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, so it must be belt driven.
   • B.It uses spring-loaded suction and discharge reed valves on each stroke.
   • C.It compresses continuously between a fixed and an orbiting scroll.✓ Answer
   • D.It pumps during only half of each revolution, so discharge pulsates.

   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 in the cylinder and blocks the discharge.
   • B.Liquid refrigerant carries a charge that shorts the motor windings.
   • C.Liquid boiling in the cylinder melts the reed valves with hot gas.
   • D.Liquid does not compress, so it breaks valves and washes off the oil.✓ 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 in cold weather.
   • B.It keeps the oil warm so refrigerant does not migrate into it.✓ Answer
   • C.It preheats the refrigerant so head pressure builds fast at start-up.
   • D.It warms the run capacitor so its capacitance stays in 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 compression ratio stays lower.
   • B.It stores surplus liquid on the high side for the metering device.
   • C.It filters acid and moisture out of the vapor entering the compressor.
   • D.It traps liquid leaving the evaporator so the compressor gets vapor.✓ 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; compressor displacement fixes it.
   • B.Condensing temperature falls because the fan moves denser cool air.
   • C.Condensing temperature and head pressure rise with the warmer air.✓ Answer
   • D.Condensing pressure falls while subcooling rises in the warmer 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, suction climbs and compressor current drops.
   • B.Head pressure holds while subcooling drops and the coil floods.
   • C.Head pressure and compression ratio climb while capacity falls off.✓ Answer
   • D.The condenser fan motor compensates, so operation stays unchanged.

   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.The condenser fan motor bearings would seize in the cold air.
   • B.Condensing pressure would fall so low that the metering device starves.✓ Answer
   • C.The outdoor coil would ice over and would need a defrost cycle.
   • D.Head pressure would climb and trip the high-pressure switch.

   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 heat is absorbed, so the coil runs below freezing and ices.✓ Answer
   • B.The blower slows while the compressor speeds up and overfeeds it.
   • C.The metering device closes and traps liquid, which then freezes.
   • D.Airflow loss raises coil temperature until condensate flashes off.

   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.
   • B.The trap floods the pan so it drains faster; pitch stops sagging.
   • C.The pan is under negative pressure, so an untrapped drain pulls air.✓ Answer
   • D.The trap stops blower pressure from pushing water into the coil.

   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 to the secondary pan.
    • B.It opens the line-voltage circuit to the outdoor condenser fan only.
    • C.It only sounds an alarm and can never interrupt the equipment.
    • D.It opens the low-voltage control circuit before the pan overflows.✓ 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 alone, because the valve fixes liquid-line pressure.
    • B.Evaporator superheat, because the valve opening is fixed at the factory.
    • C.Liquid subcooling, because the valve holds superheat nearly constant.✓ Answer
    • D.Compressor amperage, because the valve makes pressure readings useless.

    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 means nothing, so the nameplate charge is only weighed in.
    • B.Superheat stays constant under any load, so subcooling is the only check.
    • C.Superheat rises when the system is overcharged and falls when low.
    • D.Superheat varies with conditions, so it is read against a target table.✓ 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 coil drops pressure, so it taps the suction line past the bulb.✓ Answer
    • B.It feeds liquid refrigerant to the sensing bulb from the distributor.
    • C.It bleeds refrigerant around the valve to speed up every start-up.
    • D.It equalizes high and low side at shutdown from 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, in contact with oil.
    • B.Low on a vertical suction riser and left bare so it senses air.
    • C.On the liquid line just downstream of the filter-drier, insulated.
    • D.Clamped to bare horizontal suction line at 10 or 2 o'clock, 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 through the riser.
    • B.The suction line must match the liquid line so both drop the same.
    • C.Vapor velocity must stay high enough to carry oil up the riser.✓ Answer
    • D.The suction line size sets the metering device orifice size 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 drops sharply and the compressor short cycles off.
    • B.The cold line picks up heat, wasting capacity, and it sweats and drips.✓ Answer
    • C.Nothing changes; the insulation only protects the copper from damage.
    • D.The refrigerant subcools further in the suction line, adding 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 dirt, with its arrow toward the valve.✓ Answer
    • B.It strips oil from the refrigerant, with its arrow toward the condenser.
    • C.It boils the liquid into vapor, with its arrow toward the compressor.
    • D.It stores excess liquid refrigerant, and its direction 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; run position is back seated.✓ Answer
    • B.Front and back seating both leave the system open through the port.
    • C.Front seating vents the compressor to air and is the run position.
    • D.Front seating opens the service port and 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.It senses temperatures that do not represent the conditioned space.✓ Answer
    • B.It has no effect as long as the thermostat is level and wired right.
    • C.The compressor runs nonstop but still holds the space at set point.
    • D.It affects only heating, since the outdoor unit calls for cooling.

    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 joints.
    • B.It cools the compressor windings so the first start draws less.
    • C.It lowers pressure until water boils and non-condensables leave.✓ Answer
    • D.It draws dry nitrogen through the system to displace the moisture.

    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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