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Troubleshooting, Repair & Maintenance — California C-20 Practice Questions

46 questions · 20% of the C-20 HVAC Trade exam

Worked questions

7. Head pressure rises while a clean condenser's fan turns well below commanded speed. Which circuit and airflow components should be checked?

  • AAdd more refrigerant
  • Fan circuit and airflow components
  • CEvaporator drain only
  • DThermostat batteries only for the described HVAC equipment and measured operating conditions

Why B is correct

The best answer is: Fan circuit and airflow components. Insufficient condenser airflow reduces heat rejection and raises pressure. Electrical and mechanical fan diagnosis should precede any charge adjustment. The transferred operating condition requires the same design, installation, diagnostic, or safety principle in a different HVAC context. The other choices skip a prerequisite, conflict with the listed procedure, or treat a symptom without controlling its cause.

8. Total external static is above the fan rating and delivered airflow is low. Which restrictions and setup conditions should be investigated?

  • ARemove safety controls for the described HVAC equipment and measured operating conditions
  • BAdd refrigerant
  • Filters, coils, dampers, duct restrictions, and fan setup
  • DIncrease gas pressure

Why C is correct

The best answer is: Filters, coils, dampers, duct restrictions, and fan setup. High static indicates resistance or incorrect setup on the air path. Increasing speed without finding the restriction can overload the motor and worsen noise or leakage. The transferred operating condition requires the same design, installation, diagnostic, or safety principle in a different HVAC context. The other choices skip a prerequisite, conflict with the listed procedure, or treat a symptom without controlling its cause.

38 more in the bank

Answers and explanations for these are in the app.

  • A 2-square-foot duct carries 1,600 cfm. What is average air velocity?
  • A 3-square-foot duct carries 1,800 cfm. Using velocity = cfm ÷ area, what is average air velocity?
  • A branch must carry 1,200 cfm at a design velocity of 800 feet per minute. Using area = cfm ÷ velocity, what area is required?
  • A compressor has correct line voltage but opens its overload and will not start. What should be checked next?
  • A condenser coil is heavily fouled and head pressure is high. What is the appropriate first correction?
  • A cooling system does not start. What is the best first diagnostic approach?
  • A cooling system is measured at 48,000 Btu per hour. Using 12,000 Btu per hour per ton of refrigeration, what capacity does that represent?
  • A fan delivers low airflow while total external static exceeds its rated range. What should be investigated?
  • A furnace reaches high limit repeatedly with clean filters. What should be evaluated?
  • A heat pump has correct control voltage but remains in one mode. What should be evaluated?
  • A main duct is intended to carry 2,000 cfm at 500 feet per minute. What cross-sectional area follows from area = cfm ÷ velocity?
  • A room has low airflow while the branch takeoff has adequate pressure, and the concealed duct is disconnected. What is the proper repair goal?
  • A system provides 36,000 Btu per hour of cooling while using 3.10 kW of input. Using 1 kW = 3,412 Btu per hour and COP = cooling kW ÷ input kW, approximately what COP results?
  • A technician replaces filters but never records temperature, airflow, electrical, or combustion data. What value is being lost?
  • An evaporator is iced and airflow is very low. What should be evaluated before adding refrigerant?
  • Head pressure is high with a clean coil but the condenser fan is slow. What should be checked?
  • Liquid-line temperature is 95°F and condensing saturation temperature is 110°F. What is subcooling?
  • A compressor receives proper terminal voltage but cannot start and opens its overload. Which electrical and mechanical checks come next?
  • A furnace repeatedly opens the high limit although its filter is clean. Which airflow, temperature-rise, and input checks are needed?
  • A heat-pump coil remains heavily frosted in mild weather and no defrost cycle occurs. Which sequence elements should be tested?
  • A packaged unit remains off despite a thermostat demand. Which diagnostic sequence should begin the investigation?
  • A reversing valve receives correct control voltage but the heat pump never changes mode. What should be evaluated?
  • An economizer admits hot moist air while mechanical cooling is required. Which sensors and logic should be checked?
  • An iced evaporator has almost no air movement. What must be restored and stabilized before refrigerant charge is judged?
  • Combustion measurements are unsafe and occupants report illness symptoms. What action takes priority over equipment adjustment?
  • Condensing saturation temperature is 118°F and measured liquid-line temperature is 101°F. What is subcooling?
  • Defrost begins, but a fan that should stop continues to operate. Which board, relay, and wiring sequence should be examined?
  • Discharge pressure is elevated while a condenser is matted with debris. Which first correction provides meaningful follow-up readings?
  • During air balancing, a traverse records 1,750 cfm through a main whose measured internal area is 2.5 square feet. Convert the volume-flow reading to average speed.
  • Filters were changed, but the maintenance report records no operating measurements. What useful capability is lost?
  • Indoor dry-bulb temperature is satisfied but humidity remains excessive. Which loads, runtime, airflow, and control factors need evaluation?
  • Leak detection confirms a field-joint refrigerant leak. Which compliant recovery-through-recharge sequence applies?
  • Low suction is observed with a dirty filter and collapsed return duct. What should happen before charge is altered?
  • Measured carbon monoxide increases throughout a furnace cycle. What is the safe response?
  • One conductor feeding a three-phase compressor carries no current while the other phases are high. Which condition is most likely?
  • One room loses airflow because a concealed flex branch separated at a collar. What complete repair should be made?
  • Soot and heat marks surround a furnace rollout switch that has opened. What is the safe service response?
  • The suction line is 62°F while corresponding saturation temperature is 40°F. What is refrigerant superheat?

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