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The OK-MECH exam has 90 questions and runs 4 hours.
These 10 free OK-MECH questions are organized by exam domain, so you can see how each part of the Oklahoma Mechanical Trade License Exam - HVACR Contractor Unlimited blueprint is tested. Reveal the answer and explanation under each question.
An air-conditioning compressor receives low voltage while running. With its two-pole contactor pulled in, a technician measures 240 V line-to-line at the contactor input, 24 V across the energized coil, and voltage drops of 0.1 V and 18 V across the two closed power-contact poles. Which fault is directly implicated by these measurements?
Correct answer: D - Excessive resistance in one contactor power pole
A permanent split capacitor blower motor specifies a 40-µF, 370-VAC run capacitor. The failed capacitor is being replaced. All four available replacements are listed, continuous-duty AC motor-run capacitors that fit the mounting location. Select the suitable replacement rating.
Correct answer: D - 40 µF, 440 VAC
A natural-gas meter supplies a furnace rated at 100,000 Btu/h input, a water heater rated at 40,000 Btu/h input, and a unit heater rated at 60,000 Btu/h input. The common trunk serves all three before the first branch. The most remote outlet is 53 developed feet from the meter. The design uses the longest-length method with available table rows of 40, 60, and 80 ft; its fitting allowance is already accounted for. Gas heating value is 1,000 Btu/ft³. Which demand and table-length pair belongs in the common trunk's sizing calculation?
Correct answer: B - 200 ft³/h using the 60-ft table row
The balancing contractor proposes increasing a belt-driven supply fan from 1,000 to 1,200 rpm. At 1,000 rpm, the fan absorbs 2.4 brake horsepower and is driven by a 5-hp motor with a 1.0 service factor. The fan and drive are rated for the proposed speed; fan geometry, the system resistance curve, and air density will not change. Neglect drive losses. What predicted fan load and motor decision follow from this change?
Correct answer: A - About 4.15 bhp; keep the 5-hp motor.
Water stains appear beneath an insulated supply duct in a humid ceiling space. The metal duct is at 50°F, and the surrounding air has a 63°F dew point. The insulation is wet beneath a torn exterior jacket, while adjacent sections with intact jackets remain dry. The condensate drain is sound, and the duct passes its air-leakage test. What repair addresses the moisture-entry mechanism?
Correct answer: C - Replace wet insulation and reseal the exterior vapor-retarder jacket.
Every time a restaurant's new hood exhaust starts, an outward-swinging exterior door becomes difficult to open and a natural-draft water heater spills combustion products at its draft hood. Both effects disappear when the exhaust stops. The exhaust fan delivers its scheduled airflow, and inspection finds no obstruction in the water-heater vent. The water heater has been taken out of service pending correction. What single system defect most convincingly explains both observations?
Correct answer: C - Insufficient replacement air for the exhaust
A walk-in cooler with a thermostatic expansion valve is not reaching setpoint. The evaporator is clean, its fans are operating correctly, and the load is normal. Stabilized readings show low suction pressure, 28°F evaporator-outlet superheat, and 18°F subcooling at the condenser outlet. The liquid line has an unusually large pressure drop across the filter-drier, with the drier outlet noticeably colder than its inlet. Which fault best accounts for the entire set of findings?
Correct answer: C - A restricted filter-drier starving the evaporator
While checking the charge of an R-407C split system with a thermostatic expansion valve and no liquid receiver, a technician confirms normal airflow, load, and superheat. At the measured condenser-outlet pressure, the refrigerant chart gives a bubble temperature of 108°F and a dew temperature of 115°F. The liquid line is 98°F at that same location. The manufacturer's acceptable subcooling range under these conditions is 8°F to 12°F. Which charge decision follows from these readings?
Correct answer: A - No charge adjustment: 10°F of subcooling.
On a cold, damp morning, an owner reports a white cloud rising from the outdoor heat-pump coil. The technician observes the outdoor fan stop while the compressor continues running and auxiliary heat operates indoors. Frost melts from the coil; several minutes later, the outdoor fan restarts and normal heating resumes. No fault is recorded, and the sequence matches the manufacturer's defrost description. How should the technician respond to the owner's request to shut the unit off?
Correct answer: B - Leave it operating; the observed cycle is normal defrost.
During a hydronic retrofit review, an installer adds the building's full vertical rise to the calculated circuit friction head when selecting a circulator. The system is a completely filled, closed supply-and-return loop, and its fill pressure maintains positive pressure at the highest emitter. Which pump-selection principle resolves the disagreement?
Correct answer: A - Use circuit friction head at design flow; the return offsets elevation gain.
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