Oklahoma HVACR Contractor, Limited

Key Concepts

Ohm's Law & Watt's Law - Voltage (V) = Current (I) × Resistance (R) - Power (W) = Voltage (V) × Current (I) - These two laws are the mathematical foundation for nearly all electrical troubleshooting questions.

Circuit Types - Series circuit: One path for current; total resistance = sum of all resistances; if one component fails open, the entire circuit goes dead. - Parallel circuit: Multiple paths for current; total resistance decreases as more loads are added; voltage across each branch is equal. - Ladder diagrams show control logic horizontally between two power rails — used for control circuits (24V). Pictorial diagrams show physical wiring layout.

Single-Phase vs. Three-Phase Motors - Single-phase motors require a start winding or capacitor to create a rotating magnetic field (they are not self-starting). - PSC (Permanent Split Capacitor): Run capacitor always in circuit; used in fan motors and small compressors. - CSCR (Capacitor Start, Capacitor Run): Both start and run capacitors; high starting torque, efficient running. - Three-phase motors are self-starting; rotating field is created by three displaced voltages — no capacitors needed.

Key Concepts

Gas Piping Materials & Fittings

• Black steel pipe (Schedule 40) is the most common interior gas piping material. Galvanized pipe is acceptable but less common.
• CSST (Corrugated Stainless Steel Tubing) is allowed but must be bonded/grounded per local requirements; cannot be used as a vent connector.
• Copper is acceptable for LP gas but prohibited for natural gas in most jurisdictions (natural gas can contain hydrogen sulfide which corrodes copper).
• Polyethylene (PE) pipe is approved for underground exterior use only — never inside buildings or above ground.
• Use only approved fittings for each pipe type; no compression fittings on gas piping. Flare fittings acceptable on copper/CSST.
• A drip leg (sediment trap) must be installed upstream of each appliance.

Gas Piping Sizing

• Longest Length Method: Size all pipe using the total length of the longest run from meter to farthest appliance. Conservative — results in larger pipe throughout.
• Branch Length Method: Each branch segment is sized using only the length of that branch. More accurate, allows smaller pipe on short runs.
• Hybrid Pressure Method: Uses higher delivery pressure (e.g., 2 psi) with regulators at appliances, allowing smaller pipe sizes over longer runs.
• Pipe capacity is found in BTU/hr tables (IFGC/NFPA 54 Appendix tables) based on pipe diameter, length, and specific gravity of gas.

Key Concepts

Duct System Configurations - Radial: Individual ducts run directly from plenum to each outlet — simple, works well for small systems - Extended plenum: Large trunk duct maintains full size most of its length, then reduces; balances pressure well - Reducing trunk: Trunk reduces in size as branch takeoffs are made; maintains more consistent velocity - Perimeter loop: Continuous loop around perimeter fed by multiple radial ducts — best for slab-on-grade in cold climates - SDHV (Small Duct High Velocity): Uses 2"–3" flexible ducts at 1,200–2,000 fpm; requires special high-static blowers and aspirating outlets

Duct Construction & Materials - Sheet metal (galvanized steel): Most durable, used for trunk lines - Flex duct: Factory-insulated, limited to branch runs — must be fully extended (no sags), max 5–6 ft runs recommended - Duct board (fiberglass): Pre-insulated, R-4.2 typical; requires proper sealing at joints - All supply/return ducts must be sealed per IMC Chapter 6; use mastic sealant or UL 181-listed tape (NOT standard cloth duct tape)

Oklahoma HVACR Contractor, Limited Exam (20 questions, 25% of exam)

Key Concepts

Compression Refrigeration Cycle The basic vapor-compression cycle has four stages: evaporation (heat absorption) → compression → condensation (heat rejection) → expansion/metering. Refrigerant absorbs heat in the evaporator as it changes from liquid to vapor (latent heat of vaporization), and rejects heat in the condenser as it returns to liquid. Heat flows from warm to cool — the refrigerant is the medium that moves it.

Refrigerant Classification - CFCs (R-11, R-12): Fully halogenated, highest ozone depletion potential (ODP), phased out under the Montreal Protocol - HCFCs (R-22, R-123): Partial hydrogen substitution, lower ODP, being phased down (R-22 production/import banned as of January 1, 2020) - HFCs (R-134a, R-410A, R-404A): Zero ODP, but high Global Warming Potential (GWP); subject to AIM Act phase-down - HFOs (R-1234yf, R-1234ze): Near-zero ODP and low GWP; increasingly common replacements - Refrigerant blends: Zeotropic blends (R-410A, R-404A) have glide — temperature changes during phase change; azeotropic blends (R-502) behave like single-component refrigerants

Key Concepts

Forced-Air Furnace Types - Upflow: Air enters bottom, exits top — most common, used in basements/closets - Downflow (counterflow): Air enters top, exits bottom — used over crawlspaces or slab - Horizontal: Air flows side-to-side — used in attics or crawlspaces with limited vertical clearance - Multipoise: Can be installed in multiple orientations - Highboy: Upflow with blower beneath heat exchanger; Lowboy: Shorter profile, blower beside heat exchanger

Efficiency Ratings - AFUE (Annual Fuel Utilization Efficiency): Measures seasonal efficiency of fuel-burning furnaces - Noncondensing furnaces: AFUE typically 80–89%; single (primary) heat exchanger; exhaust vented through metal flue - Condensing furnaces: AFUE 90%+; primary and secondary heat exchanger; extracts latent heat from flue gases; produces liquid condensate; vented with PVC pipe (not metal flue) - Two-stage and modulating furnaces improve efficiency by varying heat output to match load

Gas-Fired Heating: Ignition & Combustion Controls - Standing pilot: Continuously burning; proven via thermocouple - Intermittent pilot (IPI): Spark-ignited pilot, proven by flame sensor before gas valve opens - Hot surface igniter (HSI): Silicon carbide or silicon nitride element glows to ignite burner directly — most common modern method - Flame rollout switch: Safety control that cuts power if flames extend outside combustion chamber - Limit switch: Shuts off burner if heat exchanger overheats; resets automatically or manually

Key Concepts

Lockout/Tagout (LOTO) - Lockout (LO): Physically locking an electrical switch or mechanism in the open (off) position so service work can be performed safely. - Tagout (TO): Placing a warning tag on a switch to inform others that service is in progress and the switch must not be changed. - OSHA requires lockout devices be used when equipment can be locked out. Tagout alone is only acceptable if it provides equivalent protection to lockout. - Never assume someone else shut off the power — personally verify the circuit is de-energized. Treat all power sources as live until you shut them off yourself. - Systems requiring LOTO include: electrical, hydraulic, pneumatic, mechanical, and thermal.

Fire Extinguisher Use — P.A.S.S. - Pull the pin - Aim at the base of the fire - Squeeze the handle - Sweep side to side - Use the correct extinguisher class: Class C for energized electrical equipment. Never use water on electrical fires.

Refrigerant Cylinder Safety - Refrigerant cylinders are pressure vessels — always store upright, keep away from heat sources, and never exceed rated pressure. - Never refill disposable cylinders; never heat a cylinder with an open flame. - ASHRAE Standard 34 classifies refrigerants by safety group (toxicity and flammability). Know that lower numbers = less hazard. - A stationary refrigerant detector is used in machine rooms to detect dangerous refrigerant concentrations.