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These 10 free TN-CMC questions are organized by exam domain, so you can see how each part of the Tennessee Full Mechanical Contractor (CMC) Exam blueprint is tested. Reveal the answer and explanation under each question.
An electric reheat coil warms dehumidified air from 55°F to 72°F. No air is mixed in, no water is added or removed, and the pressure change is negligible. Relative humidity decreases. What happens to humidity ratio and dew-point temperature across the reheater?
Correct answer: B - Humidity ratio and dew-point temperature remain unchanged.
A gas-appliance enclosure will use the one-permanent-opening combustion-air method in the 2021 IFGC. Appliance clearances and the opening's location comply with that method, and the opening communicates directly with outdoors. Combined appliance input is 180,000 Btu/h; the vent connectors have a combined cross-sectional area of 84 square inches. Each available louver has a manufacturer-certified free area equal to 60% of its gross opening. Select the smallest listed gross opening that satisfies the method.
Correct answer: D - 10 inches by 16 inches
Before a construction crew enters a mechanical vault classified as a permit-required confined space, a calibrated monitor reads 18.2% oxygen, 0% of the lower flammable limit, and no detectable concentration of the toxic gases being monitored. No one has entered, and the entry plan does not provide atmosphere-supplying respiratory protection. Which entry decision is warranted?
Correct answer: D - Withhold entry; correct the oxygen deficiency and reverify acceptable entry conditions.
A listed condensing unit is marked MCA 22 A and MAXIMUM FUSE 35 A. Its supply conductors have a final allowable ampacity of 25 A after all required adjustments, and the installer uses 30-A time-delay fuses permitted by the equipment instructions. All other circuit requirements are satisfied. A reviewer objects that a 30-A fuse cannot protect these 25-A conductors. Which response correctly applies the equipment ratings?
Correct answer: D - Accept the sizing: the conductors meet MCA and the fuses are within the marked limit.
A permanently connected potable-water makeup line serves a closed process loop containing a toxic corrosion inhibitor. The loop pump can raise downstream pressure above potable-supply pressure, and a supply interruption could also cause backsiphonage. The design requires a mechanical backflow-prevention assembly suitable for this health hazard. Which assembly addresses both pressure conditions?
Correct answer: C - A reduced-pressure-principle backflow assembly
A small music venue has design cooling loads of 23,000 Btu/h sensible and 9,000 Btu/h latent. The capacities below are manufacturer-rated values at the project's actual design conditions and approved airflow. No separate dehumidifier is planned. Which unit meets both load components?
Correct answer: C - 34,000 Btu/h total; 24,000 Btu/h sensible
An owner reports that a heat pump sometimes releases a cloud of vapor and stops its outdoor fan. During a cold, damp morning, the contractor observes the compressor continue running while the outdoor fan stops. Frost clears from the outdoor coil; the fan and normal heating restart automatically after four minutes. There is no burning odor or fault indication. How should this service call be resolved?
Correct answer: B - Record a normal defrost cycle; these observations do not indicate a repair is needed.
A TXV-controlled walk-in cooler runs continuously above setpoint. Evaporator airflow is correct, suction pressure is below its expected operating range, and evaporator outlet superheat is above the specified target. Subcooled liquid enters the liquid-line filter-drier, but the measured pressure drop across it exceeds its allowable limit and its outlet is 8°F colder than its inlet. Bubbles appear in the downstream sight glass. Which fault accounts for the complete pattern?
Correct answer: C - A restricted liquid-line filter-drier
The ventilation schedule for a single-zone office calls for 1,400 CFM of total supply air, including 260 CFM of outdoor air. Verified measurements show 1,410 CFM of supply but only 110 CFM of outdoor air. The outdoor-air path is unobstructed, and the supply outlets are already near their scheduled flows. What adjustment corrects the ventilation deficiency without over-supplying the occupied space?
Correct answer: A - Increase outdoor air and reduce recirculated air to preserve total supply.
Acceptance testing of a sprinkler system produces three recorded results: operation of the detection system alone does not admit water to the sprinkler piping; opening a sprinkler alone does not admit water; combining detection with an open sprinkler releases water into the piping. These results identify which arrangement?
Correct answer: D - A double-interlock preaction system
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