Oklahoma - HVACR CONTRACTOR UNLIMITED

Key Concepts

Ohm's Law & Watt's Law - Ohm's Law: V = I × R (Voltage = Current × Resistance) - Watt's Law: P = V × I (Power = Voltage × Current) - Derived: P = I²R and P = V²/R - 1 volt = EMF required to push 1 ampere through 1 ohm of resistance

Circuit Types - Series circuit: Same current flows through all components; total resistance = R1 + R2 + R3; voltage divides across loads - Parallel circuit: Same voltage across all branches; total resistance = 1/(1/R1 + 1/R2 + 1/R3); current divides; total current increases - Series-parallel: Combination — solve parallel sections first, then treat as series

Wiring Diagrams - Pictorial diagrams: Show physical appearance and location of components; used for installation - Ladder diagrams: Show electrical function only; control circuit on left, load on right; read top-to-bottom like rungs of a ladder; used for troubleshooting

Control System Fundamentals - Differential: The gap between cut-in and cut-out points - Cut-out: The point at which the control opens the circuit (e.g., high pressure reached, low temp reached) - Cut-in: The point at which the control closes the circuit to restart - Range adjustment shifts both cut-in and cut-out together; differential adjustment changes the gap between them

Key Concepts

Gas Piping Materials & Fittings

• Steel pipe (black or galvanized) is the standard for interior gas piping; use malleable iron or steel fittings
• CSST (Corrugated Stainless Steel Tubing) is allowed but requires bonding/grounding per local amendments; must be protected from physical damage
• Copper tubing is allowed only for LP gas where the gas does not contain hydrogen sulfide; prohibited for natural gas in most jurisdictions
• Polyethylene (PE) pipe is approved for underground exterior use only — never inside buildings
• Drip legs (sediment traps) must be installed at each appliance connection to collect moisture and debris

Gas Piping Sizing Methods

• Longest Length Method: Size all pipe using the longest pipe run from meter to the furthest appliance; conservative, simpler
• Branch Length Method: Size each branch based on its own length; more accurate, allows smaller pipe on short runs
• Hybrid Pressure Method: Uses higher system pressure (typically 2 psi) with pressure regulators at each appliance; allows smaller pipe sizes throughout
• Pipe sizing tables are based on BTU/hr capacity, pipe length, and allowable pressure drop

Key Concepts

Duct System Types

• Radial (spider) system: Individual ducts run from central plenum to each register. Simple, low static pressure, ideal for small residential.
• Extended plenum: Large central trunk extends full length; branches tap off the side. Most common residential system.
• Reducing trunk (reducing plenum): Trunk reduces in size as air is distributed; maintains more uniform static pressure throughout.
• Perimeter loop: Duct runs around the building perimeter, fed from a central plenum; excellent for slab-on-grade construction in cold climates.
• Small Duct High Velocity (SDHV): Operates at 1,200–2,000 fpm; uses 2" flexible mini-ducts with inlet velocities ~600 fpm at diffuser; requires special equipment.

Duct Construction & Materials (IMC Chapter 6 / IMC 603)

• Sheet metal: Galvanized steel is standard; gauges per SMACNA — round ducts and rectangular ducts have minimum thickness requirements based on duct size.
• Flexible duct: Limited to 5 ft maximum recommended length per run to minimize friction losses; must be fully extended (not compressed).
• Fiberglass duct board: Factory-insulated; R-value built in; joined with approved staples + foil tape.
• Joining methods: Slip joints, drive cleats, S-cleats, and standing seams for sheet metal; UL-listed tape and mastic sealant for sealing all duct types. Duct tape (cloth) is NOT code-approved for sealing.
• IMC 603.2: References ACCA Manual D as the accepted standard for residential duct system design.

Key Concepts

Compression Refrigeration Cycle The mechanical refrigeration cycle has four stages: compression (compressor raises refrigerant pressure/temperature), condensation (condenser rejects heat to ambient), metering (metering device drops pressure), and evaporation (evaporator absorbs heat from the conditioned space). Refrigerant flows heat from a low-temperature area to a high-temperature area — always moving heat, never creating cold.

Metering Devices - TXV (Thermostatic Expansion Valve): Maintains constant superheat; best for variable loads. Has a sensing bulb clamped to suction line. - Fixed orifice / piston: Simple, no moving parts; sized for one design condition. - Automatic Expansion Valve (AXV): Maintains constant evaporator pressure; used in steady-load applications. - Capillary tube: Fixed restriction; must be precisely matched to system; cannot compensate for load changes. - Electronic expansion valve (EEV): Most precise control; used in variable-capacity and heat pump systems.

Compressor Types - Reciprocating: Common, handles liquid slugging poorly; uses reed valves. - Scroll: Quiet, efficient, oil-sensitive, no valve failures; cannot run in reverse (unidirectional). - Rotary: Compact, quiet; common in small residential equipment. - Screw (helical rotary): High capacity commercial/industrial use. - Centrifugal: Large tonnage chillers; sensitive to low-load surge. - Safety devices: high-pressure cutout, low-pressure cutout, motor overload, oil pressure safety switch (crankcase pressure differential type — requires 120-second time delay on startup).

Key Concepts

Gas-Fired Heating Systems

• Burner types: atmospheric (natural draft), power (forced air), and premix. Ignition systems include standing pilot, intermittent pilot (IPI), and hot surface ignition (HSI). HSI elements are typically silicon carbide or silicon nitride rated at 120V.
• Gas valves: combination gas valves integrate pressure regulation, safety shutoff, and flow control. Two-stage and modulating valves are common on high-efficiency equipment.
• Heat exchangers: primary heat exchanger transfers combustion heat to supply air; secondary (condensing) heat exchangers recover latent heat in high-efficiency (90%+ AFUE) furnaces. Cracked heat exchangers produce CO infiltration — a critical safety defect requiring immediate shutdown.

Oil-Fired Heating Systems — Combustion Efficiency Testing

• CO₂ test: ideal range is 10–12% for No. 2 fuel oil (higher CO₂ = more complete combustion, up to a point).
• Stack temperature: net stack temperature (gross stack minus room temp) should be 300–600°F. High stack temp = heat going up the flue, poor efficiency.
• Draft: over-fire draft target is approximately –0.02 in. WC; breech draft approximately –0.04 to –0.06 in. WC.
• Smoke test: Bacharach scale 0 (clear) to 9 (black); acceptable rating is 0–1 for residential. Readings above 1 indicate incomplete combustion.
• Combustion efficiency = 100% minus flue losses (calculated using CO₂% and net stack temperature via Siegert or chart method).

Key Concepts

OSHA Regulations & Employer Reporting

• OSHA (Occupational Safety and Health Act) governs all workplace safety requirements for equipment and personal safety.
• Employers must report to OSHA every fatality, every permanently disabling injury, or any accident involving three or more hospitalized workers.
• OSHA mandates a yearly audit of workplace safety practices; however, technicians should conduct an informal hazard assessment every time they arrive at a job site (check exits, first aid, chemicals, ventilation).

Lockout/Tagout (LOTO)

• Lockout (LO): Physically locking an electrical switch or mechanism in the open (off) position so service can be performed safely. The servicing technician holds the key.
• Tagout (TO): Placing a tag on a switch/mechanism to warn others not to change its position while service is underway.
• LOTO combined: Both practices are typically used together — lock the panel open and tag it.
• OSHA standard: Use lockout devices whenever equipment can be locked out. Tagout alone is only acceptable if it provides equivalent employee protection to lockout.
• Systems requiring LOTO include: electrical, hydraulic, pneumatic, mechanical, and thermal.
• Never assume someone else shut off the power — personally verify and de-energize the circuit.