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These 10 free TABB-T questions are organized by exam domain, so you can see how each part of the TABB Technician (TABB-T) blueprint is tested. Reveal the answer and explanation under each question.
Across a dry reheat coil, air temperature rises from 55°F to 72°F and relative humidity falls. No air is mixed into the stream, no moisture is added or removed, and air pressure is effectively unchanged. Which description of the leaving air is correct?
Correct answer: A - Its humidity ratio and dew point are unchanged despite the lower relative humidity.
A cooling coil must provide at least 5.5 tons of total cooling at the specified acceptance condition. Simultaneous measurements immediately across the coil show 2,000 cfm, entering/leaving dry-bulb temperatures of 80°F/55°F, and entering/leaving enthalpies of 31/23 Btu per lb of dry air. For this calculation, use 0.075 lb of dry air per ft³ and 12,000 Btu/h per refrigeration ton. Which capacity entry and acceptance decision are justified?
Correct answer: C - 6.0 tons of total cooling; meets the stated capacity requirement.
A direct-drive supply fan delivers 8,000 cfm at 900 rpm and absorbs 5.0 brake horsepower. Required airflow is 10,000 cfm; the motor is rated 7.5 hp with a service factor of 1.0. Assume constant air density and efficiency, unchanged duct and damper configuration, and no drive-speed limitation. What do the fan laws predict for the change needed to reach design airflow?
Correct answer: A - 1,125 rpm and 9.8 bhp; exceeds the motor rating.
Measurements on either side of a gradual duct enlargement show lower velocity, higher static pressure, and slightly lower total pressure downstream. There is no fan, branch connection, or leakage between the stations, and air density is unchanged. How should these readings be interpreted?
Correct answer: B - Some velocity pressure becomes static pressure, while losses reduce total pressure.
A centrifugal water pump develops a gravel-like sound and fluctuating discharge pressure. The system has been vented, and measured flow is within the manufacturer's permitted operating range. At that flow, net positive suction head available is 7 ft, while the pump requires 12 ft. Which mechanism most directly accounts for these findings?
Correct answer: D - Vapor formation from inadequate suction head, followed by bubble collapse inside the pump.
Before a motor resistance test, a qualified technician presses STOP on the variable-frequency drive. The motor stops and the display shows 0 Hz, but the input disconnect remains closed. A coworker is about to remove the motor terminal cover. Which action is required before the resistance test proceeds?
Correct answer: D - Isolate all supplies, apply lockout/tagout, follow the discharge procedure, and verify absence of voltage.
Proportional balancing is complete on a constant-volume air system: every outlet is at approximately 80% of its own design airflow, and the index path is fully open. The fan and motor have verified capacity for the remaining adjustment. How should the technician bring the system up to design while preserving the balance already achieved?
Correct answer: C - Increase fan speed with the balanced damper settings retained, then recheck the outlets.
A pressure-independent VAV terminal is overridden to its scheduled maximum of 900 cfm. Its controller displays 900 cfm, but a calibrated inlet-duct traverse repeatedly measures 630 cfm on the same density basis. Inlet static pressure is adequate. The damper is partly open, and manually opening it produces 900 cfm. Where should troubleshooting focus?
Correct answer: C - Check the airflow pickup and tubing, and verify the sensor calibration and flow coefficient.
Leakage-test submittal: construction pressure class +4.0 in. w.g.; required test pressure +2.0 in. w.g.; allowable leakage 75 cfm; recorded result 58 cfm at +1.0 in. w.g. The apparatus can safely reach the specified test pressure. What is the correct acceptance action?
Correct answer: B - Require a leakage measurement at +2.0 in. w.g. before accepting or rejecting the section.
A chilled-water coil requires 18 gpm. Pressure readings at the same-elevation taps of its fixed-orifice flow station are 49 psig upstream and 45 psig downstream; the separate pressure drop across the coil is 9 psi. For water at these conditions, the station calibration is Q = 6√Δp, with Q in gpm and Δp in psi across the station. The branch balancing valve is partly closed. Which finding and adjustment are appropriate?
Correct answer: B - 12 gpm; open the balancing valve further and remeasure.
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