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NATE AC Installation practice questions

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Question 1Operating Conditions

A technician records a return-air dry-bulb temperature of 75°F and a supply-air dry-bulb temperature of 57°F at steady-state cooling. What is the indoor temperature split?

  1. 12°F
  2. 18°F Correct answer.
  3. 132°F
  4. 22°F

Explanation

The indoor temperature split is the return-air dry-bulb minus the supply-air dry-bulb. Subtracting 57°F from 75°F gives 18°F. This value is compared with the manufacturer's expected split at the measured indoor wet-bulb and outdoor ambient.

Question 2Operating Conditions

Which instrument is used to measure the wet-bulb temperature of air entering an evaporator coil?

  1. Sling psychrometer Correct answer.
  2. U-tube manometer
  3. Rotating vane anemometer
  4. Micron vacuum gauge

Explanation

Wet-bulb temperature reflects evaporative cooling and is measured with a psychrometer, which has wet and dry bulbs. Manometers measure pressure, anemometers measure air velocity, and micron gauges measure vacuum during evacuation.

Question 3Operating Conditions

Before recording operating pressures and temperatures on a newly installed split system, what should the technician do?

  1. Charge to the nameplate value without weighing the line set
  2. Run the system in heating mode for at least 30 minutes
  3. Operate the system until pressures and temperatures stabilize Correct answer.
  4. Check pressures with the compressor locked out

Explanation

Operating conditions must be evaluated at steady state so that pressures and temperatures are representative. The system is normally allowed to run 15 to 30 minutes, with indoor and outdoor conditions in the manufacturer's charging range, before readings are recorded.

Question 4Operating Conditions

A fixed-orifice system operates at 78°F indoor wet-bulb and 95°F outdoor dry-bulb. The charging chart calls for 12°F superheat. The suction line is 62°F and suction pressure corresponds to 50°F saturated evaporating temperature. What is the actual superheat and the needed action?

  1. 6°F; recover refrigerant to raise superheat
  2. 12°F; no charge adjustment is needed Correct answer.
  3. 12°F; add refrigerant to raise superheat
  4. 18°F; add refrigerant to lower superheat

Explanation

Superheat is suction-line temperature minus saturated suction temperature: 62°F minus 50°F equals 12°F. Since the measured superheat matches the target for the recorded indoor wet-bulb and outdoor dry-bulb, the charge is correct and should not be adjusted.

Question 5Operating Conditions

A TXV system has a liquid-line temperature of 95°F and a liquid-line pressure corresponding to 110°F saturation. What is the subcooling?

  1. 28°F
  2. 205°F
  3. 15°F Correct answer.
  4. 13°F

Explanation

Subcooling is saturated condensing (liquid-line) temperature minus liquid-line temperature. Here, 110°F minus 95°F equals 15°F. The result is compared with the manufacturer's target subcooling for the current operating conditions.

Question 6Operating Conditions

A 3-ton residential cooling system is being checked for design airflow. What indoor airflow is typically targeted?

  1. 2,400 cfm
  2. 800 cfm
  3. 1,200 cfm Correct answer.
  4. 1,600 cfm

Explanation

Nominal residential cooling airflow is about 400 cfm per ton. For a 3-ton system, 3 times 400 equals 1,200 cfm. The manufacturer's blower table should still be consulted because some equipment is designed for 350 to 450 cfm per ton.

Question 7Operating Conditions

As outdoor ambient dry-bulb rises while indoor load and airflow remain steady, how will a properly charged fixed-orifice cooling system most likely respond?

  1. Higher condensing pressure and higher amp draw Correct answer.
  2. No change in either pressure because the refrigerant circuit is sealed
  3. Lower condensing pressure with lower compressor amp draw
  4. Lower suction pressure with unchanged condensing pressure

Explanation

A higher outdoor ambient reduces the condenser's ability to reject heat, so condensing pressure and temperature rise. The compressor then works against a greater pressure difference, which typically increases amp draw; suction pressure may change only slightly.

Question 8Operating Conditions

A blower rated for 0.50 in. w.c. external static pressure is measured at 0.80 in. w.c. with a clean filter. What is the most likely effect and the appropriate next check?

  1. No airflow change; static pressure has no effect on cfm
  2. Lower airflow; add refrigerant to compensate for the reduced load
  3. Higher airflow; reduce blower speed to lower static
  4. Lower airflow; check duct and coil restrictions Correct answer.

Explanation

External static pressure above the blower rating indicates added resistance, so airflow will usually drop below design. The technician should look for undersized or restrictive ductwork, a dirty evaporator coil, or other blockages rather than adjusting the refrigerant charge.

Question 9Operating Conditions

A manufacturer requires weighing in the charge when outdoor ambient is below 55°F. Why is the subcooling method unreliable at that ambient?

  1. Subcooling is only valid for systems with fixed-orifice metering devices
  2. Subcooling increases automatically at low ambient and hides undercharge
  3. The liquid line is too hot at low ambient for subcooling to be measured
  4. Low ambient prevents stable condensing, making subcooling unreliable Correct answer.

Explanation

Subcooling charging requires the condenser to develop a stable condensing temperature under representative load. At low outdoor ambient, condensing pressure may be too low and unstable for a meaningful subcooling reading, so the charge is weighed in per the nameplate and line-set adjustment.

Question 10Operating Conditions

A technician measures 95°F condenser entering air and a condensing temperature of 120°F. What is the condenser approach, and what does the value suggest?

  1. 15°F; the system is operating within a typical approach range
  2. 25°F; the system is undercharged and requires additional refrigerant charge
  3. 25°F; restricted condenser airflow or a dirty coil Correct answer.
  4. 215°F; the pressure-temperature chart was read incorrectly

Explanation

Condenser approach is condensing temperature minus entering-air dry-bulb: 120°F minus 95°F equals 25°F. A high approach at a steady condition suggests the condenser is not rejecting heat well, often because of dirty coil surfaces, restricted airflow, or fan problems.

Question 11Operating Conditions

A technician is measuring external static pressure on a residential air handler. Where should the high-pressure port of the manometer be connected?

  1. Return-air plenum upstream of the filter
  2. Suction-line service port
  3. Condensate drain trap outlet
  4. Supply-air plenum downstream of the coil Correct answer.

Explanation

External static pressure is the pressure difference between the return-air plenum and the supply-air plenum. The high-pressure port belongs on the supply side, downstream of the coil, while the low-pressure port references the return side.

Question 12Operating Conditions

Where should a dry-bulb probe be placed when measuring supply-air temperature for a temperature-split check?

  1. In the return duct within 6 inches of the filter
  2. In the supply plenum, out of coil line of sight Correct answer.
  3. Touching the evaporator coil face
  4. Outside the building next to the condenser

Explanation

Supply-air temperature should be measured in the supply plenum or trunk, away from direct radiant influence of the coil. A probe touching the coil or placed in the return duct will not represent the air leaving the evaporator.

Question 13Operating Conditions

Which instrument is used to measure air velocity at a supply register when checking delivered airflow?

  1. Manifold gauge set
  2. Micron gauge
  3. Clamp meter
  4. Anemometer Correct answer.

Explanation

An anemometer measures air velocity at a register or grille; multiplying velocity by effective free area gives delivered airflow. Micron gauges, clamp meters, and manifold gauges do not measure air velocity.

Question 14Operating Conditions

A technician measures 52°F at the suction line at the evaporator outlet and a suction pressure that corresponds to 40°F saturation. What is the superheat?

  1. 52°F
  2. 12°F Correct answer.
  3. 92°F
  4. 40°F

Explanation

Superheat is the difference between the measured suction-line temperature and the saturation temperature at the same pressure. Here 52°F minus 40°F equals 12°F.

Question 15Operating Conditions

A technician measures a 26°F temperature split across the evaporator, which is above the typical range. The filter is clean. Which condition most likely causes this reading?

  1. Low refrigerant charge
  2. High outdoor ambient temperature
  3. An oversized return-air duct
  4. Restricted evaporator airflow Correct answer.

Explanation

A temperature split above the normal range usually means the evaporator is removing more sensible heat per unit of airflow than expected. Restricted airflow, such as a dirty coil or blower wheel, lowers airflow and raises the split.

Question 16Operating Conditions

A rooftop condenser is installed beside a parapet wall. The technician measures 105°F air entering the condenser while the local outdoor dry-bulb is 95°F. What does this indicate?

  1. Low refrigerant charge
  2. A restricted liquid line
  3. Normal condenser approach temperature
  4. Recirculated condenser air Correct answer.

Explanation

The condenser must draw outdoor air at close to ambient dry-bulb; an entering-air reading 10°F above ambient indicates hot discharge air is being recirculated. Recirculation raises condensing pressure and reduces capacity.

Question 17Operating Conditions

When measuring liquid-line temperature to calculate subcooling, where should the probe be placed?

  1. Liquid line at the evaporator inlet
  2. On the suction line at the evaporator outlet
  3. Liquid line near the condenser outlet Correct answer.
  4. On the discharge line near the compressor

Explanation

Subcooling is measured at the liquid line near the condenser outlet, after the condenser has fully condensed the refrigerant and before any pressure drop from a receiver or metering device. The probe must contact the line and be insulated.

Question 18Operating Conditions

A technician performs a total duct leakage test on a residential system. To what test pressure is the duct system typically pressurized?

  1. 250 Pa (1.0 in. w.c.)
  2. 100 Pa (0.4 in. w.c.)
  3. 500 Pa (2.0 in. w.c.)
  4. 25 Pa (0.1 in. w.c.) Correct answer.

Explanation

Residential total duct leakage tests commonly pressurize the duct system to 25 Pa, about 0.1 in. w.c., and measure leakage airflow at that pressure. Higher pressures are used in some commercial or specialized tests but are not the standard residential test pressure.

Question 19Operating Conditions

A three-phase compressor receives line voltages of 208 V, 216 V, and 224 V. What is the percent voltage imbalance, and does it exceed the typical 2% limit?

  1. 7.4%, exceeds the 2% limit
  2. 1.5%, does not exceed the 2% limit
  3. 12.0%, exceeds the 2% limit
  4. 3.7%, exceeds the 2% limit Correct answer.

Explanation

Average voltage is (208 + 216 + 224) / 3 = 216 V. The greatest deviation is 8 V, so 8 / 216 × 100 = 3.7%, which exceeds the typical 2% limit.

Question 20Operating Conditions

A return-air dry-bulb is 75°F and wet-bulb is 63°F. Using a psychrometric chart, which relative humidity is closest?

  1. 100%
  2. 60% Correct answer.
  3. 40%
  4. 80%

Explanation

At 75°F dry-bulb and 63°F wet-bulb, the wet-bulb depression is 12°F. On a psychrometric chart that intersection is near 60% relative humidity.

Question 21Operating Conditions

A technician is about to record the outdoor ambient dry-bulb temperature on a newly installed condenser so it can be compared with the manufacturer's data. Where should the probe be placed?

  1. On the cabinet top beside the fan grille, in the discharge airstream
  2. Clamped to the hot-gas line where it leaves the compressor
  3. In direct sun against the coil guard on the sunniest side of the unit
  4. In the shade at the coil inlet, at mid-coil height, clear of the fan discharge Correct answer.

Explanation

The ambient reading must represent the air actually entering the condenser coil, so the probe belongs on the air-entering side, shaded, at roughly mid-coil height. Sun-exposed surfaces, fan discharge air, and refrigerant lines all bias the number and will distort any comparison with published capacity or charging data.

Question 22Operating Conditions

A fixed-orifice split system at 80°F dry-bulb and 67°F wet-bulb indoors shows suction pressure below the expected value, superheat well above the charging-chart value, condensing temperature only slightly above ambient, and 2°F of subcooling. Which condition best fits these readings?

  1. An overcharge of refrigerant
  2. Non-condensable gases in the system
  3. A liquid-line restriction
  4. An undercharge of refrigerant Correct answer.

Explanation

Low subcooling plus a condensing temperature barely above ambient plus excessive superheat means too little refrigerant is circulating. The starved evaporator drives superheat up and suction pressure down, and the partially filled condenser has little liquid left to subcool.

Question 23Operating Conditions

To compare a new split system's measured capacity with the manufacturer's published rating tables, which set of conditions matches the standard cooling rating point?

  1. 75°F dry-bulb / 63°F wet-bulb indoors and 82°F dry-bulb outdoors
  2. 80°F dry-bulb / 67°F wet-bulb indoors and 95°F dry-bulb outdoors Correct answer.
  3. 80°F dry-bulb / 67°F wet-bulb indoors and 105°F dry-bulb outdoors
  4. 70°F dry-bulb / 58°F wet-bulb indoors and 95°F dry-bulb outdoors

Explanation

Published residential cooling ratings are based on 80°F dry-bulb and 67°F wet-bulb air entering the indoor unit with 95°F dry-bulb air entering the outdoor unit. Readings taken at other indoor or outdoor conditions must be adjusted using the manufacturer's performance tables before they are compared to the rating.

Question 24Operating Conditions

A technician measures 12°F of superheat at the evaporator outlet but 18°F at the service valves on a unit with a long, uninsulated suction line run. Which statement explains the difference?

  1. The higher reading at the service valves proves the system is overcharged with refrigerant
  2. Refrigerant cools as it travels through the suction line, so superheat should read lower at the service valves
  3. Heat gain and pressure drop along the suction line raise the superheat seen at the service valves Correct answer.
  4. The readings should be identical, so any difference means the thermostatic expansion valve is hunting

Explanation

Superheat belongs to the evaporator outlet, where the expansion valve actually controls it. Between that point and the outdoor unit the suction line picks up heat and loses a little pressure, and both effects increase the superheat calculated from the service-valve readings.

Question 25Operating Conditions

A new 3-ton system at 80°F dry-bulb and 67°F wet-bulb indoors shows a 12°F temperature split while superheat and subcooling are both on target for a thermostatic expansion valve system. The filter and evaporator coil are clean. What is the most likely explanation?

  1. Evaporator airflow is above design, so the same capacity is spread over more air Correct answer.
  2. The blower is delivering less air than design, so the coil is over-cooling the air
  3. The evaporator coil is partly blocked internally, reducing heat transfer
  4. The system is undercharged, so less refrigerant is boiling in the coil

Explanation

A low temperature split with correct superheat and subcooling points to excess airflow rather than a charge or coil problem, because a thermostatic expansion valve holds superheat nearly constant while subcooling tracks charge. More air across the same coil absorbs the same heat over a larger mass of air, shrinking the dry-bulb difference.

Question 26Operating Conditions

Which superheat value falls within the range a manufacturer typically publishes for a thermostatic expansion valve evaporator at design conditions?

  1. 25°F
  2. 0°F
  3. 10°F Correct answer.
  4. 35°F

Explanation

A thermostatic expansion valve is set to hold superheat roughly constant, commonly about 8°F to 12°F at design conditions. Superheat near zero risks liquid floodback to the compressor, while 25°F or more indicates the coil is being starved.

Question 27Operating Conditions

On a thermostatic expansion valve system the technician verifies a clean condenser, correct condenser fan operation, and 82°F outdoor ambient. Subcooling measures 22°F against a 10°F target, condensing temperature is well above ambient, and superheat sits at the low end of the published range. What is the most likely cause?

  1. A liquid-line restriction
  2. Undercharge of refrigerant
  3. Overcharge of refrigerant Correct answer.
  4. Low evaporator airflow

Explanation

Excess refrigerant backs liquid up in the condenser, so subcooling climbs and condensing temperature rises above what the ambient can support, while the valve holds superheat near its setting. With the coil verified clean and the fan confirmed operating, overcharge is the remaining explanation for the high subcooling.

Question 28Operating Conditions

A return duct passes through a 135°F attic. Dry-bulb is 75°F at the return grille, 84°F at the air-handler inlet, and 60°F in the supply plenum. Which statement correctly interprets the 24°F difference between air-handler inlet and supply?

  1. The 9°F gain in the attic duct lowers the coil's entering air temperature and reduces capacity
  2. The 24°F split proves the blower is moving more air than the design airflow
  3. The 24°F split confirms the evaporator is starved for refrigerant
  4. The split is inflated by return-duct heat gain and should not be judged against the manufacturer's coil split Correct answer.

Explanation

The coil sees the 84°F air at the air-handler inlet, so part of the measured difference is heat the return duct picked up in the attic rather than cooling delivered to the house. Judging airflow from that inflated number can lead a technician to misdiagnose a duct insulation problem as a refrigerant or blower fault.

Question 29Operating Conditions

A 15 kW electric resistance heater is operating at rated voltage with the blower running steadily. Return air at the air handler is 70°F, supply air is 100°F, and the manufacturer's blower table lists 1,600 cfm for the current tap and static. Using the sensible heat equation, what airflow is the system delivering?

  1. 1,180 cfm
  2. 790 cfm
  3. 1,580 cfm Correct answer.
  4. 2,370 cfm

Explanation

Airflow equals Btu per hour divided by 1.08 times the temperature rise. The heater delivers 15 × 3,413 = 51,195 Btu/h, and 51,195 ÷ (1.08 × 30) is about 1,580 cfm, which closely matches the blower table and confirms the duct system is not strangling the unit.

Question 30Operating Conditions

A fixed-orifice system was commissioned at 80°F dry-bulb and 67°F wet-bulb indoors. On a callback the indoor dry-bulb is still 80°F but the wet-bulb has risen to 71°F, with outdoor conditions and airflow unchanged. How do suction pressure and superheat compare with the original readings?

  1. Suction pressure lower and superheat lower
  2. Suction pressure lower and superheat higher
  3. Suction pressure higher and superheat lower Correct answer.
  4. Suction pressure higher and superheat higher

Explanation

A higher entering wet-bulb means more moisture is condensing on the coil, so the evaporator absorbs more heat and refrigerant boils faster, raising suction pressure. With a fixed orifice the increased boiling and reduced pressure differential across the piston both drive superheat down, which is why charge charts are indexed to indoor wet-bulb.

Question 31Operating Conditions

For a standard residential split air-conditioning system, the nominal airflow used for charging and performance checks is approximately how many cubic feet per minute per ton of cooling capacity?

  1. 200 cfm per ton
  2. 500 cfm per ton
  3. 300 cfm per ton
  4. 400 cfm per ton Correct answer.

Explanation

Nominal residential airflow is about 400 cfm per ton of cooling capacity. This value supports the sensible and latent heat removal the evaporator coil is rated for and is the basis for many manufacturer airflow tables. Higher or lower airflow changes the temperature split and the charge verification results.

Question 32Operating Conditions

Which instrument is designed to measure both dry-bulb and wet-bulb temperatures at the return-air opening?

  1. Digital manometer
  2. Sling psychrometer Correct answer.
  3. Electronic vacuum gauge
  4. Clamp-on ammeter

Explanation

A sling psychrometer uses wet-bulb and dry-bulb thermometers to measure both temperatures directly. Those two readings establish the entering-air condition used for capacity and charge comparisons. Manometers, ammeters, and vacuum gauges do not measure air temperature.

Question 33Operating Conditions

Which indoor dry-bulb and wet-bulb condition is commonly used as the standard rating point for residential cooling equipment?

  1. 80°F dry-bulb and 67°F wet-bulb Correct answer.
  2. 85°F dry-bulb and 72°F wet-bulb
  3. 70°F dry-bulb and 58°F wet-bulb
  4. 75°F dry-bulb and 63°F wet-bulb

Explanation

Residential cooling ratings commonly use 80°F dry-bulb and 67°F wet-bulb entering air. The wet-bulb value represents indoor humidity and strongly affects evaporator capacity. The other pairs are not the standard rating point.

Question 34Operating Conditions

A technician measures 1,050 cfm through a 3-ton evaporator. What is the airflow per ton?

  1. 250 cfm per ton
  2. 350 cfm per ton Correct answer.
  3. 325 cfm per ton
  4. 300 cfm per ton

Explanation

Divide the measured airflow by the nominal capacity: 1,050 cfm ÷ 3 tons = 350 cfm per ton. This is below the common 400 cfm per ton nominal value and may indicate a restrictive duct system or a lower blower speed. The technician should compare it with the manufacturer's airflow table before adjusting charge.

Question 35Operating Conditions

A technician measures return static pressure of −0.30 in. w.c. and supply static pressure of +0.45 in. w.c. at an air handler. What is the external static pressure?

  1. 0.75 in. w.c. Correct answer.
  2. 0.45 in. w.c.
  3. 0.30 in. w.c.
  4. 0.15 in. w.c.

Explanation

External static pressure is the difference between the supply and return static pressures at the air handler. The supply is +0.45 and the return is −0.30, so the difference is 0.45 + 0.30 = 0.75 in. w.c. The signs show the two sides of the blower.

Question 36Operating Conditions

A technician measures a suction-line temperature of 55°F and a suction pressure that corresponds to a saturation temperature of 40°F. What is the superheat?

  1. 40°F
  2. 15°F Correct answer.
  3. 5°F
  4. 25°F

Explanation

Superheat is the suction-line temperature minus the saturation temperature corresponding to suction pressure. Here, 55°F − 40°F = 15°F superheat. This confirms the refrigerant is fully vapor before the compressor.

Question 37Operating Conditions

To calculate subcooling on a TXV system, the technician measures liquid-line temperature and which other value?

  1. Suction-line temperature
  2. Suction pressure
  3. Outdoor dry-bulb temperature
  4. Liquid-line pressure converted to saturation temperature Correct answer.

Explanation

Subcooling is the difference between the saturation temperature for the liquid-line pressure and the measured liquid-line temperature. The technician must convert liquid-line pressure to its saturation temperature before subtracting. Suction pressure and outdoor temperature are separate checks.

Question 38Operating Conditions

A 3-ton air handler is rated for a maximum external static pressure of 0.50 in. w.c. at 1,200 cfm. The installed duct system measures 0.80 in. w.c. while the blower is on the same speed tap. Which operating condition should the technician expect?

  1. Airflow will decrease only if the filter is dirty
  2. Airflow will remain unchanged because the blower speed is fixed
  3. Airflow will increase and the temperature split will decrease
  4. Airflow will decrease and the temperature split will increase Correct answer.

Explanation

The blower is moving air against more resistance than its rating allows, so delivered airflow falls. With less air across the evaporator, the temperature split rises and the coil may run colder. The same speed tap cannot maintain 1,200 cfm when external static rises from 0.50 to 0.80 in. w.c.

Question 39Operating Conditions

A technician measures an average velocity of 500 fpm at a 12 in. by 12 in. supply grille. The grille has a free-area factor of 0.75. What is the approximate delivered airflow?

  1. 667 cfm
  2. 375 cfm Correct answer.
  3. 500 cfm
  4. 750 cfm

Explanation

Airflow equals velocity multiplied by the free area of the opening. A 12 in. by 12 in. grille has 1.0 ft² gross area; at a 0.75 free-area factor, the effective area is 0.75 ft². Thus 500 fpm × 0.75 ft² = 375 cfm.

Question 40Operating Conditions

A condenser coil is partially blocked by a piece of cardboard during a cooling check. Which set of operating conditions is most likely?

  1. Higher evaporator airflow and smaller temperature split
  2. Lower condensing pressure and lower subcooling
  3. Higher condensing pressure and higher subcooling Correct answer.
  4. Lower suction pressure and higher superheat

Explanation

Blocking the condenser reduces heat rejection, so condensing pressure and temperature rise. Higher condensing pressure increases the amount of subcooling at the liquid line. Evaporator airflow and suction pressure are affected mainly by indoor conditions.

Question 41System Components

In a split air-conditioning system, what is the primary function of the metering device?

  1. It reverses the direction of refrigerant flow when the system changes between heating and cooling.
  2. It removes moisture and acid from the refrigerant before the refrigerant enters the evaporator coil.
  3. It regulates refrigerant flow into the evaporator and maintains the high-to-low pressure difference. Correct answer.
  4. It stores excess liquid refrigerant in the system during low-load operation.

Explanation

The metering device creates the pressure drop between the condenser and evaporator and controls how much refrigerant enters the evaporator, which establishes superheat. Moisture and acid removal is the job of a filter drier, not the metering device. A receiver stores excess liquid, and a reversing valve changes flow direction in a heat pump.

Question 42System Components

What is the primary purpose of the liquid-line filter drier in a residential split system?

  1. It traps particles and absorbs moisture from liquid refrigerant. Correct answer.
  2. It provides a liquid seal for the evaporator coil.
  3. It dampens discharge pressure pulsations produced by the compressor.
  4. It separates lubricating oil from the refrigerant vapor leaving the evaporator coil.

Explanation

A filter drier is installed in the liquid line to capture particulate matter and absorb moisture before the refrigerant reaches the metering device. Keeping the refrigerant clean and dry protects the metering device and compressor from contamination and acid formation.

Question 43System Components

In a typical residential condensing unit, what is the function of the compressor contactor?

  1. It balances refrigerant pressure during the off cycle.
  2. It protects the compressor from excessive discharge temperature.
  3. It converts line-voltage alternating current to low-voltage direct current for the thermostat.
  4. It switches high-voltage power to the compressor and outdoor fan motor. Correct answer.

Explanation

The contactor is a relay with a low-voltage coil and high-voltage contacts. When the thermostat energizes the coil, the contacts close to supply power to the compressor and condenser fan motor; when the coil is de-energized, the contacts open.

Question 44System Components

Where should the sensing bulb of a thermostatic expansion valve be mounted?

  1. On the liquid line immediately ahead of the valve inlet, downstream of the filter drier
  2. On the discharge line within six inches of the compressor
  3. On the clean suction line at the evaporator outlet, securely clamped and insulated Correct answer.
  4. On the suction line downstream of the suction accumulator

Explanation

The TXV bulb senses suction-line temperature at the evaporator outlet so the valve can regulate superheat. It must be clamped tightly to a clean, straight section of suction line and insulated so ambient air does not influence the reading.

Question 45System Components

What is the purpose of a suction-line accumulator in a heat pump or air-conditioning system?

  1. It stores refrigerant charge in the liquid line during heating mode.
  2. It provides a mounting point for the suction line filter drier.
  3. It keeps liquid refrigerant from reaching the compressor. Correct answer.
  4. It increases refrigerant subcooling at the condenser outlet.

Explanation

An accumulator is installed in the suction line to hold liquid refrigerant that may slug or migrate during off-cycle or flooded-start conditions. It meters oil and vapor back to the compressor while preventing liquid damage.

Question 46System Components

Why is a crankcase heater used on some compressors?

  1. To warm the suction line and increase superheat at start-up
  2. To preheat the evaporator coil before defrost
  3. To prevent refrigerant migration into the crankcase during the off cycle Correct answer.
  4. To raise discharge pressure during low-ambient cooling operation

Explanation

The crankcase heater warms compressor oil so refrigerant that migrates to the crankcase during shutdown does not condense and dilute the oil. This helps prevent oil pump-out and compressor damage at start-up.

Question 47System Components

How does a variable-speed ECM blower motor respond as duct static pressure increases?

  1. It operates only at full speed whenever the thermostat calls for fan-only operation.
  2. It requires a run capacitor to determine its operating speed.
  3. It maintains a fixed speed regardless of duct static pressure.
  4. It adjusts torque to hold selected airflow as static pressure changes. Correct answer.

Explanation

An ECM blower uses electronic commutation and feedback to vary torque and speed. As external static pressure rises, the motor increases torque to maintain the programmed airflow, within its performance limits.

Question 48System Components

What does the reversing valve do in a heat pump?

  1. It prevents the compressor from starting with unequal pressures.
  2. It bypasses the outdoor coil during the defrost cycle.
  3. It regulates the amount of refrigerant that enters the indoor coil during heating.
  4. It redirects refrigerant flow so the indoor and outdoor coils swap roles. Correct answer.

Explanation

The reversing valve changes the direction of refrigerant flow between heating and cooling. This makes the indoor coil the condenser in heating and the evaporator in cooling, while the outdoor coil takes the opposite role.

Question 49System Components

A suction-line filter drier is installed after a compressor burnout. Which statement best describes its purpose and service limitation?

  1. It traps acid and debris, but adds pressure drop and must be removed after cleanup. Correct answer.
  2. It filters refrigerant vapor and can be cleaned by back-flushing with nitrogen.
  3. It increases suction superheat and should be installed downstream of the accumulator.
  4. It absorbs moisture from the liquid line and is left in place for the entire life of the system.

Explanation

A suction-line filter drier is used temporarily to capture acid, sludge, and particles before they reach the replacement compressor. Because it creates a pressure drop and can become restricted, it is normally removed after the cleanup period.

Question 50System Components

Why is a low-ambient fan-cycling control used on an air-conditioning condensing unit?

  1. To raise suction pressure for better dehumidification
  2. To reduce compressor current draw when the outdoor ambient temperature is high
  3. To prevent the evaporator coil from freezing when the indoor heat load is low
  4. To maintain enough condensing pressure to feed liquid to the metering device Correct answer.

Explanation

In low outdoor ambient, the condenser can reject more heat than needed, causing condensing pressure to fall. A fan-cycling control maintains a minimum head pressure so the metering device still receives liquid refrigerant and the compressor remains properly lubricated.