20 AC Components & Operation Practice Questions & Answers

1. 1. A technician compares a scroll compressor with a reciprocating compressor of similar capacity. Which statement correctly describes normal scroll compressor operation?

   • A.It cannot be built as a hermetic unit and must be belt driven.
   • B.It uses spring-loaded suction and discharge reed valves that seat on every stroke.
   • C.It compresses refrigerant continuously between one fixed and one orbiting scroll and has no suction or discharge reed valves.✓ Answer
   • D.It pumps during only half of each revolution, so its discharge flow is strongly pulsating.

   In a scroll compressor the gas pocket formed between the fixed and orbiting scrolls is carried inward and squeezed continuously, so no suction or discharge reed valves are needed. That continuous, low-clearance compression gives smoother flow, high volumetric efficiency, and better tolerance of small amounts of liquid than a reciprocating machine, which relies on reed valves that liquid can break.

   Source: NATE Air Conditioning Service knowledge areas, System Components — compressors (scroll and reciprocating)Report a problem with this question

2. 2. Why is liquid refrigerant that reaches the compressor cylinder or scroll set destructive?

   • A.Liquid refrigerant freezes inside the cylinder and mechanically blocks the discharge port.
   • B.Liquid refrigerant carries an electrical charge that shorts the motor windings to ground.
   • C.Liquid boiling in the cylinder raises discharge temperature high enough to melt the reed valves.
   • D.Liquid is essentially incompressible, so it can break valves, rods and scroll flanks, and it also washes the oil film off bearing surfaces and dilutes the oil charge.✓ Answer

   A compressor is designed to compress vapor. Liquid does not compress, so a slug of it produces very high mechanical force on valves, pistons, rods and scroll flanks, and liquid refrigerant also dissolves into and washes away the oil film, causing bearing wear and oil dilution. Accumulators, correct charge, adequate airflow and a working crankcase heater exist to keep liquid out of the compressor.

   Source: NATE Air Conditioning Service knowledge areas, System Components — compressors and compressor protectionReport a problem with this question

3. 3. What does a crankcase heater on an outdoor unit accomplish while the compressor is off?

   • A.It keeps frost from forming on the outdoor coil during cold weather.
   • B.It keeps the oil warm so refrigerant vapor does not condense and migrate into the crankcase, which prevents oil foaming and a flooded start.✓ Answer
   • C.It preheats the refrigerant so that head pressure builds quickly at start-up.
   • D.It warms the run capacitor so its capacitance stays within tolerance.

   Refrigerant migrates to the coldest part of a shut-down system and is readily absorbed by the oil. The crankcase heater keeps the oil above the temperature at which refrigerant will condense in it, so on start-up the crankcase does not flash into foam and the pump does not lose its lubricant or slug liquid.

   Source: NATE Air Conditioning Service knowledge areas, System Components — electrical components and crankcase heatersReport a problem with this question

4. 4. A suction-line accumulator is installed on a split system. How does it protect the compressor during normal operation?

   • A.It raises suction pressure so the compressor always operates at a lower compression ratio.
   • B.It stores surplus liquid on the high side so the metering device always receives a solid column of liquid.
   • C.It filters acid and moisture out of the vapor before it reaches the compressor.
   • D.It traps liquid refrigerant leaving the evaporator and lets it boil off gradually, metering oil back through a small orifice so the compressor is fed vapor instead of a slug.✓ Answer

   The accumulator is a low-side vessel between the evaporator and the compressor. Liquid collects in the bottom while vapor is drawn from the top through a J-tube, and a small metering hole at the bottom of that tube returns oil and a little liquid at a rate the compressor can handle. Storing liquid on the high side for a variable charge is the job of a receiver, not an accumulator.

   Source: NATE Air Conditioning Service knowledge areas, System Components — refrigerant-circuit accessories (accumulator and receiver)Report a problem with this question

5. 5. On a correctly charged air-cooled system, what happens in the condenser as outdoor air temperature rises, and why?

   • A.Condensing pressure does not change, because it is fixed by compressor displacement.
   • B.Condensing temperature falls, because the fan moves denser air through the coil.
   • C.Condensing temperature and head pressure rise, because the refrigerant must condense at a higher temperature to keep a usable temperature difference with the warmer entering air.✓ Answer
   • D.Condensing pressure falls while subcooling rises, because more heat is rejected in warm air.

   Heat only moves from the refrigerant to the air when the refrigerant is hotter than the entering air, and the rate depends on that temperature difference and on the coil surface and airflow. When entering air gets warmer, the condensing temperature — and therefore the head pressure — floats up until the difference is again large enough to reject the heat the compressor and evaporator are adding.

   Source: NATE Air Conditioning Service knowledge areas, System Components — outdoor coils and the basic cooling cycleReport a problem with this question

6. 6. An outdoor coil is packed with cottonwood seed and dirt while the system runs. What is the effect on condenser operation?

   • A.Head pressure falls while suction pressure climbs and compressor current drops.
   • B.Head pressure stays constant while liquid subcooling drops to zero and the evaporator floods.
   • C.Heat rejection is reduced, so condensing temperature and head pressure climb, compression ratio rises, cooling capacity falls and the compressor draws more current.✓ Answer
   • D.The condenser fan motor automatically compensates, so operation is unchanged until the coil is completely blocked.

   A fouled condenser reduces both airflow and effective surface, so the coil cannot reject its heat at the normal temperature difference. The refrigerant condenses at a higher temperature, head pressure and compression ratio climb, volumetric efficiency and capacity drop, discharge temperature rises and motor current increases. Cleaning the coil restores the heat-transfer surface rather than adding refrigerant.

   Source: NATE Air Conditioning Service knowledge areas, System Components — outdoor coils and planned mechanical maintenanceReport a problem with this question

7. 7. Why does a system that must provide mechanical cooling in cool outdoor weather need a head-pressure (low-ambient) control that cycles or slows the condenser fan?

   • A.Without it the condenser fan motor bearings would seize in cold air.
   • B.Without it condensing pressure falls so low that the pressure difference across the metering device cannot push enough refrigerant through, starving the evaporator and risking liquid floodback to the compressor.✓ Answer
   • C.Without it the outdoor coil would ice over in cool weather.
   • D.Without it the compressor motor would overheat because of the light load.

   A metering device is driven by the pressure difference between the liquid line and the evaporator. In cool weather the condenser rejects heat too easily, condensing pressure collapses, and that driving pressure difference disappears, so the coil is underfed, suction pressure and superheat go unstable and liquid can return to the compressor. Cycling or slowing the condenser fan holds condensing pressure up so the metering device keeps working.

   Source: NATE Air Conditioning Service knowledge areas, System Components — low-ambient and head-pressure controlsReport a problem with this question

8. 8. Airflow across an indoor coil is badly reduced by a loaded filter while the system runs in cooling. Why does the coil eventually ice up?

   • A.Less air over the coil means less heat is absorbed, so the refrigerant saturation temperature in the coil falls below freezing, condensate turns to ice, and the ice blocks still more airflow.✓ Answer
   • B.The blower slows down while the compressor speeds up, so the coil is overfed with liquid.
   • C.The metering device closes completely and traps liquid refrigerant in the coil, which then freezes.
   • D.Reduced airflow raises coil temperature until the condensate flashes to steam and refreezes farther downstream.

   Evaporator temperature is set by the balance between the heat the air delivers and the heat the compressor removes. Starve the air side and the refrigerant boils at a lower pressure and temperature; once the coil surface drops below the freezing point the condensate freezes instead of draining, and the growing ice restricts airflow further, so the problem accelerates and liquid can eventually reach the compressor.

   Source: NATE Air Conditioning Service knowledge areas, System Components — indoor coils and air-side componentsReport a problem with this question

9. 9. An indoor coil sits upstream of the blower in a draw-through air handler. Why must the condensate drain have a properly made trap, and why must the drain line be pitched away from the unit?

   • A.The trap keeps refrigerant vapor from escaping through the drain line if the coil develops a leak.
   • B.The trap raises the water level so the pan stays flooded and drains faster; pitch only keeps the pipe from sagging.
   • C.The drain pan is under negative pressure, so without a trap the blower would pull air in through the drain and hold the water back; the trap seals against that air and the pitch lets the water run away by gravity.✓ Answer
   • D.The trap keeps blower discharge pressure from pushing water back into the coil, and pitch matters only in freezing climates.

   In a draw-through arrangement the blower is downstream of the coil, so the coil and drain pan sit in negative pressure. An untrapped drain simply becomes another return-air opening: air is pulled up the pipe and the water stands in the pan. A trap with an adequate water seal blocks that airflow, and continuous fall in the drain line lets gravity carry the condensate away.

   Source: NATE Air Conditioning Service knowledge areas, System Components — indoor coils and condensate managementReport a problem with this question

10. 10. An indoor coil and its drain pan are installed in an attic above a finished ceiling. What is the function of the condensate float (safety) switch on that installation?

    • A.It opens a bypass that routes condensate into the secondary pan so the unit can keep cooling indefinitely.
    • B.It opens the line-voltage circuit to the outdoor fan only, so the indoor blower keeps running to dry the coil.
    • C.It sounds an alarm but is wired so that it can never interrupt equipment operation.
    • D.It senses rising water in the pan and opens the low-voltage control circuit so the cooling equipment shuts down before the pan can overflow.✓ Answer

    When a coil is above a finished space, an overflow damages the building, so the installation includes secondary protection: a secondary pan or secondary drain plus a float switch wired in series with the low-voltage cooling circuit. When the water rises, the switch opens the control circuit and the system stops making condensate, which is a shutdown on a safety, not a bypass to keep running.

    Source: NATE Air Conditioning Service knowledge areas, System Components — condensate management and safety controlsReport a problem with this question

11. 11. A split system uses a thermostatic expansion valve at the indoor coil. Which refrigerant-side measurement is the meaningful check of charge on that system, and why?

    • A.Suction pressure by itself, because the valve fixes liquid-line pressure.
    • B.Evaporator superheat, because the valve holds a fixed opening once it is set at the factory.
    • C.Liquid subcooling, because the valve modulates to hold evaporator superheat nearly constant across a wide range of loads, so superheat says little about how much refrigerant is in the system.✓ Answer
    • D.Compressor amperage, because the valve makes pressure readings meaningless.

    A TXV feeds the evaporator to satisfy its superheat setting, so it will show close to the same superheat whether the system is somewhat low or somewhat full. What changes with charge is the amount of liquid backed up in the condenser, which shows as liquid subcooling — that is why the manufacturer's charging procedure for a TXV system is based on subcooling and the system is charged to the value the manufacturer specifies.

    Source: NATE Air Conditioning Service knowledge areas, System Components — variable metering devices (TXV) and charging methodReport a problem with this question

12. 12. How does evaporator superheat behave on a system that meters refrigerant with a fixed orifice (piston) or capillary tube, and what does that mean for setting the charge?

    • A.Superheat is meaningless because the restriction is fixed, so the nameplate charge is simply weighed in and never adjusted.
    • B.Superheat stays constant no matter what the load is, so subcooling is the only useful charging check.
    • C.Superheat rises when the system is overcharged and falls when it is undercharged.
    • D.Superheat changes with indoor and outdoor conditions and with charge, so the technician compares measured superheat with the manufacturer's target-superheat table for the measured indoor wet-bulb and outdoor dry-bulb temperatures.✓ Answer

    A fixed restriction has no feedback: it passes whatever the pressure difference pushes through it. Low load or low charge leaves more of the coil dry and superheat climbs; high load or extra charge floods more of the coil and superheat falls. Because superheat responds directly to charge on these systems, it is the charging measurement, read against the manufacturer's target-superheat table for the conditions at the time of service.

    Source: NATE Air Conditioning Service knowledge areas, System Components — fixed metering devices and charging methodReport a problem with this question

13. 13. An evaporator is fed through a refrigerant distributor with equal-length feeder tubes. Why does its expansion valve need an external equalizer, and where is that line connected?

    • A.The distributor and coil produce a real pressure drop, so the valve must sense the pressure at the coil outlet rather than at its own inlet; the equalizer line connects to the suction line just downstream of the sensing bulb.✓ Answer
    • B.It supplies liquid refrigerant to the sensing bulb, and it connects between the distributor and the bulb.
    • C.It bleeds refrigerant around the valve to speed start-up, and it connects across the valve inlet and outlet.
    • D.It equalizes high- and low-side pressure during the off cycle, and it connects to the liquid line.

    The valve balances bulb pressure (opening) against evaporator pressure plus spring force (closing). If it reads pressure at its own outlet, the drop through the distributor and coil is mistaken for extra superheat and the valve overfeeds. The external equalizer gives the diaphragm the true coil-outlet pressure, and it is tapped into the suction line downstream of the bulb so bulb sensing is not disturbed.

    Source: NATE Air Conditioning Service knowledge areas, System Components — variable metering devices, distributors and external equalizersReport a problem with this question

14. 14. Where and how should the sensing bulb of a thermostatic expansion valve be mounted on the suction line?

    • A.On the bottom of a horizontal suction line, so that it makes contact with any oil lying in the tube.
    • B.Low on a vertical suction riser and left uninsulated so that it also senses ambient temperature.
    • C.Strapped to the liquid line just downstream of the filter-drier and insulated.
    • D.Clamped tightly to clean, bare tubing on a horizontal section of suction line near the upper side of the pipe (about the 10 or 2 o'clock position), then insulated.✓ Answer

    The bulb must read the temperature of the refrigerant vapor leaving the evaporator, nothing else. Oil and any liquid run along the bottom of the tube, so a bulb mounted there reads falsely; a loose bulb or bare bulb reads surrounding air. Tight metal-to-metal contact on the upper side of a horizontal run, plus insulation over the bulb, gives a true reading and stops the hunting or flooding that a badly located bulb causes.

    Source: NATE Air Conditioning Service knowledge areas, System Components — TXV bulb location and installationReport a problem with this question

15. 15. A long line set on a split system includes a tall vertical suction riser from a basement air handler up to a rooftop condensing unit. Why is suction-line size critical on this installation?

    • A.A larger suction line always improves oil return, because the oil drains more easily through a bigger pipe.
    • B.The suction line must be the same diameter as the liquid line so that pressure drop is equal in both lines.
    • C.Vapor velocity must stay high enough to carry oil up the riser back to the compressor: an oversized line loses velocity and traps oil in the riser, while an undersized line adds pressure drop that lowers suction pressure and capacity.✓ Answer
    • D.The suction line size sets the metering device orifice size and the two must match exactly.

    Oil leaves the compressor with the refrigerant and only comes back if the returning vapor moves fast enough to sweep it along, which is hardest on a vertical riser where gravity works against it. Too large a line drops velocity and the oil stays behind; too small a line raises friction loss, which lowers the pressure and density of the vapor reaching the compressor and costs capacity. Line sizes and any required traps come from the equipment manufacturer's line-set data for the actual length and lift.

    Source: NATE Air Conditioning Service knowledge areas, System Components — line sets, line sizing and oil returnReport a problem with this question

16. 16. What happens when the suction-line insulation on a split system is missing, crushed flat, or soaked with water?

    • A.Suction pressure falls sharply and the compressor short cycles on its low-pressure control.
    • B.The cold line picks up heat from the surrounding air, which raises suction temperature and superheat at the compressor and wastes cooling capacity, and the bare or wet line sweats and drips.✓ Answer
    • C.Nothing changes in operation; the insulation is only there to protect the copper from physical damage.
    • D.The refrigerant subcools further in the suction line, which raises system capacity.

    The suction line carries cold vapor, so any heat it picks up on the way to the compressor is cooling that never reaches the house, and it also raises compressor suction temperature. Water-logged or crushed insulation has lost most of its resistance to heat flow, and the resulting condensation drips on the structure and keeps the insulation wet, so damaged suction-line insulation is replaced rather than simply taped over.

    Source: NATE Air Conditioning Service knowledge areas, System Components — line sets and insulationReport a problem with this question

17. 17. A liquid-line filter-drier is being installed on a residential split system. What does it do in normal operation and how is it oriented?

    • A.It captures moisture, acid and particles ahead of the metering device, and it is installed with its flow arrow pointing in the direction of refrigerant flow, toward the metering device.✓ Answer
    • B.It strips the oil out of the refrigerant before the metering device, and its arrow points back toward the condenser.
    • C.It boils the remaining liquid into vapor ahead of the expansion valve, and its arrow points toward the compressor.
    • D.It stores excess liquid refrigerant, and the direction it faces does not matter.

    The drier's desiccant and filter media protect the smallest passage in the system — the metering device — and also protect the oil by holding moisture and acid, so it must be upstream of that device with the arrow following the flow. Because a drier is a deliberate but small restriction, a noticeable temperature difference across it in operation indicates that it has become plugged and needs replacement.

    Source: NATE Air Conditioning Service knowledge areas, System Components — refrigerant-circuit accessories (filter-driers)Report a problem with this question

18. 18. A compressor service valve has been turned fully in onto its front seat. What has that accomplished, and what is the valve's normal operating position?

    • A.Front seating isolates the compressor from the rest of the system and the unit must never be operated that way; the normal running position is back seated, and the stem is cracked off the back seat only to read pressure at the service port.✓ Answer
    • B.Front seating and back seating both leave the system open; the only difference is which gauge the port feeds.
    • C.Front seating vents the compressor to the atmosphere so it can be removed, and it is the normal running position.
    • D.Front seating opens the service port to the system for gauge readings, and front seated is the normal running position.

    A front-seated valve blocks the port to the system, so running the compressor front seated either dead-heads the discharge or pulls the suction into a vacuum, both damaging. Back seated is the running position because it opens the system flow and closes off the service port; mid-seating (cracking the stem off the back seat) opens the port so gauges can read system pressure while the system still operates.

    Source: NATE Air Conditioning Service knowledge areas, System Components — service valves and their positionsReport a problem with this question

19. 19. A wall thermostat for a cooling system is mounted on an exterior wall where afternoon sun strikes it, and close to a supply register. How does that location affect normal system operation?

    • A.The thermostat senses temperatures that do not represent the conditioned space, so the equipment short cycles or runs at the wrong times while the rest of the house stays uncomfortable.✓ Answer
    • B.It has no effect as long as the thermostat is level and correctly wired.
    • C.It makes the compressor run continuously at reduced capacity but still holds the space exactly at set point.
    • D.It affects only heating operation; the cooling control is unaffected because the cooling call comes from the outdoor unit.

    A thermostat controls the whole system from one point, so it must sense air that is representative of the occupied space: an interior wall, out of direct sun and drafts, away from supply air, doors, lamps, appliances and exterior-wall conduction. Sun on the cover makes it call for cooling that the house does not need; supply air blowing on it satisfies it early and cuts the cycle short, and both leave the space uncomfortable while the equipment cycles poorly.

    Source: NATE Air Conditioning Service knowledge areas, System Components — thermostats and control locationReport a problem with this question

20. 20. What does a deep evacuation physically accomplish in a system before the refrigerant charge is added?

    • A.It pressurizes the system enough to reveal leaks at the brazed and flared joints.
    • B.It cools the compressor windings so that the first start draws less current.
    • C.It lowers the pressure enough that water inside the system boils to vapor at ambient temperature and is pulled out along with air and other non-condensables, which would otherwise raise head pressure and combine with the oil to form acids.✓ Answer
    • D.It removes any refrigerant left in the system so that a new charge can be weighed in accurately.

    Water boils at a lower temperature as pressure falls, so pulling the system deep into a vacuum turns any liquid water into vapor that the pump can remove, while also removing air and other non-condensables. Non-condensables collect in the condenser and raise head pressure, and moisture attacks the oil and can freeze at the metering device — which is why recovery, evacuation to a measured deep vacuum, and a decay check are done before charging, never a purge of refrigerant to the atmosphere.

    Source: NATE Air Conditioning Service knowledge areas, Installation — evacuation and chargingReport a problem with this question

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