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OH HVAC Contractor practice questions

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Question 1HVAC and Refrigeration Trade Competency

A technician reads a manifold gauge set on an R-410A system operating at 118 psig suction. Using the pressure-temperature relationship for R-410A, what is the approximate saturated evaporator temperature?

  1. 25°F
  2. 40°F Correct answer.
  3. 68°F
  4. 52°F

Explanation

R-410A boils at roughly 118 psig when the saturated temperature is near 40°F, which is why typical A/C suction pressures run higher than R-22 for the same coil temperature. A technician comparing measured suction pressure to this reference confirms whether the evaporator is operating in a normal range.

Question 2HVAC and Refrigeration Trade Competency

Which safety practice is required before brazing refrigerant lines on a system that has contained refrigerant?

  1. Cap both ends and braze under positive pressure
  2. Vent refrigerant into the space to equalize pressure
  3. Leave a slight refrigerant charge to prevent oxidation
  4. Purge the system with nitrogen and verify it is open to atmosphere Correct answer.

Explanation

Residual refrigerant plus heat forms toxic decomposition products, and sealed lines can rupture from thermal expansion. Purging with dry nitrogen and confirming the section is open prevents both hazards and limits internal oxidation scale.

Question 3HVAC and Refrigeration Trade Competency

A compressor draws high amps and trips its internal overload shortly after startup, and the suction line is hot to the touch. Which condition best fits these symptoms?

  1. A restricted liquid line drier
  2. A failed run capacitor
  3. A locked rotor or mechanically seized compressor Correct answer.
  4. Low indoor airflow across the evaporator

Explanation

A seized or locked compressor cannot rotate, so it draws locked-rotor current and overheats until the internal protector opens. The hot suction line reflects the lack of refrigerant being pumped rather than a charge problem.

Question 4HVAC and Refrigeration Trade Competency

For a residential split system, the recommended superheat at the evaporator when using a fixed-orifice metering device and a TXV is best described how?

  1. TXV systems are charged to a target subcooling, while fixed-orifice systems are charged to a target superheat Correct answer.
  2. Fixed-orifice systems are charged to subcooling, and TXV systems to superheat
  3. Neither can be charged by temperature method and must use weight only
  4. Both are charged to the same superheat target regardless of metering type

Explanation

A TXV maintains superheat on its own, so the charge is verified by subcooling at the condenser. A fixed orifice cannot regulate superheat, so the technician sets charge using the superheat target appropriate to indoor and outdoor conditions.

Question 5HVAC and Refrigeration Trade Competency

What does a measured subcooling of 2°F on a TXV system most likely indicate?

  1. Restriction in the liquid line
  2. Overcharge of refrigerant
  3. Undercharge of refrigerant Correct answer.
  4. Normal operation

Explanation

Low subcooling means the condenser is not stacking enough liquid, which points to an undercharged system. A properly charged TXV system typically shows subcooling in the range specified by the manufacturer, commonly around 8 to 12°F.

Question 6HVAC and Refrigeration Trade Competency

A heat pump in heating mode has frost covering the entire outdoor coil. Which component most directly controls the defrost cycle initiation?

  1. The outdoor fan capacitor
  2. The indoor blower limit switch
  3. The defrost control board using coil and ambient sensors or a timer Correct answer.
  4. The reversing valve solenoid

Explanation

Defrost initiation is governed by the defrost control board, which uses coil temperature sensors, ambient sensors, or accumulated run-time logic to decide when to switch to defrost. The board then energizes the reversing valve and often disables the outdoor fan.

Question 7HVAC and Refrigeration Trade Competency

A technician is evacuating a system and the micron gauge stalls at about 1,500 microns and slowly rises after the pump is isolated. What is the most probable cause?

  1. The vacuum pump oil is new and the system is dry
  2. The system is fully evacuated and ready to charge
  3. The micron gauge is reading in inches of mercury
  4. There is a leak or moisture still evaporating in the system Correct answer.

Explanation

A rising micron reading after isolation signals either a leak admitting air or moisture continuing to boil off under vacuum. A tight, dry system holds a low micron level when the pump is valved off.

Question 8HVAC and Refrigeration Trade Competency

Which refrigerant is classified as A2L under ASHRAE Standard 34?

  1. R-717 (ammonia)
  2. R-32 Correct answer.
  3. R-410A
  4. R-134a

Explanation

A2L refrigerants are mildly flammable with low toxicity; R-32 falls in this class and is a component of R-410A. Technicians must apply A2L handling and leak-detection practices when servicing these systems.

Question 9HVAC and Refrigeration Trade Competency

A ducted system has a measured external static pressure of 0.9 in. w.c. against a rated 0.5 in. w.c. What is the most likely consequence?

  1. Lower compressor amp draw
  2. Reduced airflow and possible coil freezing Correct answer.
  3. Improved dehumidification without penalty
  4. Increased airflow and higher sensible capacity

Explanation

Higher-than-rated external static pressure means the blower is pushing against excessive resistance, so delivered airflow drops. Low airflow across a cold evaporator can drive coil temperature below freezing and cause icing.

Question 10HVAC and Refrigeration Trade Competency

Which device protects a hermetic compressor against sustained low suction pressure caused by a loss of charge?

  1. Crankcase heater
  2. High-pressure switch
  3. Internal thermal overload only
  4. Low-pressure switch Correct answer.

Explanation

A low-pressure switch opens the control circuit when suction pressure falls below a setpoint, stopping the compressor before it overheats from lack of refrigerant. This is the standard loss-of-charge protection on residential and light commercial equipment.

Question 11HVAC and Refrigeration Trade Competency

A technician must size a replacement liquid-line filter drier for a system converted from R-22 to a POE-lubricated refrigerant. Which practice is correct?

  1. Install a filter drier rated for the new refrigerant and compatible with POE oil Correct answer.
  2. Omit the drier because POE oil absorbs moisture on its own
  3. Install a suction-line drier only and skip the liquid-line drier
  4. Reuse the existing drier after blowing it out with nitrogen

Explanation

POE oil is hygroscopic and can form acids with residual moisture, so a new drier compatible with the refrigerant and lubricant is required. Reusing an old drier risks releasing trapped moisture and contaminants into the system.

Question 12HVAC and Refrigeration Trade Competency

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

  1. To keep oil warm so refrigerant does not migrate and dilute it Correct answer.
  2. To preheat refrigerant before the metering device
  3. To reduce discharge temperature during operation
  4. To raise suction pressure at startup

Explanation

During the off cycle, refrigerant can migrate to the crankcase and condense in the oil, thinning it and risking slugging on startup. The heater keeps the oil warmer than the coldest system part so refrigerant stays out of the sump.

Question 13HVAC and Refrigeration Trade Competency

A commercial walk-in cooler with a TXV shows normal superheat but the box cannot pull down below 45°F. The evaporator is clean and the compressor runs continuously. Which check is most appropriate next?

  1. Increase the TXV superheat setting
  2. Replace the condenser fan motor
  3. Verify door seals, infiltration, and defrost schedule Correct answer.
  4. Recheck the refrigerant charge by weight only

Explanation

When refrigeration-side readings are normal, the load side must be examined; failed door gaskets, frequent door openings, or a defrost cycle running too long can overwhelm capacity. These load factors explain a box that will not reach temperature despite correct superheat.

Question 14HVAC and Refrigeration Trade Competency

Which statement correctly describes the relationship between superheat and evaporator efficiency?

  1. Lower superheat always improves efficiency regardless of risk
  2. Superheat is measured only at the condenser outlet
  3. Superheat has no effect on coil capacity
  4. Higher superheat means more of the coil is used for boiling refrigerant Correct answer.

Explanation

Superheat represents refrigerant vapor heated beyond its boiling point, meaning the coil section producing it is no longer absorbing latent heat. A larger superheat value indicates less of the coil is actively boiling refrigerant, reducing effective capacity.

Question 15HVAC and Refrigeration Trade Competency

A technician measures 0°F superheat at the evaporator outlet on a fixed-orifice system. What risk does this present?

  1. Excessive superheat causing compressor overheating
  2. Liquid refrigerant flooding back to the compressor Correct answer.
  3. Loss of condenser subcooling
  4. Reduced refrigerant flow through the orifice

Explanation

Zero superheat means saturated vapor, or possibly liquid droplets, leaving the evaporator and traveling to the compressor. Liquid slugging dilutes oil and can damage valves and bearings.

Question 16HVAC and Refrigeration Trade Competency

Which action best reduces the risk of non-condensables after a system repair?

  1. Triple evacuate with dry nitrogen breaks and hold below 500 microns Correct answer.
  2. Charge refrigerant before evacuation to save time
  3. Use only a manifold gauge to confirm vacuum
  4. Pull a vacuum for five minutes and charge

Explanation

Triple evacuation with nitrogen breaks helps remove moisture and air, and holding below 500 microns confirms the system is dry and tight. Non-condensables raise head pressure and reduce capacity if left in the system.

Question 17HVAC and Refrigeration Trade Competency

In a refrigerant piping diagram, what does an inverted trap on a suction riser accomplish?

  1. It prevents oil from draining back into the compressor during off cycles Correct answer.
  2. It reduces pressure drop across the evaporator
  3. It increases refrigerant velocity to carry oil up the riser
  4. It separates liquid from vapor before the compressor

Explanation

An inverted trap at the top of a suction riser keeps oil from flowing back down the riser into the compressor during shutdown. Properly sized risers plus traps protect the compressor from oil slugging.

Question 18HVAC and Refrigeration Trade Competency

What is the function of a receiver in a refrigeration system?

  1. To regulate evaporator superheat
  2. To increase suction pressure
  3. To remove moisture from the refrigerant
  4. To store liquid refrigerant and accommodate charge variations Correct answer.

Explanation

A receiver holds excess liquid refrigerant so the system can respond to load changes without starving or overfeeding the evaporator. It is common on systems with a thermostatic expansion valve and a condenser that can vary in liquid inventory.

Question 19HVAC and Refrigeration Trade Competency

A technician finds a burned hermetic compressor with acidic oil. Which step is essential before installing the replacement?

  1. Flush the system with water and dry it
  2. Install a suction-line filter drier and perform an acid test after running Correct answer.
  3. Reuse the old contactor and capacitor
  4. Charge the system with the old refrigerant recovered from the failed unit

Explanation

Acid and debris from a burnout can contaminate the new compressor, so suction filtration plus post-run acid testing verifies cleanup. Replacing the liquid-line drier and confirming the oil is clean protects the replacement.

Question 20HVAC and Refrigeration Trade Competency

What does a thermal expansion valve regulate?

  1. Refrigerant mass flow to maintain superheat at the evaporator outlet Correct answer.
  2. Condenser subcooling
  3. Compressor discharge temperature
  4. Indoor blower speed

Explanation

The TXV senses evaporator outlet temperature and pressure and modulates the valve opening to hold a target superheat. This matches refrigerant flow to the actual load on the coil.

Question 21HVAC and Refrigeration Trade Competency

A technician must determine the correct refrigerant line size for a 40-foot liquid-line run. Which factor most directly determines the minimum pipe diameter?

  1. The color of the insulation
  2. Pressure drop and refrigerant velocity needed to carry oil Correct answer.
  3. The size of the condenser fan
  4. The thermostat wire gauge

Explanation

Liquid-line sizing balances pressure drop against velocity and charge; suction-line sizing must also ensure oil return. Manufacturer piping tables account for these factors based on capacity and length.

Question 22HVAC and Refrigeration Trade Competency

Which condition would cause a high discharge pressure on an air-cooled condenser?

  1. Low refrigerant charge
  2. Dirty condenser coil Correct answer.
  3. Failed evaporator fan
  4. Restricted liquid line

Explanation

A dirty coil reduces heat rejection, so the condenser cannot condense refrigerant at normal pressure and head pressure rises. Cleaning the coil restores the design approach temperature.

Question 23HVAC and Refrigeration Trade Competency

A technician is diagnosing a heat pump that heats poorly in cold weather but cools normally in summer. Which component is the most likely suspect?

  1. Indoor blower motor
  2. Liquid-line drier
  3. Outdoor thermistor used for defrost Correct answer.
  4. Reversing valve

Explanation

A faulty outdoor coil sensor can cause the defrost control to misjudge frost conditions, leaving ice on the coil and reducing heating capacity. Cooling performance can remain normal because defrost is not used in cooling mode.

Question 24HVAC and Refrigeration Trade Competency

What is the correct interpretation of a 10°F approach temperature on an air-cooled condenser?

  1. The liquid line is 10°F warmer than the condenser
  2. The evaporator is 10°F colder than the return air
  3. The refrigerant condensing temperature is 10°F above the outdoor ambient Correct answer.
  4. The suction line is 10°F warmer than the evaporator

Explanation

Condenser approach is the difference between condensing temperature and entering air temperature. A high approach indicates poor heat transfer from a dirty coil or fan problem.

Question 25HVAC and Refrigeration Trade Competency

A technician is testing a compressor's start winding with an ohmmeter. The reading between common and start is 5 ohms and common to run is 2 ohms. What should the reading between start and run be approximately?

  1. 7 ohms Correct answer.
  2. 2.5 ohms
  3. 3 ohms
  4. 10 ohms

Explanation

The start and run windings are in series through the common terminal, so the start-to-run resistance equals the sum of the two winding readings. Adding 5 and 2 gives about 7 ohms.

Question 26HVAC and Refrigeration Trade Competency

A technician is brazing a copper suction line and notices the joint turns black and sooty during heating. What is the most likely cause?

  1. The brazing alloy contains too much silver.
  2. The torch is oxidizing the copper due to a lack of shielding gas. Correct answer.
  3. The copper tubing has absorbed moisture from the air.
  4. The nitrogen purge flow is too high.

Explanation

When copper is heated in air without a shielding gas, oxygen reacts with the copper surface to form black copper oxide (soot). A nitrogen purge prevents this oxidation by displacing oxygen around the joint. High silver content or moisture would not produce the black sooty appearance.

Question 27HVAC and Refrigeration Trade Competency

Which refrigerant is classified as A1 under ASHRAE Standard 34?

  1. R-717
  2. R-1234yf
  3. R-410A Correct answer.
  4. R-32

Explanation

R-410A is a zeotropic blend of R-32 and R-125 that is classified as A1, meaning lower toxicity and no flame propagation. R-32 and R-1234yf are A2L (lower flammability), and R-717 (ammonia) is B2L, which indicates higher toxicity and flammability.

Question 28HVAC and Refrigeration Trade Competency

A technician is checking a compressor's run capacitor with a digital multimeter that has a capacitance mode. The capacitor is rated 35 µF. The meter reads 12 µF. What is the most appropriate action?

  1. Add a second capacitor in parallel to boost the total.
  2. Replace the capacitor with one of the same rating. Correct answer.
  3. Leave it in service because the compressor still starts.
  4. Recharge the capacitor using a capacitor charger.

Explanation

A run capacitor that reads significantly below its rated capacitance has failed and must be replaced with one of the same microfarad rating and voltage. Operating with a weak capacitor can cause the compressor to overheat and draw high current. Capacitors are not rechargeable, and adding another in parallel changes the circuit design.

Question 29HVAC and Refrigeration Trade Competency

What is the primary function of a thermostatic expansion valve (TXV) in a refrigeration system?

  1. To adjust the condenser fan speed based on outdoor temperature.
  2. To control the compressor's discharge temperature.
  3. To regulate refrigerant flow to maintain a set superheat at the evaporator outlet. Correct answer.
  4. To maintain a constant evaporator pressure.

Explanation

A TXV modulates refrigerant flow in response to the superheat measured at the evaporator outlet, keeping the evaporator properly fed without flooding the compressor. It does not hold a constant evaporator pressure; that is the role of an evaporator pressure regulator. It also does not control discharge temperature or condenser fan speed.

Question 30HVAC and Refrigeration Trade Competency

A technician is troubleshooting a heat pump that runs but does not heat. The outdoor unit is running, but the indoor coil is cold and the supply air is not warm. Which component is most likely faulty?

  1. The reversing valve solenoid. Correct answer.
  2. The outdoor fan motor.
  3. The defrost control board.
  4. The indoor blower motor.

Explanation

If the reversing valve does not shift to the heating position, the system will continue to operate in cooling mode, making the indoor coil cold and delivering cool air. The outdoor fan and defrost board affect efficiency but would not prevent heating entirely. A faulty indoor blower would cause poor airflow, but the coil would not be cold in heating mode.

Question 31HVAC and Refrigeration Trade Competency

A technician measures a 12°F temperature difference between the return air and supply air on a residential forced-air furnace. Which condition does this indicate?

  1. The airflow is too high, resulting in a low temperature rise. Correct answer.
  2. The furnace is undersized for the duct system.
  3. The furnace is overfired and producing excessive heat.
  4. The furnace is operating within its normal temperature rise range.

Explanation

A temperature rise of 12°F is well below the typical 35–65°F range for a residential furnace, which usually indicates excessive airflow or a low firing rate. High airflow can be caused by a blower set too fast or ductwork that is oversized. Overfiring would produce a high temperature rise, not a low one.

Question 32HVAC and Refrigeration Trade Competency

Which safety practice is required when working with a recovery cylinder that has been used for a different refrigerant?

  1. Ensure the cylinder is labeled with the refrigerant it contains and is not overfilled. Correct answer.
  2. Fill the cylinder to 90% of its gross weight.
  3. Paint the cylinder a different color to indicate the new refrigerant.
  4. Evacuate the cylinder to 500 microns before use.

Explanation

Recovery cylinders must be labeled with the type of refrigerant they contain to prevent mixing, and they must never be overfilled beyond the safe fill limit (typically 80% of the cylinder's gross weight for liquids). Evacuation is not required for recovery cylinders, and painting does not replace proper labeling.

Question 33HVAC and Refrigeration Trade Competency

A technician is sizing a gas furnace for a house with a calculated heat loss of 60,000 Btu/h. The selected furnace has an input of 80,000 Btu/h and an AFUE of 80%. What is the furnace's output capacity?

  1. 80,000 Btu/h
  2. 60,000 Btu/h
  3. 64,000 Btu/h Correct answer.
  4. 48,000 Btu/h

Explanation

Output capacity equals input multiplied by AFUE: 80,000 Btu/h × 0.80 = 64,000 Btu/h. This output exceeds the 60,000 Btu/h heat loss, so the furnace is adequately sized. The other values either ignore AFUE or miscalculate the multiplication.

Question 34HVAC and Refrigeration Trade Competency

What is the purpose of a condensate trap on a condensing furnace?

  1. To collect and drain condensate while maintaining a water seal. Correct answer.
  2. To prevent flue gases from escaping into the conditioned space.
  3. To increase the efficiency of the furnace.
  4. To reduce the noise of the induced draft blower.

Explanation

A condensate trap creates a water seal that allows condensate to drain while preventing flue gases from being drawn into the cabinet and circulated through the ductwork. It does not directly improve efficiency or reduce noise. Flue gas prevention is a safety function, but the trap's primary purpose is to manage condensate.

Question 35HVAC and Refrigeration Trade Competency

A technician is installing a new refrigerant line set and must select the correct size for a 40-foot liquid line run. Which factor most directly determines the liquid line size?

  1. The refrigerant velocity in the suction line.
  2. The ambient temperature at the condenser.
  3. The size of the evaporator coil.
  4. The pressure drop due to friction and elevation. Correct answer.

Explanation

Liquid line sizing is based primarily on limiting pressure drop to about 2–3 psi, which is affected by friction losses and elevation changes. Suction line velocity determines oil return and is a separate consideration. Evaporator coil size and ambient temperature influence system performance but are not used to size the liquid line.

Question 36HVAC and Refrigeration Trade Competency

Which device is used to prevent refrigerant from migrating to the compressor during the off cycle?

  1. A liquid-line solenoid valve Correct answer.
  2. A crankcase heater
  3. A discharge muffler
  4. A suction accumulator

Explanation

A liquid-line solenoid valve closes when the compressor stops, preventing refrigerant from flowing into the evaporator and migrating back to the compressor. A crankcase heater helps keep oil warm but does not stop migration. A suction accumulator separates liquid but does not prevent migration during off cycles.

Question 37HVAC and Refrigeration Trade Competency

A technician is diagnosing a system with a high superheat and a low suction pressure. The evaporator is not frosted. Which condition is most likely?

  1. A dirty evaporator coil.
  2. A restricted liquid line or a faulty TXV. Correct answer.
  3. A failed condenser fan motor.
  4. An overcharge of refrigerant.

Explanation

High superheat with low suction pressure indicates that the evaporator is starved of refrigerant. This can be caused by a restriction in the liquid line or a TXV that is stuck closed or improperly adjusted. An overcharge would cause high suction pressure and low superheat, while a dirty coil would typically cause low suction pressure with normal to low superheat.

Question 38HVAC and Refrigeration Trade Competency

What is the maximum allowable concentration of refrigerant vapor in an occupied space according to ASHRAE Standard 15?

  1. It is always 1,000 ppm regardless of refrigerant.
  2. It depends on the refrigerant's toxicity and flammability classification. Correct answer.
  3. It is determined by the system's charge size only.
  4. It is set by the EPA for all refrigerants at 0.5% by volume.

Explanation

ASHRAE Standard 15 sets allowable concentration limits based on the refrigerant's safety classification (A1, A2L, B1, etc.) and the volume of the occupied space. There is no single universal limit; the standard provides a table of refrigerant concentration limits (RCL) for each refrigerant. The EPA does not set a universal 0.5% limit.

Question 39HVAC and Refrigeration Trade Competency

A technician is brazing a joint on a system that contains a small amount of refrigerant. What is the first step to take before applying heat?

  1. Wrap the joint with a wet rag.
  2. Open the service valves to relieve pressure.
  3. Purge the line with nitrogen.
  4. Recover the refrigerant and evacuate the line. Correct answer.

Explanation

Before brazing, all refrigerant must be recovered and the section evacuated to prevent the formation of toxic compounds and to avoid pressure buildup. Purging with nitrogen is done during brazing to prevent oxidation, but it does not remove refrigerant. Opening service valves would release refrigerant, which is illegal and unsafe.

Question 40HVAC and Refrigeration Trade Competency

A technician is checking a PSC condenser fan motor that does not run. The capacitor is good and voltage is present at the motor terminals. What is the next step?

  1. Replace the motor.
  2. Replace the capacitor again.
  3. Check the thermostat wiring.
  4. Check the motor windings for continuity. Correct answer.

Explanation

With voltage present and a good capacitor, the next step is to check the motor windings for continuity and resistance. An open winding would indicate a failed motor. Replacing the motor without testing is not diagnostic, and the thermostat does not control the condenser fan directly.

Question 41HVAC and Refrigeration Trade Competency

What is the primary purpose of a suction accumulator in a refrigeration system?

  1. To increase the suction pressure.
  2. To store excess refrigerant during low load.
  3. To prevent liquid refrigerant from reaching the compressor. Correct answer.
  4. To separate oil from the refrigerant.

Explanation

A suction accumulator is installed in the suction line to trap any liquid refrigerant that may not have boiled off in the evaporator, allowing it to vaporize before reaching the compressor. It does not store excess refrigerant (that is the receiver's function) or increase suction pressure. Oil separation is handled by an oil separator.

Question 42HVAC and Refrigeration Trade Competency

A technician is commissioning a new walk-in cooler and measures 8°F of subcooling at the condenser outlet. The system uses a TXV. What does this indicate?

  1. The system is undercharged.
  2. The system has a restriction in the liquid line.
  3. The system is properly charged and operating efficiently. Correct answer.
  4. The condenser is not rejecting enough heat.

Explanation

Subcooling of 8°F at the condenser outlet is within the typical range for a properly charged TXV system (usually 5–15°F). It indicates that the condenser is condensing refrigerant and providing solid liquid to the TXV. Undercharge would show low subcooling, and a restriction would cause high superheat with low subcooling.

Question 43HVAC and Refrigeration Trade Competency

Which component is used to protect a compressor from excessively high discharge pressure?

  1. A crankcase heater
  2. A thermal overload
  3. A low-pressure switch
  4. A high-pressure switch Correct answer.

Explanation

A high-pressure switch opens the control circuit when discharge pressure exceeds a set limit, stopping the compressor to prevent damage. A low-pressure switch protects against low suction pressure, a thermal overload protects against overheating, and a crankcase heater keeps oil warm.

Question 44HVAC and Refrigeration Trade Competency

A technician is installing a ducted system and needs to determine the equivalent length of a 6-inch round duct with two 90-degree elbows and a 10-foot straight run. What is the primary reason for calculating equivalent length?

  1. To select the correct duct size based on friction loss. Correct answer.
  2. To determine the required clearance from combustibles.
  3. To calculate the weight of the duct for support.
  4. To determine the duct's insulation thickness.

Explanation

Equivalent length accounts for the pressure drop of fittings (elbows, tees) in terms of straight duct length, allowing the technician to size ductwork so that the total friction loss stays within the blower's available static pressure. It does not determine insulation thickness, duct weight, or clearance requirements.

Question 45HVAC and Refrigeration Trade Competency

A technician is testing a thermocouple on a standing-pilot gas furnace. The pilot flame is steady and envelops the thermocouple tip, but the pilot goes out when the gas valve is turned to ON. What is the most likely cause?

  1. The thermocouple is generating insufficient millivolts. Correct answer.
  2. The gas valve is faulty.
  3. The pilot orifice is clogged.
  4. The thermostat is miswired.

Explanation

If the pilot goes out when the main valve opens, the thermocouple is likely producing too few millivolts to hold the safety magnet in the gas valve closed against the increased demand. A clogged pilot orifice would affect the pilot flame regardless of the main valve position. A faulty gas valve or miswired thermostat would not cause the pilot to extinguish when the main burner lights.

Question 46HVAC and Refrigeration Trade Competency

What is the function of a heat anticipator in a mechanical thermostat?

  1. To reduce the number of cycles by anticipating the space temperature change. Correct answer.
  2. To turn on the blower immediately when the thermostat calls for heat.
  3. To provide a delay before the compressor starts.
  4. To calibrate the thermostat's thermometer.

Explanation

The heat anticipator is a small resistance heater inside the thermostat that warms the bimetal element slightly during the heating cycle, causing the thermostat to open the circuit before the space reaches the setpoint. This reduces overshoot and short cycling. It does not control the blower or compressor delay.

Question 47HVAC and Refrigeration Trade Competency

A technician is troubleshooting a system with a low suction pressure and a low superheat. The evaporator coil is partially frosted. What is the most likely cause?

  1. A faulty TXV that is stuck open.
  2. A failed evaporator fan motor. Correct answer.
  3. A restricted liquid line.
  4. An overcharge of refrigerant.

Explanation

Low suction pressure with low superheat and partial frosting indicates that the evaporator is not absorbing enough heat, often due to reduced airflow from a failed fan motor. A restricted liquid line would cause high superheat, an overcharge would cause high suction pressure, and a TXV stuck open would cause low superheat but typically normal to high suction pressure.

Question 48HVAC and Refrigeration Trade Competency

What is the purpose of a Schrader valve core in a refrigeration system?

  1. To provide a service port for charging and pressure measurement. Correct answer.
  2. To regulate the flow of refrigerant to the evaporator.
  3. To allow refrigerant to flow only in one direction.
  4. To relieve excess pressure in the system.

Explanation

A Schrader valve core is a spring-loaded valve that allows access to the system for charging, recovery, and pressure measurement while preventing refrigerant loss when the service hose is removed. It does not control flow direction, regulate refrigerant flow, or act as a pressure relief device.

Question 49HVAC and Refrigeration Trade Competency

A technician is measuring the static pressure of a duct system and obtains a reading of 0.5 in. w.c. at the supply plenum and -0.2 in. w.c. at the return plenum. What is the total external static pressure?

  1. 0.7 in. w.c. Correct answer.
  2. 0.3 in. w.c.
  3. 0.2 in. w.c.
  4. 0.5 in. w.c.

Explanation

Total external static pressure is the sum of the absolute values of the supply and return static pressures: 0.5 + 0.2 = 0.7 in. w.c. This value is compared to the blower's rated static pressure to determine if the duct system is restrictive. The other answers either subtract the values or use only one reading.

Question 50HVAC and Refrigeration Trade Competency

Which of the following is a requirement for a refrigerant recovery cylinder before it can be filled?

  1. It must be painted a specific color.
  2. It must be evacuated to 500 microns.
  3. It must be DOT-approved and labeled for the refrigerant being recovered. Correct answer.
  4. It must be filled with nitrogen.

Explanation

Recovery cylinders must meet DOT specifications and be labeled with the type of refrigerant they contain. They do not need to be evacuated or filled with nitrogen before use. While color coding is common, it is not a substitute for proper labeling and is not a regulatory requirement.