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HVAC Excellence Heat Pump study materials
HVAC Excellence Heat Pump study resources

HVAC Excellence Heat Pump practice questions

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Question 1Heat Pump Fundamentals and Refrigeration Cycle

In a vapor-compression heat pump, which component raises the refrigerant's pressure and temperature so that it can reject heat to the conditioned space?

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

Explanation

The compressor is the pump of the cycle; it draws low-pressure vapor from the evaporator and compresses it to a higher pressure and temperature so the condenser can reject heat. The metering device does the opposite, dropping pressure between the condenser and evaporator.

Question 2Heat Pump Fundamentals and Refrigeration Cycle

Which refrigerant state should exist at the outlet of the evaporator in a properly charged heat pump?

  1. Subcooled liquid
  2. A saturated liquid-vapor mixture near 50 percent quality
  3. Superheated vapor with a small amount of superheat Correct answer.
  4. Two-phase mixture with liquid carryover

Explanation

The evaporator must fully boil the refrigerant so vapor, not liquid, returns to the compressor. A small superheat margin confirms complete evaporation and protects the compressor from slugging.

Question 3Heat Pump Fundamentals and Refrigeration Cycle

During the heating season, the outdoor coil of an air-source heat pump performs which function?

  1. It acts as the condenser and rejects heat to outdoor air
  2. It acts as the evaporator and absorbs heat from outdoor air Correct answer.
  3. It stores liquid refrigerant for defrost
  4. It bypasses refrigerant directly to the suction line

Explanation

In heating mode the reversing valve makes the outdoor coil the evaporator, so refrigerant boils there at a temperature below outdoor air and absorbs heat. The indoor coil becomes the condenser and delivers that heat to the space.

Question 4Heat Pump Fundamentals and Refrigeration Cycle

What is the primary purpose of the reversing valve in a heat pump?

  1. To modulate compressor capacity in response to load
  2. To control refrigerant charge during defrost
  3. To reduce suction-line pressure before the compressor
  4. To swap the roles of the indoor and outdoor coils between heating and cooling Correct answer.

Explanation

The reversing valve redirects discharge and suction gas so the indoor coil can be either the condenser (heating) or the evaporator (cooling). That single change of flow direction is what makes one circuit serve both seasons.

Question 5Heat Pump Fundamentals and Refrigeration Cycle

A technician measures the refrigerant entering the metering device as a saturated liquid at 100 psig and the leaving mixture at 40 psig. What has the metering device accomplished?

  1. It increased the mass flow rate through the compressor
  2. It removed moisture from the refrigerant
  3. It dropped refrigerant pressure and temperature to feed the evaporator Correct answer.
  4. It raised refrigerant enthalpy to increase heating capacity

Explanation

The metering device throttles refrigerant from condensing pressure to evaporating pressure, and the temperature falls with the pressure. The flash gas formed helps distribute refrigerant evenly through the evaporator.

Question 6Heat Pump Fundamentals and Refrigeration Cycle

A heat pump in heating mode shows a suction pressure well below normal and a frosted outdoor coil that never fully clears. Which condition best explains this?

  1. A failed indoor blower motor
  2. A stuck reversing valve in cooling
  3. Restricted refrigerant flow through the outdoor coil or a low charge Correct answer.
  4. An overcharge of refrigerant

Explanation

Low suction pressure combined with persistent frost points to starved evaporator capacity, either from a restriction or insufficient charge. The coil cannot absorb enough heat, so its temperature falls below freezing and frost accumulates.

Question 7Heat Pump Fundamentals and Refrigeration Cycle

Which statement correctly describes the pressure-enthalpy relationship across the condenser?

  1. Pressure stays essentially constant while enthalpy decreases as heat is rejected Correct answer.
  2. Pressure rises while enthalpy increases
  3. Pressure and enthalpy both remain unchanged
  4. Pressure drops sharply while enthalpy stays constant

Explanation

In the condenser, refrigerant rejects heat at nearly constant pressure, so enthalpy falls while pressure is set by saturation conditions. The large enthalpy drop is what the airstream picks up as delivered heat.

Question 8Heat Pump Fundamentals and Refrigeration Cycle

What does superheat measured at the evaporator outlet indicate about the evaporator?

  1. The amount of liquid stored in the receiver
  2. The condensing temperature of the system
  3. How completely the coil is being fed with refrigerant Correct answer.
  4. The compressor's volumetric efficiency

Explanation

Superheat is the difference between suction-line temperature and evaporating temperature. A small, stable superheat means the coil is well fed; excessive superheat means the coil is starved and not using its full surface.

Question 9Heat Pump Fundamentals and Refrigeration Cycle

A heat pump's indoor coil ices during the heating cycle while the outdoor unit runs normally. Which cause is most consistent with this symptom?

  1. Restricted indoor airflow across the condenser coil Correct answer.
  2. A defective outdoor fan motor
  3. Too much superheat at the compressor
  4. Low outdoor ambient temperature alone

Explanation

In heating mode the indoor coil is the condenser, so restricted airflow prevents adequate heat rejection and the coil can drop below freezing. Checking the filter, blower, and ductwork is the first diagnostic step.

Question 10Heat Pump Fundamentals and Refrigeration Cycle

Which refrigerant property makes it suitable for use in a heat pump that must operate across a wide outdoor temperature range?

  1. Complete miscibility with mineral oil at all temperatures
  2. A saturation curve that still produces useful pressure differences at low ambient Correct answer.
  3. A boiling point well above typical indoor temperatures
  4. A very high freezing point

Explanation

The refrigerant must still evaporate and produce a workable pressure ratio when outdoor air is cold. A favorable saturation curve keeps capacity and compression ratio within the compressor's operating envelope.

Question 11Heat Pump Fundamentals and Refrigeration Cycle

A technician reads a suction-line temperature of 55 F and an evaporating temperature of 40 F on a system in heating mode. What is the superheat?

  1. 55 F
  2. 40 F
  3. 95 F
  4. 15 F Correct answer.

Explanation

Superheat equals suction-line temperature minus evaporating temperature, so 55 minus 40 gives 15 F. That value tells the technician how far past saturation the vapor has traveled.

Question 12Heat Pump Fundamentals and Refrigeration Cycle

Why is a defrost cycle necessary on an air-source heat pump operating in cold weather?

  1. Moisture in outdoor air freezes on the outdoor coil and blocks airflow Correct answer.
  2. Refrigerant oil thickens and must be warmed
  3. The indoor coil accumulates condensate that must be drained
  4. The compressor needs periodic cooling between cycles

Explanation

When the outdoor coil runs below freezing, airborne moisture frosts the fins and insulates the coil, cutting capacity. Defrost temporarily reverses the cycle so hot discharge gas melts the frost and restores heat transfer.

Question 13Heat Pump Fundamentals and Refrigeration Cycle

Which condition would cause a heat pump to run a defrost cycle too frequently?

  1. A dirty indoor air filter
  2. A loose indoor blower belt
  3. An overcharged system with high subcooling
  4. A faulty outdoor coil sensor or defrost control board Correct answer.

Explanation

Defrost initiation depends on coil temperature and pressure signals interpreted by the control board. A failed sensor or board can misread coil conditions and call for defrost when no frost exists, wasting capacity.

Question 14Heat Pump Fundamentals and Refrigeration Cycle

What is the function of the accumulator in a heat pump system?

  1. It prevents liquid refrigerant from reaching the compressor during off-cycle migration or flooding Correct answer.
  2. It removes non-condensable gases from the circuit
  3. It stores subcooled liquid for the metering device
  4. It regulates the reversing valve pilot pressure

Explanation

The accumulator sits on the suction line and traps liquid so only vapor enters the compressor. It meters oil and liquid back gradually, protecting the compressor from slugging during transient conditions.

Question 15Heat Pump Fundamentals and Refrigeration Cycle

A heat pump delivers adequate heating but the indoor coil shows very high subcooling and the compressor cycles on high pressure. Which condition is most likely?

  1. A failed outdoor fan
  2. Undercharge of refrigerant
  3. Overcharge or restricted indoor airflow Correct answer.
  4. A stuck-open metering device

Explanation

High subcooling with high head pressure indicates excess liquid in the condenser or an inability to reject heat. Both overcharge and restricted indoor airflow produce that pattern in heating mode.

Question 16Heat Pump Fundamentals and Refrigeration Cycle

Which sequence correctly traces refrigerant flow in the cooling mode of a heat pump?

  1. Compressor, indoor coil, accumulator, metering device, outdoor coil
  2. Compressor, outdoor coil, metering device, indoor coil, compressor Correct answer.
  3. Compressor, reversing valve, accumulator, outdoor coil, compressor
  4. Compressor, indoor coil, metering device, outdoor coil, compressor

Explanation

In cooling mode the outdoor coil is the condenser and the indoor coil is the evaporator. Hot gas leaves the compressor, condenses outdoors, passes through the metering device, and evaporates indoors before returning to the compressor.

Question 17Heat Pump Fundamentals and Refrigeration Cycle

What is the purpose of the suction-line accumulator combined with a crankcase heater during the off cycle?

  1. To keep liquid refrigerant out of the compressor oil sump Correct answer.
  2. To lower the condensing temperature
  3. To raise the heating capacity of the outdoor coil
  4. To speed up the defrost cycle

Explanation

During off cycles, refrigerant migrates to the coldest point, often the compressor crankcase. A crankcase heater boils off liquid so oil stays viscous and lubricating, while the accumulator limits liquid return during operation.

Question 18Heat Pump Fundamentals and Refrigeration Cycle

Which statement best explains why a heat pump's heating capacity decreases as outdoor temperature falls?

  1. The evaporator's temperature difference to outdoor air shrinks, reducing heat absorbed Correct answer.
  2. The condenser becomes smaller in cold weather
  3. The compressor's displacement increases at low ambient
  4. The metering device closes completely below 40 F

Explanation

Heating capacity depends on the temperature lift between outdoor air and the evaporating refrigerant. As outdoor air cools, that difference narrows, so less heat is absorbed and capacity declines.

Question 19Heat Pump Fundamentals and Refrigeration Cycle

A heat pump in heating mode has a suction pressure that is higher than normal and the compressor is sweating. What is the most likely cause?

  1. The reversing valve is leaking internally, bypassing hot gas to the suction Correct answer.
  2. The outdoor coil is frosted
  3. The indoor blower is running too fast
  4. The system is low on charge

Explanation

An internal leak in the reversing valve lets high-pressure discharge gas bleed into the suction side, raising suction pressure and temperature. The compressor may sweat or overheat because the returning vapor is unusually warm.

Question 20Heat Pump Fundamentals and Refrigeration Cycle

Which tool is used to determine the evaporating and condensing temperatures of a heat pump?

  1. A clamp meter only
  2. An anemometer only
  3. A vacuum gauge only
  4. A pressure gauge set with a refrigerant temperature-pressure chart Correct answer.

Explanation

Evaporating and condensing temperatures are saturation temperatures read from pressures with a gauge set and converted using the refrigerant's temperature-pressure relationship. Superheat and subcooling are then calculated from line temperatures.

Question 21Heat Pump Fundamentals and Refrigeration Cycle

What is the function of the liquid-line filter-drier in a heat pump circuit?

  1. To separate oil from refrigerant
  2. To store excess refrigerant charge
  3. To trap moisture and particulate contamination that could block the metering device Correct answer.
  4. To reduce discharge pulsation

Explanation

The filter-drier contains a desiccant that removes moisture and a screen that catches debris. Keeping the circuit clean protects the metering device from blockage and prevents acid formation in the oil.

Question 22Heat Pump Fundamentals and Refrigeration Cycle

Why is a heat pump's coefficient of performance typically greater than one in heating mode?

  1. It moves heat rather than generating it by direct conversion Correct answer.
  2. It uses a larger compressor than a furnace
  3. It burns fuel more efficiently than a furnace
  4. It operates at a lower pressure ratio than a furnace

Explanation

A heat pump does not create heat; it transports heat from outdoor air to the space. Because the delivered heat exceeds the compressor work input, the ratio of heat out to energy in can exceed one.

Question 23Heat Pump Fundamentals and Refrigeration Cycle

A technician finds a heat pump that heats poorly, with low suction pressure, low subcooling, and visible bubbles in the liquid line sight glass. What is the likely diagnosis?

  1. The indoor blower is oversized
  2. The system is undercharged Correct answer.
  3. The system is overcharged
  4. The reversing valve is stuck in heating

Explanation

Low suction pressure, low subcooling, and bubbles in the sight glass together indicate insufficient refrigerant charge. Adding charge until subcooling reaches the manufacturer's target corrects the condition.

Question 24Heat Pump Fundamentals and Refrigeration Cycle

Which statement about the compressor's role in the refrigeration cycle is accurate?

  1. It directly cools the refrigerant by expanding it
  2. It removes heat from the refrigerant through its housing
  3. It maintains the pressure difference that allows the evaporator to absorb heat and the condenser to reject it Correct answer.
  4. It controls the amount of refrigerant entering the evaporator

Explanation

The compressor establishes and maintains the high-side and low-side pressures that make heat absorption and rejection possible. Without that pressure difference, the cycle cannot move heat against the indoor-outdoor temperature gradient.

Question 25Heat Pump Fundamentals and Refrigeration Cycle

A heat pump runs but the indoor coil stays at ambient temperature in heating mode while the outdoor unit operates. Which component is most suspect?

  1. The defrost board timing out early
  2. The liquid-line filter-drier being clean
  3. The reversing valve failing to switch to the heating position Correct answer.
  4. The indoor blower running at high speed

Explanation

If the reversing valve stays in cooling position, the indoor coil remains the evaporator and cannot deliver heat. Checking valve solenoid voltage and pilot pressure is the appropriate diagnostic step.

Question 26Heat Pump Fundamentals and Refrigeration Cycle

In a heat pump operating in the cooling mode, which component rejects heat to the outdoor air?

  1. The outdoor coil, which acts as the condenser Correct answer.
  2. The indoor coil, which acts as the condenser
  3. The accumulator, which acts as the condenser
  4. The reversing valve, which acts as the condenser

Explanation

In cooling mode the refrigerant flow is arranged so the outdoor coil receives hot high-pressure discharge vapor and condenses it, rejecting heat to outdoor air. The indoor coil then serves as the evaporator and absorbs heat from the conditioned space.

Question 27Heat Pump Fundamentals and Refrigeration Cycle

A heat pump uses a thermostatic expansion valve. What condition at the evaporator outlet does the valve's sensing bulb control?

  1. The subcooling of liquid leaving the condenser
  2. The pressure drop across the reversing valve
  3. The temperature of the discharge line
  4. The superheat of vapor leaving the evaporator Correct answer.

Explanation

The TXV bulb is clamped to the suction line at the evaporator outlet and senses the refrigerant temperature there. Combined with the evaporator pressure, this regulates the valve opening to hold a target superheat at the coil outlet.

Question 28Heat Pump Fundamentals and Refrigeration Cycle

Which metering device relies on a small fixed orifice and has no moving parts or sensing bulb?

  1. Thermostatic expansion valve
  2. Fixed-bore orifice (piston) Correct answer.
  3. Automatic expansion valve
  4. Electronic expansion valve

Explanation

A fixed-bore piston meters refrigerant through a drilled orifice of set size, so the flow rate depends on the pressure difference across it. It has no modulating element, bulb, or moving parts, unlike thermostatic, electronic, or automatic valves.

Question 29Heat Pump Fundamentals and Refrigeration Cycle

Which statement correctly describes the state of refrigerant entering the compressor under normal operating conditions?

  1. A low-pressure liquid at saturation
  2. A high-pressure vapor with high superheat
  3. A low-pressure vapor with a small amount of superheat Correct answer.
  4. A high-pressure subcooled liquid

Explanation

The compressor should receive vapor at low-side pressure with enough superheat to ensure no liquid droplets reach the valves. Superheat is deliberately maintained at the evaporator outlet, so the suction gas is slightly above its saturation temperature.

Question 30Heat Pump Fundamentals and Refrigeration Cycle

A heat pump in heating mode has a suction pressure lower than the manufacturer's chart predicts for the measured outdoor temperature, and the suction line is warm rather than cold. Which condition best fits these readings?

  1. An overcharge of refrigerant in the system
  2. A restricted liquid line ahead of the metering device
  3. A restricted suction line or evaporator starving the compressor Correct answer.
  4. A condenser fan motor running at full speed

Explanation

Low suction pressure with a warm suction line points to a supply restriction on the low side, so the compressor is pulling vapor faster than the evaporator can feed it. The line warms because little cold refrigerant is flowing through it.

Question 31Heat Pump Fundamentals and Refrigeration Cycle

A technician measures an outdoor ambient of 40 F and a saturated suction temperature of 25 F on a heat pump in heating mode. Which statement about the evaporator is accurate?

  1. The coil is not absorbing heat because the refrigerant is colder than the air Correct answer.
  2. The 15 F difference proves the refrigerant is superheated by 15 F at the coil inlet
  3. The coil must be frosted because the refrigerant is below freezing
  4. The 15 F difference is the coil's approach, and the evaporator is absorbing heat from outdoor air

Explanation

The evaporator absorbs heat only when the refrigerant saturation temperature is below the surrounding air, which is the case here: 25 F refrigerant versus 40 F air. That temperature difference is what drives heat transfer from the outdoor air into the coil.

Question 32Heat Pump Fundamentals and Refrigeration Cycle

A heat pump's reversing valve solenoid is energized in the heating mode on a particular model. The unit heats in cooling mode instead. Which component should be checked first?

  1. The liquid-line filter-drier
  2. The indoor blower motor
  3. The compressor run capacitor
  4. The reversing valve solenoid coil and its wiring Correct answer.

Explanation

If the valve does not shift when the mode changes, refrigerant continues to flow as in the opposite mode, so the unit heats when cooling is called. The solenoid coil and its control wiring are the first check because they command the pilot pressure that moves the valve.

Question 33Heat Pump Fundamentals and Refrigeration Cycle

Why does a heat pump's heating capacity fall as outdoor temperature drops, even though the compressor runs continuously?

  1. The lower evaporating temperature reduces refrigerant density and the heat absorbed per pound Correct answer.
  2. The indoor coil becomes a condenser only at mild outdoor temperatures
  3. Compressor speed decreases automatically as ambient falls
  4. The mass flow of refrigerant rises as the outdoor coil gets colder

Explanation

As outdoor air cools, the evaporator runs at a lower saturation temperature, which lowers suction pressure and vapor density. Less refrigerant mass is pumped and each pound absorbs less heat, so capacity drops.

Question 34Heat Pump Fundamentals and Refrigeration Cycle

A heat pump's compressor is running but the suction line is at room temperature and the suction pressure is near zero. Which conclusion is most reasonable?

  1. The reversing valve is stuck in the cooling position
  2. The condenser is rejecting too much heat
  3. The system is fully charged and operating normally
  4. The compressor is pumping but no refrigerant is reaching it Correct answer.

Explanation

Near-zero suction pressure with a warm suction line means the compressor is moving almost nothing, so the low side is starved or empty. The likely causes are a total restriction, a closed service valve, or a loss of charge.

Question 35Heat Pump Fundamentals and Refrigeration Cycle

During a heating-mode check, the technician finds high head pressure, high subcooling, and a condenser (indoor coil) that is only slightly warm. Which condition is most consistent with these readings?

  1. Restriction in the suction line
  2. Failed defrost termination thermostat
  3. Undercharge of refrigerant
  4. Overcharge or excess refrigerant in the circuit Correct answer.

Explanation

Extra refrigerant backs up liquid in the condenser, raising subcooling and head pressure while limiting the coil surface available for heat rejection. The indoor coil then feels only slightly warm despite the high discharge pressure.

Question 36Heat Pump Fundamentals and Refrigeration Cycle

Which refrigerant property most directly determines the pressure at which a heat pump evaporator will operate at a given coil temperature?

  1. The refrigerant's color and odor additives
  2. The refrigerant's lubricant miscibility
  3. The refrigerant's saturation pressure-temperature relationship Correct answer.
  4. The refrigerant's dielectric strength

Explanation

For any pure refrigerant, saturation pressure and temperature are linked, so choosing an evaporating temperature fixes the corresponding pressure. Technicians use pressure-temperature charts built on this relationship to interpret gauge readings.

Question 37Heat Pump Fundamentals and Refrigeration Cycle

A heat pump has a suction-line temperature of 30 F at the compressor inlet while the outdoor ambient is 45 F. The suction line is frosted back to the compressor. What does this indicate?

  1. A high superheat condition from undercharge
  2. Normal operation with adequate superheat
  3. A restricted discharge line
  4. Liquid refrigerant flooding back to the compressor Correct answer.

Explanation

Frost on the suction line at the compressor means the refrigerant there is at or below freezing, which happens when liquid is returning instead of superheated vapor. Floodback risks oil dilution and compressor damage, so the charge and metering device should be checked.

Question 38Heat Pump Fundamentals and Refrigeration Cycle

A heat pump with a piston-type metering device is installed at a different elevation than the outdoor unit. Why does the manufacturer supply different piston sizes for heating and cooling?

  1. Because the piston must be changed with each defrost cycle
  2. Because the piston material expands differently in heating mode
  3. Because the piston only meters refrigerant in cooling mode
  4. Because the required flow rate differs between the two modes and the pressure drops differ Correct answer.

Explanation

In heating mode the outdoor coil is the evaporator and the indoor coil is the condenser, so the flow direction and the pressure difference across the metering device change. Manufacturers select piston sizes that meter correctly in each direction.

Question 39Heat Pump Fundamentals and Refrigeration Cycle

A heat pump in heating mode shows a suction pressure slightly above normal, a discharge pressure below normal, and a compressor that is hot to the touch but not pumping effectively. Which failure best explains the pattern?

  1. A dirty outdoor coil
  2. A worn compressor with internal bypassing of discharge back to suction Correct answer.
  3. A failed outdoor fan motor
  4. An overcharged system

Explanation

When compressor valves or internal seals wear, high-pressure discharge leaks back into the low side, so suction pressure rises and head pressure falls. The compressor runs hot because the gas is recompressed without useful heat rejection.

Question 40Heat Pump Fundamentals and Refrigeration Cycle

A technician is troubleshooting a heat pump that cools adequately but provides little heat, and notes that the outdoor coil is colder than the outdoor air in heating mode. Which interpretation is correct?

  1. The compressor is bypassing internally
  2. The outdoor coil is acting as a condenser and rejecting heat
  3. The reversing valve has failed and the system is stuck in cooling
  4. The system is behaving as an evaporator outdoors, which is normal for heating mode Correct answer.

Explanation

In heating mode the outdoor coil is the evaporator, so it must be colder than the outdoor air to absorb heat from it. That reading confirms the refrigerant circuit is in heating mode, so the poor heating must be traced to another cause such as charge, airflow, or compressor performance.

Question 41Heat Pump Components and Electrical Controls

A residential heat pump uses a compressor contactor whose coil is rated 24 VAC. Which transformer tap should feed the control circuit?

  1. The 208 VAC primary winding
  2. The 24 VAC secondary winding Correct answer.
  3. The 480 VAC tap on the primary
  4. The 240 VAC line terminals

Explanation

Control circuits in residential heat pumps operate on the low-voltage secondary of the transformer, typically 24 VAC, because the thermostat, contactor coil, and safety switches are all rated for that level. The primary taps connect to the incoming line voltage and must never feed the control loop.

Question 42Heat Pump Components and Electrical Controls

What is the primary function of the reversing valve in an air-source heat pump?

  1. It raises discharge pressure before the condenser
  2. It stores excess refrigerant during the heating cycle
  3. It changes refrigerant flow direction between heating and cooling Correct answer.
  4. It meters liquid refrigerant into the evaporator coil

Explanation

The reversing valve is a four-way solenoid valve that swaps the connections between the compressor discharge and suction lines and the indoor and outdoor coils, allowing the unit to switch between heating and cooling. Metering, storage, and pressure raising are handled by other components.

Question 43Heat Pump Components and Electrical Controls

A heat pump defrost control board uses a demand-defrost strategy. Which input most directly triggers a defrost cycle?

  1. A rise in indoor return-air temperature above setpoint
  2. A drop in outdoor coil temperature combined with a pressure or temperature switch Correct answer.
  3. A drop in compressor suction superheat
  4. A rise in liquid-line subcooling

Explanation

Demand defrost monitors the outdoor coil condition, usually through a coil temperature sensor plus a pressure switch or delta-T sensing, and initiates defrost only when frost buildup is actually detected. Time-temperature boards, by contrast, run on accumulated compressor runtime.

Question 44Heat Pump Components and Electrical Controls

A technician measures 0 VAC across the contacts of a compressor contactor while the thermostat is calling and the compressor is running. What does this indicate?

  1. The contactor is welded open
  2. The coil circuit is open
  3. The transformer secondary has failed
  4. The contacts are closed and conducting Correct answer.

Explanation

A voltage reading across closed contacts should be near zero because the contacts present almost no resistance. If the coil were open or the contacts open, the full control voltage would appear across the contacts. A failed transformer would remove voltage from the entire control circuit.

Question 45Heat Pump Components and Electrical Controls

Which component protects a heat pump compressor from operating with low suction pressure caused by a refrigerant loss?

  1. Defrost thermostat
  2. High-pressure switch
  3. Crankcase heater
  4. Low-pressure switch Correct answer.

Explanation

A low-pressure switch opens the control circuit when suction pressure falls below a set threshold, which commonly indicates a loss of charge or a restriction. This prevents the compressor from running with inadequate refrigerant and overheating.

Question 46Heat Pump Components and Electrical Controls

A heat pump in heating mode has a suction-line accumulator. What is its purpose?

  1. To bypass the reversing valve during defrost
  2. To increase subcooling at the indoor coil
  3. To store liquid refrigerant for the cooling cycle
  4. To prevent liquid refrigerant from reaching the compressor Correct answer.

Explanation

The accumulator is installed on the suction line to trap any liquid refrigerant and oil that carry over from the evaporator, releasing refrigerant vapor to the compressor. This protects the compressor from liquid slugging, which is especially important during low-load heating operation and defrost transitions.

Question 47Heat Pump Components and Electrical Controls

During a defrost cycle, why does the auxiliary heat strip typically energize?

  1. To warm the crankcase oil before restart
  2. To increase refrigerant pressure at the outdoor coil
  3. To offset the cold supply air produced while the outdoor unit defrosts Correct answer.
  4. To speed up the reversing valve changeover

Explanation

While defrosting, the heat pump temporarily reverses to cooling mode at the indoor coil, which delivers cold air to the space. The control board energizes auxiliary electric heat to temper supply air and maintain comfort during that period.

Question 48Heat Pump Components and Electrical Controls

What is the correct function of the crankcase heater on an outdoor heat pump compressor?

  1. To keep oil warm and drive off refrigerant that migrated during the off cycle Correct answer.
  2. To prevent frost on the outdoor coil
  3. To raise suction pressure at startup
  4. To heat the compressor discharge line

Explanation

During the off cycle, refrigerant can migrate and condense in the compressor crankcase, diluting the oil. The crankcase heater warms the oil so that refrigerant boils off and lubricating ability is preserved at startup.

Question 49Heat Pump Components and Electrical Controls

A heat pump thermostat has terminals R, Y, O/B, G, W2, and C. Which terminal energizes the reversing valve for a unit configured to heat in the O/B energized state?

  1. W2
  2. G
  3. O/B Correct answer.
  4. Y

Explanation

The O/B terminal controls the reversing valve solenoid. Depending on whether the equipment is configured for O (energized in cooling) or B (energized in heating), the thermostat applies 24 VAC to that terminal in the appropriate mode.

Question 50Heat Pump Components and Electrical Controls

Which device provides a time delay before the compressor can restart after a thermostat interruption?

  1. Anti-short-cycle timer Correct answer.
  2. Blower time-delay relay
  3. Lockout relay
  4. Defrost thermostat

Explanation

An anti-short-cycle timer, often built into the defrost or control board, enforces a minimum off time to allow refrigerant pressures to equalize before restart. This reduces the risk of compressor overload and hard starting.