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AC practice questions

31 questions from the hvac service and troubleshooting practice test.

AC practice test

10 questions, graded as you go, with the reason for every answer. No sign-up.

What this covers

Sample questions

A technician arrives to find the evaporator coil and suction line encased in ice with the system still running. What should be done before any refrigerant readings are taken?

  1. Add refrigerant until the suction pressure rises to its normal range
  2. Raise the blower speed and take readings right away
  3. Recover the charge and weigh in the nameplate amount
  4. Shut off the compressor and run the blower until the coil thaws
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D. Shut off the compressor and run the blower until the coil thaws

Ice blocks airflow and insulates the coil, so every pressure and temperature taken on an iced coil is misleading. Adding refrigerant because suction is low is the classic mistake: the low pressure may be caused by airflow, and the system could end up overcharged.

Reference: Service procedures; frozen evaporator diagnosis

A fixed-orifice air conditioner is running on an 85°F day. Suction saturation temperature is 25°F, superheat is 35°F and subcooling is 2°F. What do the readings indicate?

  1. The air filter is badly clogged with dust
  2. The condenser coil is dirty
  3. The system is low on refrigerant
  4. The system is overcharged
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C. The system is low on refrigerant

Low suction with high superheat shows the evaporator is starved, and almost no subcooling shows there is little liquid stored in the condenser: together that is an undercharge. A clogged filter also lowers suction pressure but gives low superheat, because the coil has little heat to boil the refrigerant.

Reference: Refrigerant-side diagnosis; superheat and subcooling

A fixed-orifice system has a suction saturation temperature of 29°F, superheat of 2°F and subcooling of 14°F. The filter was replaced yesterday. What should be checked next?

  1. The evaporator coil face and blower wheel for dirt
  2. The system for a refrigerant leak
  3. The liquid line drier for a restriction
  4. The condenser coil for blocked fins
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A. The evaporator coil face and blower wheel for dirt

Low suction with very low superheat and healthy subcooling means refrigerant is not picking up enough heat: an indoor airflow problem. With a new filter the next suspects are a matted coil and a dirty blower wheel. A leak or a restriction would both show high superheat.

Reference: Refrigerant-side diagnosis; low evaporator airflow

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