Oklahoma HVACR Contractor Unlimited

Key Concepts

Ohm's Law and Watt's Law (Ch. 12, Sec. 12.1.3; Ch. 13, Sec. 13.1) - E = I × R; R = E ÷ I; I = E ÷ R. With voltage constant, current and resistance are inversely related — lower the resistance, current increases. - Watt's law: P = I × E. A 20 A load at 120 V consumes 2400 W.

Series circuits (Ch. 12, Sec. 12.4.2) — Single path for current; the same current flows through every load. If any one device opens (switch, fuse, or safety limit), the entire circuit is broken and no current reaches the compressor. This is why series circuits are used as safety circuits.

Voltage drop (Ch. 12, Sec. 12.4.5) — VD = R_L × I_L. In a series circuit, VD1 + VD2 + VD3 + … = total applied voltage (V_T).

Series-parallel circuits (Ch. 12, Sec. 12.4.4) — Start with the farthest combination of loads and work backward to find total resistance, so total current can be calculated.

Transformers (Ch. 12, Sec. 12.7) — A control transformer steps line voltage (typically 120 V or 240 V) down to a lower voltage for the control circuit. Low-voltage control circuits usually operate at 24 V (Ch. 17, Sec. 17.1.1). Low-voltage control wiring is governed by NEC 725.

Oklahoma HVACR Contractor Unlimited · 12 questions (13% of exam)

Key Concepts

Code scope and pressures - The code covers piping systems for natural gas at 125 psig or less and LP-gas at 20 psig or less, except as provided in Section 402.7 — IFGC 2024 §101.2.3 (2018 ed.: §101.2.2). - Maximum operating pressure for piping inside buildings shall not exceed 5 psig, except where listed conditions are met (welded/brazed joints, ventilated chase, boiler/mechanical rooms, etc.) — §402.7.

Piping materials - Cast-iron pipe shall not be used — §403.3.1 (2018 ed.: §403.4.1). Steel, stainless steel and wrought iron shall be not lighter than Schedule 10 — §403.3.2. Aluminum-alloy pipe shall not be used in exterior locations or underground — §403.3.4. PVC and CPVC shall not be used to supply fuel gas — §403.5. Plastic pipe is limited to outdoors underground only — §404.17.1.

Installation - Shield plates: steel, thickness not less than 0.0575 inch (No. 16 gage) — §404.7.3; required where piping through holes/notches is less than 1½ inches from the framing member face receiving the membrane, extending not less than 4 inches each side — §404.7.1. - Support spacing per Table 415.1: ½-inch steel = 6 ft; ¾-inch or 1-inch = 8 ft; 1¼-inch or larger horizontal = 10 ft; 1¼-inch or larger vertical = every floor level. - Burial depth: 12 inches minimum below grade; 8 inches for individual lines…

Key Concepts

Duct system types - Radial duct system — individual branch ducts run directly from the central plenum to each supply outlet, with no trunk line (Modern Refrigeration and Air Conditioning, p. 672 (22nd ed.; 755 in the 20th)). - Perimeter loop system — used with downflow furnaces in homes without basements; ductwork is laid out in the concrete slab, branch ducts connect to a perimeter duct, and diffusers connect at intervals along the floor (p. 672 (22nd ed.; 755 in the 20th)). - SDHV (small duct, high-velocity) — small-diameter ducts, supply air to the inside perimeter of the building, return air originating from the center (p. 666 (22nd ed.; 748 in the 20th)).

Duct construction and installation (IMC 2024, Chapter 6) - 603.2 Duct sizing — ducts in a single dwelling unit shall be sized per ACCA Manual D, the appliance manufacturer's installation instructions, or other approved methods. All other buildings: ASHRAE Handbook of Fundamentals. - 603.3 Classification — positive or negative 0.5, 1.0, 2.0, 3.0, 4.0, 6.0, or 10.0 inches w.c. - 603.4.2 Duct lap — crimp joints for round and oval metal ducts lapped not less than 1 inch, male end extending into the adjoining duct in the direction of airflow. - 603.5 Nonmetallic ducts — Class 0 or Class 1 material, comply with UL 181; air temperature shall not exceed 250°F (121°C). - 603.5.1 Gypsum ducts —…

Key Concepts

Cycle dividers. The compressor is one of the two dividers between the low side and the high side; it draws low-pressure vapor from the evaporator and pushes high-pressure vapor into the high side. The metering device is the other divider at the opposite end of the cycle (pp. 126–128 (22nd ed.; 91–93 in the 20th)).

Metering devices. Capillary tube systems do not use liquid receivers; receivers are found on larger systems with low-side float or expansion valve metering (p. 96). A capillary tube on a static (fanless) condenser must be about 10% longer than on a fan-cooled system (Ch. 20, p. 144 (22nd ed.; 473 in the 20th)). When a capillary tube is replaced, install a new liquid line filter-drier (Ch. 20, p. 146 (22nd ed.; 475 in the 20th)); "All systems should have a liquid line filter-drier installed" (Section 6.4.4, p. 58 (22nd ed.; 123 in the 20th)). A distributor splits flow into several parallel evaporator paths, reducing pressure drop and evening out coil temperature (pp. 1088–1089 (22nd ed.; 490–491 in the 20th)).

Oklahoma HVACR Contractor Unlimited

Key Concepts

Electric resistance heating - 1 kW = 3,413 Btu/hr; to convert Btu/hr to kW, divide by 3,413 (50,000 ÷ 3,413 = 14.65 kW). (Modern Refrigeration and Air Conditioning, p. 832 (22nd ed.; 1194 in the 20th)) - A sequencer closes/opens contacts on a time delay to energize heating elements one at a time — typically about 30 seconds apart — to minimize initial current draw and prevent disturbances such as flickering lights. (pp. 834 (22nd ed.; 1196 in the 20th), 1205) A sequencer whose contacts fail to close leaves its element de-energized, causing low heat output. (pp. 841–844 (22nd ed.; 1205–1206 in the 20th)) - A double-pole thermostat is safest for 240 V baseboard because it opens both legs, reducing risk of grounds or shorts; single-pole opens only one leg. (p. 846 (22nd ed.; 1210 in the 20th)) - Line-voltage thermostats (120 Vac or 240 Vac) wire directly to electric baseboard resistance heaters with no step-down transformer. (p. 952) - Troubleshooting: elements heating and cooling as the blower cycles = short cycling, anticipator rating set too high (also: high-limit opening/closing, incorrect low voltage, motor overload cycling). (p. 849 (22nd ed.; 1214 in the 20th))

Key Concepts

OSHA and employer reporting. The national law covering workplace safety is the Occupational Safety and Health Act (OSHA), which covers both equipment safety requirements and the personal safety of the technician. Companies must report to OSHA "every fatality or permanently disabling injury or any accident involving three or more hospitalized injuries" (Modern Refrigeration and Air Conditioning, Section 2.1 Safety and the Government). Safety certifications: the OSHA 10-hour class is intended for entry-level workers; the 30-hour class is designed for workers with some safety responsibility (Section 2.1 (22nd ed.; 2.4.8 in the 20th) Safety Certifications).

Electrical safety / LOTO. Lockout/tagout (LOTO) practices eliminate or reduce electrical hazards (Chapter 2 (22nd ed.; 2 in the 20th) Summary; Section 2.2.1). Never stand in wet or damp areas when checking live connections; do not wear jewelry when working on live circuits. Extension cords for power tools should be connected to a GFCI receptacle — a fast-acting breaker that shuts off power in as little as 1/40 of a second. Extension cords must be outdoor type, rated for 600 V, with an insulated grounding conductor (Sections 2.2.1, 2.4.6).