Hawaii C-40 HVAC Exam Prep FREE Sample Exam — Fixed Set

No-cool call on an R-410A system with verified normal airflow. Readings: suction 105 psig, head pressure near normal at 285 psig, suction superheat 26 degrees F, liquid subcooling 18 degrees F, and a 7-degree temperature drop measured across the liquid-line filter drier with light frost just downstream of it. What is the most likely cause?

Low refrigerant charge, which would starve the evaporator but also leave subcooling very low because little liquid backs up in the condenser, unlike the 18 degrees measured here

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An overfeeding expansion valve, which would drive superheat down toward zero and flood the suction line rather than hold superheat at 26 degrees

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Noncondensables in the system, which would push head pressure and subcooling up together and would not create a localized temperature drop and frost at one component

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A restricted liquid-line filter drier

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A homeowner asks why the new R-454B air handler runs its blower for a while after a leak sensor trips, even though cooling has stopped. What is the correct explanation?

The blower is removing residual heat from the electric heat strips, a normal cool-down sequence that happens to coincide with the sensor alarm event

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The blower pressurizes the duct system so refrigerant vapor is forced back through the leak point into the coil, resealing the system temporarily

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The blower dilutes and disperses leaked refrigerant through the duct system so room concentration stays low

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The blower run is a fault in itself, because mitigation design requires the air handler to shut down every moving part the instant a sensor trips

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A system shows suction 94 psig, superheat 22°F, subcooling 18°F, head 330 psig, and a 9°F temperature drop measured across the liquid-line filter drier with light frost forming on its outlet side only. Airflow is normal and the evaporator is clean. What is the correct repair decision?

Replace the liquid-line filter drier

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Add refrigerant until superheat falls, which would mask the starvation while pressures downstream of the restriction remained low and the real blockage stayed in place

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Replace the TXV, since a starved evaporator always means the valve has failed, even when a 9°F drop across the drier localizes the restriction upstream of it

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Raise condenser airflow to lower the head pressure, which does nothing for a liquid-line blockage that is already proven by the temperature drop

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A furnace with a new flame sensor and a clean burner still drops out seconds after light-off intermittently. Flame signal fluctuates between 0.8 and 3 µA, gas pressure and sensor position are correct, and electrical checks find hot and neutral reversed at the furnace junction box (120 V from neutral to ground). What is the most likely cause?

A cracked heat exchanger, which disturbs flames and raises CO but does not reverse electrical polarity or make the microamp signal unstable

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A weak gas valve coil dropping out under load, which would show manifold pressure collapsing at dropout rather than a fluctuating flame signal

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A defective new flame sensor, which is unlikely because the signal fluctuates with the wiring fault and the sensor was just replaced with no change

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Reversed polarity defeating flame rectification

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A homeowner complains the heat pump ‘stopped working’ at 20 degrees F because supply air feels cool and run times are long, yet the house holds setpoint without auxiliary heat running excessively. What is the best explanation?

The reversing valve must be leaking because supply air should feel as hot as a gas furnace

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Heat pump capacity and supply-air temperature fall as outdoor temperature drops, so long run times with lukewarm air can be normal if setpoint is maintained

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The system is in permanent defrost

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The thermostat should force strip heat below 40 degrees F in all cases

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Duct leakage results are reported at 25 Pa, about 0.10 in. w.c., even though operating duct pressures vary by location and system. Why is 25 Pa used?

It is a standardized reference pressure

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It is the exact pressure found at every register in every residential system, so test results at 25 Pa need no interpretation to reflect operating leakage rates

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It is the highest pressure a residential duct can withstand, chosen so the test doubles as a structural proof test of the duct joints and seams

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It is the pressure at which leakage stops changing with pressure, so readings taken there equal the leakage that occurs at any other operating pressure

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In an inverter air conditioner, what does the inverter section do after the incoming AC supply is rectified?

Stores the DC permanently in a battery bank so the compressor can run during a utility outage without any generator connected to the system

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Steps the DC down to thermostat voltage so the indoor control board and the compressor can share one low-voltage power supply safely

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Synthesizes variable-frequency, variable-voltage AC, so compressor motor speed can be varied continuously with the load

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Converts the DC back to fixed 60 Hz AC at a higher voltage, because the compressor motor is a standard single-speed induction design

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A furnace clocks at an input 20% above its rating plate after a gas-valve replacement. The most likely setup causes are:

Manifold pressure set too high or wrong-size orifices installed

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A vent connector one size larger than the minimum, because a larger vent increases draft and pulls extra gas through the burner orifices

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A sediment trap with a long capped leg, because the trap volume adds a reserve of gas that the meter counts as additional furnace input

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A thermostat set several degrees above room temperature, because a larger temperature difference makes the gas valve deliver more fuel per hour

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A CO alarm sounds in a home with gas appliances and occupants report headaches that ease outdoors. What is the correct immediate response?

Open one window near the alarm, reset it to silence the alert, and stay in the house to see whether it sounds again over the next several hours

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Turn the thermostat up to burn off the carbon monoxide

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Leave the building, get fresh air, and call emergency services or the gas utility before re-entering

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Run the kitchen exhaust for an hour while staying inside to monitor

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Why must a refrigerant cylinder never be heated with an open flame or filled beyond its rated capacity?

It improves charging accuracy

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Overfilling or heating can raise pressure to rupture, and flame can decompose refrigerant into toxic gases

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It only affects the cylinder paint

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Cylinders are designed for any pressure

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An 80% furnace is replaced with a 96% condensing furnace reusing the old B-vent chimney path. What venting change is required?

The B-vent can be kept unchanged because furnace vent categories are interchangeable whenever the vent diameter stays the same

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The old vent can be kept if the installer raises the flue temperature by removing the new furnace's secondary heat exchanger

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The new furnace needs its listed plastic or approved vent system; its cool, condensing exhaust cannot use the old B-vent as-is

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The chimney should be capped and the new furnace vented into the attic space, where its exhaust is too cool to cause any harm

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A reach-in cooler shows high evaporator superheat, low subcooling, and bubbles in the liquid line sight glass, with a clean condenser and normal airflow. What is the most likely diagnosis?

The system is undercharged, so flash gas forms in the liquid line and the evaporator is starved

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The system is overcharged, so excess liquid backs up in the condenser and floods the evaporator coil

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The compressor valves are bypassing, so discharge gas leaks back and superheats the suction vapor

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The evaporator fan is running backward, so air short-circuits the coil and leaves the refrigerant cold

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Hawaii C-40 HVAC Exam Prep FREE Sample Exam — Fixed Set

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