Mississippi Residential HVAC

Key Concepts

Duct Insulation R-Value Requirements (Unconditioned Spaces) The IMC and energy codes require minimum duct insulation R-values based on location. For residential systems, ducts in unconditioned attics, crawl spaces, and basements typically require R-6 to R-8 insulation. Ducts in conditioned spaces may require less or none. Mississippi follows IECC climate zone requirements — know that higher R-values apply to ducts in vented attics versus interior unconditioned spaces.

Duct Insulation Materials - Wrap (blanket/batt): Flexible fiberglass or mineral wool applied externally to the duct - Liner: Applied to the interior surface of the duct; reduces noise and provides insulation - Duct board: Rigid fiberglass panels used to fabricate or encase ducts; acts as both structure and insulation - **Spray…

Key Concepts

Duct support requirements are governed by IMC 2021 Section 603 (duct construction and installation). All supports must be sized and spaced to carry the duct weight without sagging or distortion.

Hanger Spacing by Duct Type: - Rectangular ducts: Maximum 10-foot intervals horizontally; supports required within 2 feet of each elbow and within 4 feet of each branch intersection - Round ducts: Maximum 10-foot intervals for diameters ≥ 4 inches - Flat oval ducts: Follow rectangular duct rules based on major dimension

Upper Attachment Methods to Structure: - Concrete inserts – cast-in-place; used for concrete ceilings/slabs - Powder-actuated fasteners – shot into concrete or steel; must be approved for the load - Beam clamps – attach to structural…

Key Concepts

Vibration Isolation (IMC 301.13) When vibration isolation is used for HVAC equipment or appliances, an approved means of supplemental restraint must also be used to provide support and restraint. Isolation alone is not sufficient — you must add supplemental restraint. This is a critical paired requirement.

Seismic Resistance (IMC 301.18) Where earthquake loads apply per the International Building Code, mechanical system supports, anchorage, and bracing must be designed and installed for seismic forces per IBC Chapter 16. Mississippi has limited seismic risk but the code requirement still applies where triggered by IBC.

Key Concepts

Heat Transfer Methods - Conduction: Heat moves through a solid material (e.g., heat flowing through a wall or window glass). Rate depends on material conductivity and temperature difference. - Convection: Heat moves via fluid (air or water) movement. Forced-air and hydronic systems rely on convection to distribute heat. - Radiation: Heat transfers via electromagnetic waves without a medium (e.g., solar gain through windows, radiant floor heat).

Sensible vs. Latent Heat - Sensible heat: Changes dry-bulb temperature — no phase change. Measured with a standard thermometer. - Latent heat: Changes moisture content (humidity) without changing temperature — involves phase change (evaporation/condensation). In HVAC, latent loads come from occupants, infiltration, and appliances. - Per Manual J: Fenestration and opaque…

Key Concepts

Refrigerant Classification Types (ASHRAE Standard 34) - CFC (chlorofluorocarbons): R-11, R-12 — highest ODP, fully phased out under the Montreal Protocol - HCFC (hydrochlorofluorocarbons): R-22 — moderate ODP; production ended January 1, 2020 in the U.S.; only reclaimed/recovered R-22 may be used for servicing existing equipment - HFC (hydrofluorocarbons): R-410A, R-134a — zero ODP, but high GWP; currently dominant but subject to AIM Act phasedown - HFO (hydrofluoroolefins): R-1234yf, R-1234ze — near-zero ODP and very low GWP; emerging replacements - HC (hydrocarbons): R-290 (propane), R-600a (isobutane) — zero ODP, very low GWP, but highly flammable (Class A3) - Inorganic: R-717 (ammonia), R-744 (CO₂), R-718 (water) — ammonia systems governed by IIAR 2/3/4/5, not IMC Chapter 11 (per…

Key Concepts

Design Conditions (HTD / CTD) - Heating Temperature Difference (HTD) = Indoor design temp − Outdoor winter design temp - Cooling Temperature Difference (CTD) = Outdoor summer design temp − Indoor design temp - Manual J defaults: 70°F indoor heating, 75°F indoor cooling, outdoor values pulled from Manual J Table 1A for the specific location - Equipment is sized for design conditions — not extreme or part-load conditions (extreme events occur only a few dozen hours/year)

Thermal Resistance & Transmission Values - R-value: Resistance to heat flow (higher = better insulator); units = °F·ft²·hr/BTU - U-value: Rate of heat transmission; U = 1/R (lower = better insulator) - K-value: Thermal conductivity of a material per inch of…

Key Concepts

Airflow Measurement Instruments - Anemometer: Measures air velocity directly at grilles/registers; used with flow hoods for supply/return CFM readings - Pitot tube + manometer: Measures velocity pressure (VP) in duct traverses to calculate airflow; requires multiple traverse points across the duct cross-section - Flow hood (capture hood): Measures total CFM directly at diffusers/registers — most common field balancing tool - Manometer: Measures static pressure differential; used for duct leakage testing and filter pressure drop

Duct Leakage Testing (Duct Blaster) - CFM25: Duct leakage measured at 25 pascals of pressure — the standard test condition for residential duct leakage - Total leakage = leakage from entire duct system; leakage to outside = only leakage escaping the conditioned envelope…

(4 questions, 5% of exam)

Key Concepts

Thermostat Types & Sensing Elements

• Bimetal strip/coil: Two dissimilar metals bonded together; bends with temperature change. Used in mechanical stats and snap-disc limit switches.
• Thermistor: Semiconductor that changes electrical resistance with temperature. Used in electronic/digital thermostats. Negative temperature coefficient (NTC) thermistors decrease resistance as temperature rises.
• Mercury switch: Glass bulb containing mercury; tilts to make/break circuit. Requires level mounting. Being phased out due to environmental concerns.
• Millivolt system: Thermocouple or thermopile generates 250–750 mV; no external transformer needed. Common on gas fireplaces and standing-pilot wall heaters. The thermostat itself carries the full circuit current.

Key Concepts

Compressor Types

• Reciprocating (piston): Uses pistons; common in older residential systems; can be hermetic or semi-hermetic
• Rotary: Uses a rotating vane; compact, quiet; common in small window units and heat pumps
• Scroll: Two interlocking spiral scrolls; most common in modern residential split systems; more efficient and quieter than reciprocating; no suction/discharge valves needed
• Hermetic: Motor and compressor sealed in a welded housing; not field-serviceable; most residential units
• Semi-hermetic (serviceable hermetic): Bolted housing; motor and compressor can be accessed for field repair; less common in residential

Key Concepts

Gas Furnace Ignition Systems

• Standing pilot: Continuous flame; thermocouple proves pilot and holds gas valve open
• Intermittent pilot (IP): Spark-ignited pilot on each call for heat; no continuous flame
• Direct spark ignition (DSI): Spark ignites main burner directly; no pilot
• Hot surface ignition (HSI): Silicon carbide or silicon nitride igniter glows to ignite main burner; most common in modern furnaces

Gas Furnace Safety Controls

• High-limit switch: Opens on excessive heat (typically 200°F+); shuts off burner to prevent overheating; auto or manual reset
• Rollout switch: Detects flame rollout from heat exchanger; manual reset required — indicates cracked heat exchanger or blocked flue
• Pressure switch: Proves inducer motor operation via…

Key Concepts

Scope (IMC Chapter 8): Chapter 8 governs factory-built chimneys, chimney liners, vents, and connectors for non-gas appliances. Gas-fired appliances fall under the International Fuel Gas Code (IFGC), not IMC Chapter 8.

Appliance Venting Categories: - Category I: Non-positive vent pressure, flue gas temp high enough to avoid excessive condensation (natural draft; most common residential gas furnaces/boilers). Uses Type B vent. - Category II: Non-positive pressure, low temp — condensing likely. Requires corrosion-resistant venting. - Category III: Positive vent pressure, low condensation risk. Must be sealed, listed for positive pressure (IMC §801.9). - Category IV: Positive pressure, condensing. Requires sealed, corrosion-resistant vent systems rated for positive pressure.

Key Concepts

Scope (IMC Chapter 7 / IFGC) - Solid-fuel appliances: follow manufacturer's installation instructions - Oil-fired appliances: follow NFPA 31 - Gas-fired appliances: governed by the International Fuel Gas Code (IFGC), not IMC Chapter 7 directly - Chapter 7 does not apply to fireplaces, fireplace stoves, or direct-vent appliances

Types of Combustion Air - Primary air: mixes with gas before or at the burner; controls flame character and combustion stability - Secondary air: enters the combustion zone around the flame; supports complete combustion - Excess air: air supplied beyond stoichiometric requirements; prevents incomplete combustion but reduces efficiency if excessive - Complete combustion requires the correct ratio of all three; incomplete combustion produces CO (carbon monoxide) — a critical…

Key Concepts

Duct Construction Standards (IMC Chapter 6 / SMACNA) - Rectangular duct gauge selection and reinforcement are based on pressure classification (e.g., ±0.5", ±1.0", ±2.0" w.g.). Higher pressure = heavier gauge + closer reinforcement spacing. - Round and oval ducts: wall thickness and seam type (longitudinal vs. spiral) are determined by diameter and pressure class. Spiral seam ducts generally allow thinner walls than longitudinal seam for the same diameter. - Transverse joints on round duct must be mechanically fastened and sealed.

Duct Sealing (IMC Section 603.9) - All joints, longitudinal seams, and connections in duct systems must be securely fastened and sealed with approved sealant, mastic, or listed tape. - Duct tape (cloth-backed pressure-sensitive) is NOT listed for…

Key Concepts

Natural Ventilation (IMC Section 402) - Openable area must equal at least 4% of the floor area being ventilated (Section 402.2) - Rooms without direct outdoor openings ventilated through an adjoining space require an unobstructed opening of at least 8% of the interior floor area, minimum 25 sq ft (Section 402.3) - Openings below grade require a horizontal clear space of 1.5× the depth of the opening (Section 402.4)

Mechanical Ventilation (IMC Section 403) - Every occupied space must be ventilated by natural or mechanical means (Section 401.2) - Dwelling units meeting IECC or ASHRAE 90.1 air leakage requirements must use mechanical ventilation - Group R-2, R-3, R-4 occupancies may use exhaust-only, supply-only, or combination systems (Section…

(2 Questions | 2% of Exam | Reference: 29 CFR 1926, OSHA Construction)

Key Concepts

Personal Protective Equipment (PPE) — §1926.20(f)(1) - Employer must provide PPE to each employee individually. Every failure to provide PPE to an employee is a separate violation. - PPE selection must match the specific hazard (respiratory, eye, head, hand, foot protection).

Fall Protection — Subpart M - Required at heights of 6 feet or more on construction sites. - Three accepted systems: guardrail systems, safety net systems, and personal fall arrest systems (PFAS). - PFAS components: full-body harness, lanyard, and anchorage point rated to support 5,000 lbs per employee (or designed with 2:1 safety factor by a qualified person).

Key Concepts

Gas Piping Materials

• Black steel/wrought iron pipe – standard for interior gas piping; threaded fittings
• Copper tubing – allowed for natural gas only where not prohibited by local code; NOT for LP gas (propane reacts with copper)
• CSST (Corrugated Stainless Steel Tubing) – flexible, requires bonding/grounding; cannot be used as a drip leg or sediment trap
• Polyethylene (PE) pipe – approved for underground exterior use only; never inside buildings or above ground

Gas Pipe Sizing Methods

• Longest Length Method – measure total length from meter/regulator to the farthest outlet; size all pipe segments from a single table lookup using that length
• Branch Length Method – each segment is sized…

Key Concepts

LP Gas Basics - LP gas (propane/butane) is heavier than air and will settle in low areas — critical for safety and installation decisions - LP gas has a BTU content of approximately 2,500 BTU/ft³ (vs. natural gas at ~1,000 BTU/ft³); this affects orifice sizing and burner conversion - Flammability range: 2.15% to 9.6% in air for propane — outside this range, combustion will not occur - Ignition temperature for LP gas is approximately 920°F–1,020°F

Piping Materials - Approved materials: Schedule 40 black steel pipe, galvanized steel, brass, copper (only where not prohibited), and CSST (Corrugated Stainless Steel Tubing) - Aluminum pipe is not permitted for LP gas systems - Copper tubing is acceptable for LP gas…

Key Concepts

Fuel Oil Grades & Properties

• Grade No. 2 fuel oil is the standard for residential heating — most exam questions default to this grade
• Key properties to know:
• Flash point: minimum 100°F (the temperature at which oil vapors ignite) — critical for safe storage
• Pour point: temperature below which oil will not flow — affects cold-weather operation
• Heat content: approximately 140,000 BTU/gallon for No. 2 fuel oil
• Viscosity: resistance to flow; higher grades (No. 4, 5, 6) are heavier and require preheating

Key Concepts

Gas Piping Materials (IFGC)

• Steel pipe: Black steel (Schedule 40) is the most common gas piping material for interior installations. Galvanized is acceptable but not required.
• Copper: Acceptable for gas only where the gas is not corrosive to copper; prohibited for LP gas in many jurisdictions. Must be Type K or L.
• CSST (Corrugated Stainless Steel Tubing): Flexible gas piping requiring bonding/arc-fault protection — this is a heavily tested point. CSST must be bonded to the electrical grounding system to protect against lightning-induced arc damage. Each manufacturer has specific bonding requirements.
• Polyethylene (PE): Approved for underground/exterior use only; never inside buildings or above ground.