Tennessee CMC-C Mechanical HVAC/Refrigeration Contractor

Tennessee CMC-C Exam — 7 Questions (7%)

Key Concepts

Ohm's Law & Power Formulas - E = I × R (Voltage = Current × Resistance) - P = E × I (Watts = Volts × Amps) - P = I² × R and P = E²/R - 1 HP = 746 watts - kVA = (V × A) / 1,000 (single-phase) | kVA = (V × A × 1.732) / 1,000 (three-phase)

Series vs. Parallel Circuits - Series: Total resistance = R1 + R2 + R3; current is the same throughout; voltage divides - Parallel: Total resistance = 1/(1/R1 + 1/R2 + 1/R3); voltage is the same across all branches; current divides

CMC-C Tennessee Mechanical – HVAC/Refrigeration Contractor | 14 Questions (14%)

Key Concepts

Refrigeration Piping

• Oil return is critical in refrigerant piping design. Suction lines must be pitched toward the compressor (minimum ¼" per 10 feet) and sized for adequate velocity to carry oil back (minimum ~700 FPM in suction risers).
• Double suction risers are used when system capacity varies widely — prevents oil logging at low load.
• Liquid lines must be subcooled to prevent flash gas; avoid excessive vertical rise without accounting for pressure drop.
• Brazing (silver alloy) is required for refrigerant piping joints — soldering is not acceptable for most refrigerants. Nitrogen purging during brazing prevents oxidation scale inside tubing.
• ACR copper tubing is dehydrated and sealed at the factory; Type L or ACR copper is standard for refrigerant lines.

Tennessee CMC-C Exam — 10 Questions (10% of Exam)

Key Concepts

Compression Refrigeration Cycle The standard vapor-compression cycle has four components: compressor, condenser, metering device (expansion valve or orifice), and evaporator. The refrigerant absorbs heat at the evaporator (low-pressure side) and rejects heat at the condenser (high-pressure side). Heat always flows from high temperature to low temperature — never the reverse without work input.

Gas Laws and Pressure-Temperature Relationships - Boyle's Law: At constant temperature, pressure and volume are inversely proportional (P₁V₁ = P₂V₂) - Charles's Law: At constant pressure, volume and temperature are directly proportional (V₁/T₁ = V₂/T₂) - Gay-Lussac's Law: At constant volume, pressure and absolute temperature are directly proportional (P₁/T₁ = P₂/T₂) - All gas law calculations must use absolute temperature — Rankine (°F + 460) or Kelvin (°C + 273) - Refrigerant PT…

Tennessee CMC-C Exam Study Brief

Key Concepts

Compression Refrigeration Cycle (4 Phases) The cycle moves heat from a low-temperature space to a higher-temperature environment using refrigerant phase changes: 1. Compression – Compressor draws in low-pressure vapor and compresses it to high-pressure, high-temperature vapor 2. Condensing – High-pressure vapor releases heat and condenses to liquid in the condenser 3. Metering – Metering device drops pressure, causing flash gas and lowering refrigerant boiling point 4. Evaporating – Low-pressure liquid absorbs heat from the conditioned space and vaporizes

High Side vs. Low Side - High side: Compressor discharge → condenser → liquid receiver → liquid line → metering device inlet - Low side: Metering device outlet → evaporator → accumulator → suction line → compressor inlet - High-side pressure = condensing pressure; Low-side pressure = evaporating pressure

Key Concepts

Refrigeration Cycle Components (Four Core Components)

• Compressor: Pumps low-pressure vapor refrigerant, compresses it into high-pressure, high-temperature vapor. Divides low side from high side.
• Condenser: Releases heat from high-pressure vapor; refrigerant exits as high-pressure liquid.
• Metering device: Reduces pressure and controls refrigerant flow into the evaporator (TXV, fixed orifice, cap tube).
• Evaporator: Low-pressure liquid absorbs heat and changes to low-pressure vapor; the "cooling" happens here.
• Supporting components: Oil separator (after compressor), liquid receiver (stores liquid after condenser), filter-driers (remove moisture/contaminants), sight glass (indicates moisture and charge level), accumulator (prevents liquid slugging at compressor suction).

Key Concepts

Gas Types & Properties

• Natural gas: lighter than air, rises and dissipates; specific gravity < 1.0
• LP gas (propane): heavier than air, sinks and pools in low areas — critical for safety and installation decisions
• LP gas has higher BTU content per cubic foot than natural gas; orifice sizing differs between the two
• Flammability limits define the range where combustion occurs — mixture too lean (too much air) or too rich (too much fuel) will not sustain combustion

Combustion Fundamentals

• Combustion requires three elements: fuel + oxygen + ignition temperature (combustion triangle)
• Stoichiometric combustion: theoretically perfect air-to-fuel ratio — complete combustion with no excess air
• Complete combustion products: CO₂ + H₂O + heat
• Incomplete combustion produces CO (carbon monoxide) — a deadly hazard
• Excess air improves…

Tennessee CMC-C Mechanical – HVAC/Refrigeration Contractor Exam

Key Concepts

Appliance Venting Categories - Category I: Non-positive vent pressure, flue gas temp high enough to avoid excessive condensation. Uses Type B double-wall vent. Most common residential gas appliances. - Category II: Non-positive pressure, subject to condensation. Requires corrosion-resistant vent. - Category III: Positive vent pressure, not subject to condensation. Requires sealed, pressure-rated vent. - Category IV: Positive pressure, subject to condensation. Requires corrosion-resistant, sealed vent (e.g., PVC for condensing furnaces ≥90% AFUE).

Vent Connector Materials & Clearances - Type B gas vent: minimum 1" clearance to combustibles - Single-wall metal connector: minimum 6" clearance to combustibles - Single-wall connectors must not pass through walls, floors, or ceilings (Type B required for penetrations) - Maximum connector length should not exceed 75% of the vertical height of the chimney or…

Key Concepts

Duct Pressure Classification & Material Selection

• Ducts are classified by static pressure: low pressure (up to 2" WG), medium pressure (2–6" WG), and high pressure (6–10" WG)
• Material selection must match pressure class — sheet metal (galvanized steel, aluminum) is standard; flexible duct is limited in length and must be fully extended (no sagging or compression)
• Ductboard and flexible duct are not permitted in high-pressure applications

Duct Sealing & Leakage Testing

• All duct joints and seams must be sealed with approved mastic, mastic-plus-mesh tape, or UL 181-listed tape — standard cloth duct tape is NOT acceptable
• Duct leakage testing is required for new systems; leakage must not exceed 4% of system airflow for residential applications (per ACCA/ASHRAE standards)
• Pressure testing is typically performed at 25 pascals (0.1" WG) static…

Key Concepts

Design Conditions (HTD / CTD) - Indoor Design Temperature (IDT): Typically 70°F heating, 75°F cooling (Manual J defaults) - Outdoor Design Temperature (ODT): From Manual J Table 1A for the specific location - Heating Temperature Difference (HTD): IDT − ODT (heating season) - Cooling Temperature Difference (CTD): ODT − IDT (cooling season) - Grains Difference: Difference in moisture content (grains/lb) between outdoor and indoor air; drives the latent cooling load for infiltration and ventilation

Fenestration Loads (Windows, Glass Doors, Skylights) - Use HTM values (Heat Transfer Multipliers) from Manual J tables — combines U-value and temperature difference into a single multiplier (BTU/hr·ft²) - Heating HTM = U-value × HTD - Cooling HTM accounts for solar heat gain coefficient (SHGC), orientation, and shading - Overhang adjustments: Reduce cooling HTM when an overhang provides shading; Manual J provides multipliers…

Key Concepts

Lockout/Tagout (LOTO)

• Lockout (LO): Physically lock an energy-isolating device (electrical switch, breaker) in the open/off position using a padlock — technician keeps the key.
• Tagout (TO): Place a warning tag on the device when locking is not possible; must provide equivalent protection.
• OSHA requires lockout devices to be used whenever possible; tagout alone is only acceptable if it offers equivalent employee protection.
• Applies to all energy sources: electrical, hydraulic, pneumatic, mechanical, and thermal.
• Never rely on others to de-energize equipment — the servicing technician must personally lock out the circuit.
• Always assume a circuit is live until you personally shut it off.