South Carolina Residential HVAC Practice Exam

Description

Sample Questions

Question 1. A residential contractor is verifying blower performance on a gas furnace with an 80,000 Btu/h input rating and 80 percent efficiency. The measured temperature rise across the heat exchanger is 40°F, which falls within the nameplate rise range. Using the sensible heat formula with the standard air factor of 1.08, approximately how much airflow is the furnace delivering?

A. 1,852 CFM
B. 1,481 CFM
C. 1,600 CFM
D. 2,000 CFM

Correct Answer: B. 1,481 CFM

Answer Explanation:
Start with furnace output, because airflow depends on the heat actually delivered to the air stream. Output equals 80,000 × 0.80, or 64,000 Btu/h. The sensible heat equation, Q = 1.08 × CFM × ΔT, rearranges to CFM = Q ÷ (1.08 × ΔT). That gives 64,000 ÷ (1.08 × 40), or about 1,481 CFM. The 1.08 factor comes from standard air density and specific heat, and it assumes near sea-level conditions. The result should then be compared with the blower performance table and the nameplate rise range, because a rise outside that range points to a speed tap, filter, or duct restriction problem that needs correcting.

Why the Other Options Are Incorrect:
A. 1,852 CFM — Incorrect. This uses the 80,000 Btu/h input instead of the 64,000 Btu/h output, so it overstates the heat delivered to the air.
C. 1,600 CFM — Incorrect. This divides output by the temperature rise and drops the 1.08 factor, which leaves the formula incomplete.
D. 2,000 CFM — Incorrect. This divides input by the temperature rise and leaves out both the efficiency and the 1.08 factor.

Quick Study Guide:
Use output, not input, in airflow calculations. Multiply input by efficiency, then divide by 1.08 times the temperature rise. Verify that the measured rise falls within the nameplate range and compare the result with the blower table. A low rise suggests too much airflow, while a high rise points to restricted airflow, a dirty filter, or undersized ductwork.

Question 2. A technician is servicing a 3-ton split-system air conditioner that uses a thermostatic expansion valve (TXV). The outdoor temperature is 90°F, and the filter and indoor coil are clean with proper airflow. Gauge readings show low suction pressure and low discharge pressure. Superheat measures 24°F, and subcooling measures 3°F against a manufacturer target of 10°F. Which condition is most likely?

A. Restricted evaporator airflow
B. Refrigerant overcharge
C. Dirty condenser coil
D. Low refrigerant charge

Correct Answer: D. Low refrigerant charge

Answer Explanation:
Low pressures on both sides of the system, high superheat, and low subcooling point to too little refrigerant. With less liquid reaching the condenser outlet, subcooling falls. With less refrigerant feeding the evaporator, the vapor leaving the coil picks up extra heat, which raises superheat. On a TXV system, subcooling is the primary charging measurement, so the shortfall against the 10°F target is especially telling. The technician should locate and repair the leak, evacuate as needed, and recharge by subcooling to the manufacturer’s specification. Adding refrigerant without finding the leak only delays the next failure and wastes refrigerant.

Why the Other Options Are Incorrect:
A. Restricted evaporator airflow — Incorrect. Poor airflow can lower suction pressure, but it tends to produce low superheat, and the clean filter and coil rule it out here.
B. Refrigerant overcharge — Incorrect. An overcharge raises subcooling and discharge pressure, which is the opposite of these readings.
C. Dirty condenser coil — Incorrect. A fouled condenser drives head pressure up, but discharge pressure in this case is low.

Quick Study Guide:
Learn the pattern for an undercharge: low suction pressure, low head pressure, high superheat, and low subcooling. Overcharge shows high subcooling and high head pressure. Charge TXV systems by subcooling and fixed-orifice systems by superheat. Rule out airflow and coil problems first, then find and repair the leak before recharging.

Question 3. An HVAC contractor is sizing circuit conductors for a 240-volt, single-phase condensing unit. The nameplate lists a compressor rated-load current of 18.5 amperes and a condenser fan motor full-load current of 1.5 amperes. Using the NEC method for a motor-compressor combined with another motor, what is the minimum conductor ampacity?

A. 24.6 amperes
B. 20.0 amperes
C. 23.1 amperes
D. 25.0 amperes

Correct Answer: A. 24.6 amperes

Answer Explanation:
NEC Article 440 requires conductors serving a motor-compressor and other motors to carry the sum of the compressor rated-load current and the fan motor current, plus 25 percent of the largest motor in the group. The compressor is the largest motor, so 25 percent of 18.5 amperes equals about 4.6 amperes. Adding 18.5, 1.5, and 4.6 gives roughly 24.6 amperes. In practice, the equipment nameplate usually lists a minimum circuit ampacity, and the installer should use that marked value when available. The calculation still matters on the exam because it shows that the added 25 percent applies only to the largest motor, not to every load on the circuit, and the result must then be matched to a standard conductor size.

Why the Other Options Are Incorrect:
B. 20.0 amperes — Incorrect. This simply adds the two currents and leaves out the 25 percent addition for the largest motor.
C. 23.1 amperes — Incorrect. This applies 125 percent to the compressor alone and omits the fan motor current.
D. 25.0 amperes — Incorrect. This applies 125 percent to the sum of both loads, which overstates the requirement because only the largest motor receives the added 25 percent.

Quick Study Guide:
For multiple motors, add every motor’s current, then add 25 percent of the largest one. Do not multiply the whole sum by 1.25. Use the nameplate’s minimum circuit ampacity and maximum overcurrent protection when they are marked. Remember that conductor ampacity and overcurrent device size are separate calculations with separate rules.

Question 4. A contractor is replacing a 100,000 Btu/h input gas furnace located in a closet. The closet is a confined space, and the home is not of unusually tight construction. Combustion air will come from adjoining interior spaces that provide enough combined volume, using two permanent openings. What is the minimum free area required for each opening under the standard indoor method of the model fuel gas code?

A. 25 square inches
B. 50 square inches
C. 100 square inches
D. 200 square inches

Correct Answer: C. 100 square inches

Answer Explanation:
When a confined space draws combustion air from inside the building, the standard method calls for two permanent openings, one within 12 inches of the top of the enclosure and one within 12 inches of the bottom. Each opening needs at least 1 square inch of free area for every 1,000 Btu/h of total input, and never less than 100 square inches. For a 100,000 Btu/h furnace, that equals 100 square inches per opening. The upper opening lets heat escape, while the lower opening supplies air. Louvers and grilles reduce free area, so the installer must use the manufacturer’s free-area rating, not the nominal opening size, when selecting them.

Why the Other Options Are Incorrect:
A. 25 square inches — Incorrect. This reflects the 1 square inch per 4,000 Btu/h ratio used for openings connected directly to the outdoors, not for indoor air.
B. 50 square inches — Incorrect. This reflects the 1 square inch per 2,000 Btu/h ratio for horizontal ducts to the outdoors, and it also falls below the 100 square inch minimum.
D. 200 square inches — Incorrect. This doubles the requirement, possibly by adding both openings together instead of sizing each one.

Quick Study Guide:
Memorize the ratios: 1 square inch per 1,000 Btu/h for indoor air, with a 100 square inch minimum, and 1 per 4,000 Btu/h for direct outdoor openings. Place openings within 12 inches of the top and bottom. Always check the combined volume of connected spaces and use the grille’s free area, not its overall size.

Question 5. After a compressor replacement, a technician evacuates a residential heat pump to 500 microns, closes the valve to isolate the vacuum pump, and watches the micron gauge. The reading climbs to 1,800 microns within a few minutes and then levels off and stays steady. What does this indicate?

A. A leak is present, and nitrogen pressure testing must be completed before evacuation continues.
B. Moisture or other contaminants are still vaporizing in the system, so evacuation should continue.
C. The system is fully dehydrated because the gauge reading stabilized.
D. The vacuum pump oil must be changed because the reading rose after isolation.

Correct Answer: B. Moisture or other contaminants are still vaporizing in the system, so evacuation should continue.

Answer Explanation:
A rise that climbs and then levels off is the classic sign of moisture or trapped contaminants boiling off under vacuum. As pressure rises, the water vapor reaches equilibrium and the gauge stabilizes, but the level remains too high to call the system dry. The technician should continue evacuating, consider breaking the vacuum with dry nitrogen and repeating, and replace the filter drier if contamination is suspected. A true leak behaves differently, because the reading keeps rising steadily toward atmospheric pressure. Charging a system that was not properly dehydrated can cause acid formation, compressor failure, and restricted metering devices, so the evacuation target and standing test must be met before refrigerant is introduced.

Why the Other Options Are Incorrect:
A. A leak is present, and nitrogen pressure testing must be completed — Incorrect. A leak produces a continuous rise that does not level off, so a stabilized reading points away from a leak.
C. The system is fully dehydrated because the gauge reading stabilized — Incorrect. A steady reading at 1,800 microns is still far above the 500-micron target, so the system is not ready.
D. The vacuum pump oil must be changed — Incorrect. Contaminated pump oil limits how low the pump can pull while running, but it does not cause a rise after the system is isolated.

Quick Study Guide:
Use a micron gauge and evacuate to 500 microns, then isolate the pump and watch the reading. A rise that levels off suggests moisture, so keep evacuating. A rise that never stops suggests a leak. Replace the filter drier when contamination is likely, and use properly maintained pump oil. Compare readings to the manufacturer’s evacuation specification.