HVAC Certification Practice Questions

1. Compressor
2. Condenser
3. Evaporator
4. Metering device

Answer: C. The evaporator is the low-pressure, low-temperature coil where liquid refrigerant boils, absorbing heat from the air passing over it. The condenser does the opposite, rejecting heat as the high-pressure vapor condenses back into a liquid.

Topic: Refrigeration cycle

1. Low-pressure, low-temperature vapor
2. High-pressure, subcooled liquid
3. Low-pressure mixture of liquid and vapor
4. High-pressure, high-temperature superheated vapor

Answer: D. The compressor takes in low-pressure superheated vapor from the suction line and discharges high-pressure, high-temperature superheated vapor to the condenser. Subcooled liquid leaves the condenser, and the low-pressure liquid-vapor mixture is what comes out of the metering device into the evaporator.

Topic: Refrigeration cycle

1. Raise the pressure of the vapor leaving the evaporator
2. Reject heat from the refrigerant to the outdoor air
3. Drop the liquid's pressure and control flow into the evaporator
4. Separate oil from the refrigerant before the compressor

Answer: C. The metering device, whether a TXV, fixed orifice or electronic valve, creates the pressure drop between the high and low sides: liquid refrigerant flashes into a cold mixture of liquid and vapor as it enters the evaporator. Raising pressure is the compressor's job and rejecting heat is the condenser's.

Topic: Refrigeration cycle

1. Superheat at the suction line
2. Subcooling at the liquid line
3. Compressor amperage only
4. Discharge line temperature

Answer: A. Fixed-orifice systems are charged by superheat, compared with the manufacturer's chart for the indoor wet-bulb and outdoor dry-bulb conditions. Systems with a thermostatic expansion valve are charged by subcooling, because the TXV keeps adjusting to hold superheat roughly constant no matter how much charge is in the system.

Topic: Superheat & subcooling

1. 8°F
2. 12°F
3. 40°F
4. 92°F

Answer: B. Superheat is the actual line temperature minus the saturation temperature at the same pressure: 52°F − 40°F = 12°F. Adding the two numbers, or reporting the saturation temperature by itself, are the usual mistakes.

Topic: Superheat & subcooling

1. 3°F, undercharged: add refrigerant
2. 3°F, overcharged: recover refrigerant
3. 13°F, overcharged: recover refrigerant
4. 13°F, undercharged: add refrigerant

Answer: A. Subcooling is the condensing saturation temperature minus the liquid line temperature: 110°F − 107°F = 3°F, well under the 10°F target. With airflow and coils already verified, low subcooling on a TXV system means too little refrigerant backed up in the condenser, so add charge slowly while watching subcooling; high subcooling would point to an overcharge.

Topic: Superheat & subcooling

1. 200 CFM per ton
2. 400 CFM per ton
3. 600 CFM per ton
4. 1,200 CFM per ton

Answer: B. About 400 CFM per ton (12,000 BTU/h) is the standard design airflow, so a 3-ton system should move roughly 1,200 CFM. Humid climates often use closer to 350 CFM per ton to remove more moisture, and low airflow shows up in the field as low suction pressure and a frosted coil.

Topic: Airflow & psychrometrics

1. 10,800 BTU/h
2. 21,600 BTU/h
3. 43,200 BTU/h
4. 90,000 BTU/h

Answer: B. Sensible heat in BTU/h = 1.08 × CFM × ΔT = 1.08 × 1,000 × 20 = 21,600 BTU/h. This counts only the temperature change; total heat including moisture removal uses 4.5 × CFM × Δh with the enthalpy change from a psychrometric chart, which is why 90,000 BTU/h (4.5 × 1,000 × 20) is wrong here.

Topic: Airflow & psychrometrics

1. 0%
2. 50%
3. 80%
4. 100%

Answer: D. The wet bulb is cooled by evaporation, so it reads lower than the dry bulb whenever the air can still absorb moisture. Equal readings mean no evaporation is possible: the air is saturated, relative humidity is 100%, and the dew point equals the dry-bulb temperature.

Topic: Airflow & psychrometrics

1. FAN, 5 µF
2. HERM, 5 µF
3. HERM, 45 µF
4. C, 45 µF

Answer: C. The larger section, here 45 µF, is wired to the compressor's start winding through the terminal marked HERM (hermetic); the 5 µF section on FAN serves the condenser fan motor, and C is the common terminal shared by both. Discharge a capacitor before you test it with a meter.

Topic: Electrical & controls

1. R
2. G
3. W
4. Y

Answer: D. R carries 24 V from the transformer, Y calls for cooling and energizes the contactor coil, G runs the indoor blower, and W calls for heat; C is the common that returns to the transformer. The contactor coil is low-voltage, but its contacts switch line voltage to the compressor and condenser fan.

Topic: Electrical & controls

1. The contacts are closed and carrying current
2. The 24 V coil is shorted
3. The run capacitor is weak
4. The contacts are open or burned and not passing current

Answer: D. Voltage measured across a switch or a set of contacts means the contacts are open: the meter is reading line voltage through the load. Closed, healthy contacts read close to 0 V across them. Since the coil is energized, the contacts themselves are stuck or burned and the contactor should be replaced.

Topic: Electrical & controls

1. 3.5 in. w.c.
2. 7 in. w.c.
3. 10.5 in. w.c.
4. 0.5 psig

Answer: A. Natural gas furnaces typically run a manifold pressure of 3.5 inches of water column, measured at the gas valve outlet with a manometer. Around 7 in. w.c. is a typical supply pressure at the furnace inlet, and 10 to 11 in. w.c. is the usual manifold pressure for LP gas; always set the value shown on the rating plate.

Topic: Gas heat & heat pumps

1. A dirty flame sensor giving a weak rectification signal
2. A faulty gas valve
3. Manifold pressure set too high
4. An open high-limit switch

Answer: A. The furnace proves flame by flame rectification, a tiny DC microamp current that flows through the flame to the sensor; an oxidized sensor or a poor burner ground drops that signal below the control's threshold, so it closes the gas valve even though the burners lit. Clean the sensor and check the microamp reading. An open limit switch would block ignition altogether, and a bad gas valve would not deliver gas on every try.

Topic: Gas heat & heat pumps

1. Reverses the direction the compressor turns
2. Bypasses the metering device during the defrost cycle
3. Redirects refrigerant flow so the coils swap condenser and evaporator roles
4. Reverses the outdoor fan to melt frost

Answer: C. The four-way reversing valve routes compressor discharge gas either to the outdoor coil (cooling) or to the indoor coil (heating); the compressor always turns the same way. During defrost the valve shifts to cooling mode with the outdoor fan off, so hot gas melts the frost on the outdoor coil while auxiliary heat tempers the supply air.

Topic: Gas heat & heat pumps