb temperature
  3. To compensate for the pressure drop across the evaporator coil Correct answer.
  4. To provide a path for hot gas to bypass the condenser during defrost

Explanation

In a large evaporator the pressure at the outlet is lower than at the inlet, so the bulb pressure alone would not balance correctly. An external equalizer applies the true outlet pressure to the underside of the diaphragm so the valve maintains proper superheat.

Question 8HVACR Fundamentals and Refrigeration Theory

Subcooling measured at the condenser outlet is 12°F when the design value is 10°F. What does this suggest about the liquid line charge?

  1. The system has slightly more liquid seal at the condenser than design Correct answer.
  2. The compressor valves are leaking and reducing capacity
  3. The condenser is rejecting too much heat and must be bypassed
  4. The system is undercharged and the valve is starving the evaporator

Explanation

Subcooling reflects how much liquid is stacked in the condenser; a value above design generally indicates a slightly overcharged or well-sealed condenser with adequate liquid. Small deviations are not faults, but excess subcooling combined with high head pressure points to overcharge.

Question 9HVACR Fundamentals and Refrigeration Theory

In the refrigeration cycle, which component separates the high-pressure side from the low-pressure side?

  1. The metering device Correct answer.
  2. The condenser
  3. The accumulator
  4. The compressor

Explanation

The metering device is the dividing point: high-pressure liquid enters and a low-pressure liquid-vapor mixture leaves. The compressor also creates the pressure difference, but it is the boundary between the low-pressure suction side and the high-pressure discharge side rather than the device that drops pressure to the evaporator.

Question 10HVACR Fundamentals and Refrigeration Theory

A system uses R-410A, which has a glide of about 0.2°F. How should a technician interpret this glide during charging?

  1. The refrigerant behaves essentially as a zeotrope and must be charged as a liquid
  2. The refrigerant behaves essentially as an azeotrope, so vapor charging is acceptable Correct answer.
  3. The glide requires a dedicated fractionating recovery cylinder
  4. The glide means the evaporator must be operated below atmospheric pressure

Explanation

R-410A is a near-azeotropic blend with very small temperature glide, so its behavior is close to a single-component refrigerant and vapor charging causes negligible fractionation. A true zeotrope with large glide would require liquid charging to avoid changing the blend composition.

Question 11HVACR Fundamentals and Refrigeration Theory

What happens to the boiling temperature of a refrigerant as its pressure increases?

  1. It remains constant regardless of pressure
  2. It increases along with the saturation pressure Correct answer.
  3. It decreases in direct proportion to pressure
  4. It changes only when the refrigerant is superheated

Explanation

Saturation temperature and saturation pressure rise together; this relationship is the basis of the pressure-temperature chart. That is why raising suction pressure raises the evaporating temperature and why head pressure controls condensing temperature.

Question 12HVACR Fundamentals and Refrigeration Theory

A technician reads a suction pressure of 60 psig on an R-134a system and a suction line temperature of 50°F. Using a P-T chart that shows 60 psig corresponds to about 52°F saturation, what is the superheat?

  1. 52°F
  2. -2°F Correct answer.
  3. 10°F
  4. 2°F

Explanation

Superheat is measured line temperature minus saturation temperature at the same pressure: 50°F minus 52°F equals -2°F. A negative value means the refrigerant is still partially liquid at the measurement point, indicating floodback rather than superheat.

Question 13HVACR Fundamentals and Refrigeration Theory

Why is a crankcase heater used on many compressors?

  1. To raise the discharge pressure above the condenser's saturation point
  2. To preheat the oil so it can absorb more refrigerant
  3. To warm the suction gas and increase volumetric efficiency
  4. To keep refrigerant from condensing in the crankcase during the off cycle Correct answer.

Explanation

During the off cycle, cold refrigerant can migrate to and condense in the crankcase, diluting the oil. A crankcase heater keeps the oil warm enough to prevent this migration and the lubrication problems it causes.

Question 14HVACR Fundamentals and Refrigeration Theory

What is the effect of excessive liquid subcooling entering the metering device?

  1. The valve can feed more refrigerant because no flash gas forms in the liquid line Correct answer.
  2. The evaporator superheat increases sharply and the coil starves
  3. The compressor suction pressure drops below the design value
  4. The valve loses its pressure differential and stops feeding refrigerant

Explanation

Subcooling keeps the liquid from flashing before it reaches the valve, so the full liquid mass is available to be metered. More liquid at the valve inlet means the valve can maintain capacity with less flash gas and more stable superheat control.

Question 15HVACR Fundamentals and Refrigeration Theory

Which statement best describes the difference between a reciprocating and a scroll compressor?

  1. A scroll compressor relies on a centrifugal impeller, while a reciprocating compressor uses a rotary vane
  2. A scroll compressor compresses gas between two spiral elements, while a reciprocating compressor uses pistons in cylinders Correct answer.
  3. A scroll compressor has no moving parts, while a reciprocating compressor has pistons and valves
  4. A scroll compressor uses pistons driven by a crankshaft, while a reciprocating compressor uses two orbiting spirals

Explanation

A scroll compressor traps gas between a fixed and an orbiting spiral and squeezes it toward the center, while a reciprocating compressor uses pistons moving in cylinders with suction and discharge valves. Both are positive-displacement machines but differ in mechanism and in how gas is trapped and compressed.

Question 16HVACR Fundamentals and Refrigeration Theory

A heat pump in heating mode operates with an outdoor coil temperature below freezing. What is the purpose of the defrost cycle?

  1. To lower the compressor discharge temperature during cold weather
  2. To reverse the refrigerant flow permanently so the system cools
  3. To melt frost that insulates the outdoor coil and blocks airflow Correct answer.
  4. To raise the indoor coil temperature so the space heats faster

Explanation

Frost on the outdoor coil insulates the fins and restricts airflow, which reduces heat absorption and capacity. The defrost cycle temporarily reverses the cycle or uses hot gas to melt the frost, then returns to normal heating.

Question 17HVACR Fundamentals and Refrigeration Theory

Using the ideal vapor-compression model, which process is assumed to occur at constant entropy?

  1. Evaporation in the evaporator
  2. Compression in the compressor Correct answer.
  3. Condensation in the condenser
  4. Expansion through the metering device

Explanation

The ideal compressor is modeled as an isentropic (constant-entropy) compression, which sets the minimum work required. Real compression is non-isentropic, so actual discharge enthalpy and temperature are higher than the ideal model predicts.

Question 18HVACR Fundamentals and Refrigeration Theory

What does a compressor's volumetric efficiency describe?

  1. The percentage of liquid refrigerant the compressor can tolerate
  2. The ratio of evaporator capacity to condenser capacity
  3. The ratio of discharge pressure to suction pressure at design conditions
  4. The ratio of actual suction gas volume delivered to the theoretical displacement volume Correct answer.

Explanation

Volumetric efficiency compares the volume of vapor actually drawn in and delivered against the geometric displacement of the compressor. It falls as the pressure ratio rises because clearance gas re-expands and because suction gas is heated before the valves close.

Question 19HVACR Fundamentals and Refrigeration Theory

A refrigerant cylinder is stored in a hot truck and its pressure rises. What is the safest correct action?

  1. Open the valve fully to relieve internal pressure to the atmosphere
  2. Submerge the cylinder in water to cool it rapidly
  3. Move the cylinder to a cooler, ventilated area and allow it to cool Correct answer.
  4. Vent refrigerant until the pressure returns to the label value

Explanation

Refrigerant cylinders are pressure vessels whose pressure tracks ambient temperature; the correct response is to relocate the cylinder to a cooler, well-ventilated location and let it cool naturally. Venting refrigerant to atmosphere is both unsafe and regulated against.

Question 20HVACR Fundamentals and Refrigeration Theory

Which statement about the relationship between evaporating temperature and compressor capacity is correct?

  1. Lower evaporating temperature raises capacity because the pressure ratio decreases
  2. Evaporating temperature has no effect on capacity as long as superheat is stable
  3. Higher evaporating temperature reduces capacity because the compressor works harder
  4. Lower evaporating temperature reduces capacity because suction gas density falls Correct answer.

Explanation

As evaporating temperature drops, suction pressure and suction gas density fall, so each stroke moves less mass and capacity decreases. The pressure ratio also rises, which lowers volumetric efficiency and further reduces capacity.

Question 21HVACR Fundamentals and Refrigeration Theory

What is the function of an accumulator in a low-temperature refrigeration system?

  1. To store liquid refrigerant on the high side and feed the evaporator by gravity
  2. To separate oil from the discharge gas before it enters the condenser
  3. To prevent liquid refrigerant from reaching the compressor by trapping it on the suction line Correct answer.
  4. To reduce the condensing pressure during low-ambient operation

Explanation

An accumulator is installed on the suction line to catch any liquid that leaves the evaporator and allow it to boil off before vapor continues to the compressor. It protects the compressor from slugging in systems where floodback is possible.

Question 22HVACR Fundamentals and Refrigeration Theory

Which of the following best explains why a zeotropic refrigerant blend has a temperature glide during phase change?

  1. It contains a small amount of non-condensable gas that changes the pressure
  2. Its components have different boiling points, so composition shifts as it evaporates Correct answer.
  3. Its components react chemically during evaporation and change the pressure
  4. It is a single-component refrigerant with a very wide saturation band

Explanation

A zeotropic blend is a mixture of refrigerants with different boiling points, so the more volatile component boils off first and the remaining liquid composition changes. This shifting composition causes the saturation temperature to change at constant pressure, which is glide.

Question 23HVACR Fundamentals and Refrigeration Theory

What is the main reason for using a filter-drier in the liquid line?

  1. To raise the suction pressure and improve compressor capacity
  2. To remove moisture and particulate contamination that could form acids or plug the metering device Correct answer.
  3. To separate refrigerant vapor from liquid before the condenser
  4. To increase the subcooling by cooling the liquid before it reaches the valve

Explanation

A filter-drier contains desiccant and a filter element that capture moisture and debris, protecting the metering device from plugging and preventing moisture-driven acid formation. Moisture in a system can also freeze at the metering device and block flow.

Question 24HVACR Fundamentals and Refrigeration Theory

A technician finds the suction line sweating and the evaporator partially frosted near the inlet. Which condition is most consistent with these symptoms?

  1. Undercharge causing high superheat throughout the coil
  2. Overcharge causing the condenser to flood
  3. A restriction or overfeeding that leaves liquid refrigerant entering the coil Correct answer.
  4. A failed condenser fan motor raising head pressure

Explanation

Frost near the evaporator inlet with a sweating suction line suggests liquid refrigerant is present well into the coil, often from overfeeding or a restriction that causes flashing. That liquid boils off and chills the coil below freezing near the inlet.

Question 25HVACR Fundamentals and Refrigeration Theory

What does a compressor's compression ratio compare?

  1. Discharge temperature to suction temperature
  2. Gauge discharge pressure to gauge suction pressure
  3. Absolute discharge pressure to absolute suction pressure Correct answer.
  4. Mass flow at the condenser to mass flow at the evaporator

Explanation

Compression ratio is absolute discharge pressure divided by absolute suction pressure, so gauge readings must be converted to absolute by adding atmospheric pressure. High ratios reduce volumetric efficiency and raise discharge temperature.

Question 26HVACR Fundamentals and Refrigeration Theory

In a basic vapor-compression refrigeration cycle, which component absorbs heat from the conditioned space and boils the refrigerant?

  1. The compressor
  2. The condenser
  3. The expansion valve
  4. The evaporator Correct answer.

Explanation

The evaporator is the heat-exchange coil where low-pressure liquid refrigerant boils into vapor, absorbing heat from the surrounding space. The condenser rejects that heat outdoors, the expansion valve drops pressure, and the compressor raises pressure. Heat absorption at the evaporator is what produces the cooling effect.

Question 27HVACR Fundamentals and Refrigeration Theory

According to the pressure-temperature relationship, what happens to a refrigerant's saturation temperature when its pressure is lowered?

  1. The saturation temperature decreases Correct answer.
  2. The saturation temperature remains unchanged
  3. The saturation temperature increases
  4. The saturation temperature fluctuates unpredictably

Explanation

Saturation temperature and saturation pressure are directly related: as pressure falls, the boiling point falls with it. This is why the evaporator operates at low pressure to achieve a low boiling temperature. Lowering pressure does not leave the saturation temperature unaffected.

Question 28HVACR Fundamentals and Refrigeration Theory

Which refrigerant safety classification under ASHRAE Standard 34 designates a refrigerant with no flame propagation and lower toxicity?

  1. B1
  2. A2
  3. A1 Correct answer.
  4. B2

Explanation

ASHRAE 34 uses a letter for toxicity (A = lower, B = higher) and a number for flammability (1 = no flame propagation, 2 = lower flammability, 3 = higher flammability). A1 therefore combines lower toxicity with no flame propagation. Classes beginning with B indicate higher toxicity, and classes 2 or 3 indicate flammability.

Question 29HVACR Fundamentals and Refrigeration Theory

A compressor's compression ratio is calculated by comparing which two pressures?

  1. Suction pressure to evaporator pressure
  2. Absolute discharge pressure to absolute suction pressure Correct answer.
  3. Discharge pressure to condenser pressure
  4. Gauge discharge pressure to gauge suction pressure

Explanation

Compression ratio is the absolute discharge pressure divided by the absolute suction pressure. Absolute pressures must be used because gauge readings ignore atmospheric pressure and would distort the ratio. Ratios using gauge pressures or comparing points on the same side of the system are not valid.

Question 30HVACR Fundamentals and Refrigeration Theory

A technician measures an evaporator superheat of 2°F on a system with a design superheat of 10°F. What does this most likely indicate?

  1. The expansion valve is underfeeding refrigerant
  2. The expansion valve is overfeeding refrigerant Correct answer.
  3. The system is low on refrigerant charge
  4. The condenser is rejecting too much heat

Explanation

Superheat is the temperature rise of vapor above its saturation temperature at the evaporator outlet. A very low superheat means little vapor is superheating, so the valve is admitting more refrigerant than the evaporator can fully boil-overfeeding. Underfeeding or low charge would raise superheat, not lower it.

Question 31HVACR Fundamentals and Refrigeration Theory

Why does a thermostatic expansion valve with an external equalizer line respond more accurately to evaporator pressure?

  1. It senses pressure at the evaporator outlet rather than at the valve inlet Correct answer.
  2. It measures the temperature of the liquid line
  3. It bypasses the valve orifice during high load
  4. It eliminates the need for a sensing bulb

Explanation

An external equalizer connects the underside of the valve diaphragm to the evaporator outlet, so the valve references the true suction pressure at the coil exit. This compensates for pressure drop through the distributor and coil, which would otherwise make the valve open too far. The sensing bulb is still required, and the equalizer does not bypass the orifice.

Question 32HVACR Fundamentals and Refrigeration Theory

A low-temperature refrigeration system uses an accumulator. What is its primary function?

  1. To prevent liquid refrigerant from reaching the compressor Correct answer.
  2. To store excess refrigerant during pump-down
  3. To increase suction pressure before the compressor
  4. To subcool liquid refrigerant before the metering device

Explanation

An accumulator is installed in the suction line to trap any liquid refrigerant that leaves the evaporator and allow it to boil off before it reaches the compressor. This protects the compressor from slugging and oil dilution. It is not a receiver, a pressure booster, or a subcooler.

Question 33HVACR Fundamentals and Refrigeration Theory

Which statement best explains why a zeotropic refrigerant blend exhibits a temperature glide during phase change?

  1. It operates above its critical temperature during normal cycling
  2. Its components have different boiling points and change composition as they vaporize Correct answer.
  3. It is formulated with a lubricant that alters its saturation curve
  4. It contains a small amount of non-condensable gas

Explanation

A zeotropic blend is a mixture of refrigerants with different volatilities. As the mixture boils or condenses, the more volatile component changes proportion in the liquid and vapor phases, so the saturation temperature shifts and produces glide. Non-condensables, lubricant, or supercritical operation are separate phenomena.

Question 34HVACR Fundamentals and Refrigeration Theory

A technician finds that the suction line is sweating and the evaporator is frosted near its inlet. Which condition is most consistent with these symptoms?

  1. The system is low on refrigerant charge
  2. The expansion valve is overfeeding refrigerant Correct answer.
  3. The compressor valves are leaking
  4. The condenser fan is not running

Explanation

Frost at the evaporator inlet with a sweating suction line indicates that liquid refrigerant is flooding the coil faster than it can boil, so the valve is overfeeding. Low charge would starve the coil and produce high superheat with little frost. A failed condenser fan or leaking compressor valves would show different pressure and temperature patterns.

Question 35HVACR Fundamentals and Refrigeration Theory

A system has a measured subcooling of 14°F at the condenser outlet with a design value of 10°F. What does this most likely suggest?

  1. The system is undercharged with refrigerant
  2. The system is overcharged with refrigerant Correct answer.
  3. The condenser airflow is excessive
  4. The metering device is stuck open

Explanation

Subcooling increases when more liquid refrigerant backs up in the condenser, which is typical of an overcharge. A higher-than-design subcooling with normal operating pressures points to excess charge. Undercharge would reduce subcooling, and a stuck-open metering device would tend to flood the evaporator rather than raise subcooling.

Question 36HVACR Fundamentals and Refrigeration Theory

Which statement correctly describes the difference between a reciprocating compressor and a scroll compressor?

  1. A scroll compressor operates only with centrifugal force, while a reciprocating compressor uses rotary vanes
  2. A reciprocating compressor has no suction or discharge valves, while a scroll compressor does
  3. A scroll compressor uses two interleaved spirals to compress refrigerant continuously, while a reciprocating compressor uses pistons in cylinders Correct answer.
  4. A scroll compressor uses pistons in cylinders, while a reciprocating compressor uses two interleaved spirals

Explanation

A scroll compressor compresses refrigerant between two spiral wraps, one fixed and one orbiting, producing smooth continuous compression. A reciprocating compressor uses pistons moving in cylinders with suction and discharge valves. The two designs differ in mechanism, valve arrangement, and vibration characteristics.

Question 37HVACR Fundamentals and Refrigeration Theory

In the ideal vapor-compression model, which process is assumed to occur at constant entropy?

  1. Evaporation in the evaporator
  2. Condensation in the condenser
  3. Compression in the compressor Correct answer.
  4. Expansion through the metering device

Explanation

The ideal vapor-compression cycle treats compression as isentropic, meaning no heat transfer and no internal irreversibility. Condensation and evaporation are treated as constant-pressure processes, and expansion is treated as isenthalpic. Constant entropy is therefore associated with the compressor.

Question 38HVACR Fundamentals and Refrigeration Theory

A heat pump in heating mode has an outdoor coil temperature below freezing and begins to frost. What is the purpose of the defrost cycle?

  1. To temporarily reverse refrigerant flow and melt frost from the outdoor coil Correct answer.
  2. To open the reversing valve and switch to cooling permanently
  3. To bypass the outdoor coil and draw heat from indoor air
  4. To increase compressor speed and raise heating capacity

Explanation

During heating, the outdoor coil is the evaporator and can drop below freezing, causing frost that blocks airflow and reduces capacity. The defrost cycle temporarily reverses the refrigerant flow so the outdoor coil becomes the condenser and hot gas melts the frost. It is not a permanent mode change or a capacity boost.

Question 39HVACR Fundamentals and Refrigeration Theory

A technician calculates a compression ratio of 12:1 on a system designed for 8:1. Which condition is most likely responsible?

  1. Low suction pressure combined with high discharge pressure Correct answer.
  2. A normal suction pressure with a low discharge pressure
  3. High suction pressure combined with low discharge pressure
  4. A normal discharge pressure with a high suction pressure

Explanation

Compression ratio rises when the spread between absolute discharge and suction pressure widens. Low suction pressure or high discharge pressure, or both, will increase the ratio above design. The other combinations narrow the spread and would lower the ratio.

Question 40HVACR Fundamentals and Refrigeration Theory

A refrigerant cylinder is left in a hot service truck and its internal pressure rises sharply. What is the safest correct action?

  1. Move the cylinder to a cooler, ventilated area and allow it to stabilize before use Correct answer.
  2. Submerge the cylinder in water to cool it quickly
  3. Vent refrigerant slowly to reduce pressure inside the cylinder
  4. Open the cylinder valve fully to relieve pressure through the service hose

Explanation

A refrigerant cylinder is a pressure vessel, and heat raises internal pressure. The safe response is to relocate it to a cooler, well-ventilated place and let it return to a normal temperature and pressure. Venting refrigerant is illegal in many jurisdictions and dangerous, and rapid cooling or uncontrolled release can damage the cylinder or cause injury.

Question 41Electrical and Controls

On a residential split system, which conductor color is conventionally used for the 24V control (hot) wire in the low-voltage circuit?

  1. Blue
  2. Red Correct answer.
  3. White
  4. Green

Explanation

In standard low-voltage thermostat cable, red is the transformer's hot (24V) conductor and is switched through the thermostat to the other wires. Green is conventionally fan (G), white is heat (W), and blue is commonly used for common (C).

Question 42Electrical and Controls

A technician measures 0 volts across the contacts of a normally open relay while the coil is energized and the load is not running. What is the most likely conclusion?

  1. The transformer secondary is shorted
  2. The thermostat is calling for continuous fan
  3. The coil is open and the relay never pulled in
  4. The relay contacts are closed and the load circuit is intact through the contacts Correct answer.

Explanation

Zero volts across closed contacts means the two points are at the same potential, which happens when the contacts are closed and the load is not running. If the contacts were open, the meter would read the supply voltage across them.

Question 43Electrical and Controls

What is the primary function of a low-voltage transformer in a gas furnace control circuit?

  1. Regulate gas manifold pressure
  2. Step line voltage down to 24V for controls Correct answer.
  3. Convert AC to DC for the blower motor
  4. Provide ignition spark voltage

Explanation

The control transformer reduces line voltage (typically 120V or 240V) to 24V AC to operate thermostats, relays, and gas valves. Ignition and gas regulation are handled by separate components.

Question 44Electrical and Controls

A blower motor is controlled by a PSC motor and a fan relay. With a call for fan, the motor hums but does not spin. Which measurement best isolates a failed run capacitor?

  1. Check the capacitor microfarad value against its rating with a meter Correct answer.
  2. Measure voltage at the thermostat R and G terminals
  3. Measure the transformer primary voltage
  4. Check the gas valve voltage

Explanation

A failed run capacitor is a common cause of a humming PSC motor that will not start. Comparing measured microfarads to the nameplate rating confirms whether the capacitor has failed.

Question 45Electrical and Controls

Which device provides a safety lockout by sensing flame through the principle of flame rectification?

  1. Inducer pressure switch
  2. Hot surface igniter
  3. Rollout limit switch
  4. Flame sensor rod Correct answer.

Explanation

The flame sensor rod uses flame rectification: the flame conducts current in one direction, and the control board reads that microamp signal to prove flame. The igniter, pressure switch, and limit switch serve different functions.

Question 46Electrical and Controls

A technician finds the low-voltage fuse blown repeatedly after replacing it. What is the correct diagnostic approach?

  1. Bypass the fuse temporarily to observe operation
  2. Replace the transformer and the board together
  3. Isolate and test the control circuit for a short to ground Correct answer.
  4. Install a larger fuse to prevent nuisance blowing

Explanation

A repeatedly blown low-voltage fuse indicates a short in the 24V circuit, such as a pinched wire or failed component. The correct approach is to isolate sections and locate the fault rather than oversizing or bypassing protection.

Question 47Electrical and Controls

What does a 24V common (C) wire connection provide to a smart thermostat?

  1. A signal to the blower relay
  2. A switched call for heating
  3. A ground reference for the outdoor unit only
  4. A return path for continuous thermostat power Correct answer.

Explanation

The C wire completes the 24V circuit back to the transformer so the thermostat can draw continuous power for its electronics. It is not a switched call; the R wire provides the hot side.

Question 48Electrical and Controls

A condenser fan motor on a heat pump is wired through a defrost control board. During defrost, the fan is off. Which control action explains this?

  1. The defrost board opens the fan circuit to keep discharge air from being cold Correct answer.
  2. The board resets the high-pressure switch
  3. The board closes the fan relay to boost airflow
  4. The board energizes the reversing valve to switch to cooling

Explanation

During defrost, the outdoor fan is stopped so the coil warms quickly and the defrost cycle ends sooner, preventing cold air delivery. The reversing valve and other actions are separate functions.

Question 49Electrical and Controls

Which meter function should be used to measure current draw of a compressor while it is operating?

  1. An ohmmeter across the run winding
  2. A voltmeter in parallel with the compressor
  3. A megohmmeter across the terminals
  4. A clamp-on ammeter around the common conductor Correct answer.

Explanation

A clamp-on ammeter measures current without breaking the circuit by sensing the magnetic field around the conductor. Voltmeters measure voltage, ohmmeters measure resistance, and megohmmeters test insulation.

Question 50Electrical and Controls

A pressure switch on an induced-draft furnace fails to close when the inducer starts. Which condition should be checked first?

  1. That the pressure switch sensing tube is clear and properly connected Correct answer.
  2. That the flame sensor is clean
  3. That the thermostat is calling for cooling
  4. That the gas valve is opening

Explanation

The pressure switch depends on a clear sensing tube and proper inducer airflow to close. Blockage or disconnection of the tube is a common cause of failure to close. The gas valve and flame sensor are downstream of proving combustion airflow.