Mississippi Residential HVAC

Key Concepts

Flame spread and smoke-developed indices (IMC 2021, Section 604.3). Duct coverings and linings, including adhesives where used, shall have a flame spread index not more than 25 and a smoke-developed index not more than 50, when tested in accordance with ASTM E84 or UL 723, using the specimen preparation and mounting procedures of ASTM E2231.

Spray polyurethane foam exception (IMC 2021, Section 604.3 Exception). Spray polyurethane foam insulation applied to the exterior of ducts in attics and crawl spaces shall have a flame spread index not greater than 25 and a smoke-developed index not greater than 450. The relaxed limit is on the smoke-developed index only; the flame spread limit is unchanged.

Key Concepts

Scope and basic support rule. "Rigid round, rectangular, and flat oval metal ducts shall be installed with support systems indicated in Tables 5-1 to 5-3 and Figures 5-1 to 5-10. They shall be installed as required to maintain alignment. Horizontal ducts shall have a support within 2 ft (0.61 m) of each elbow and within 4 ft (1.2 m) of each branch intersection. Upper attachments to structures shall have an allowable load not more than one-fourth of the failure (proof test) load but are not limited to the specific methods shown here." (SMACNA HVAC Duct Construction Standards, 4th ed., §5.1.1)

Key Concepts

Code requirements (IMC 2021, Chapter 3) - §301.13 Vibration isolation: "Where vibration isolation of equipment and appliances is employed, an approved means of supplemental restraint shall be used to accomplish the support and restraint." - §301.18 Seismic resistance: Where earthquake loads are applicable in accordance with the International Building Code, mechanical system supports, anchorage and bracing shall be designed and installed for seismic forces in accordance with Chapter 16 of the IBC. - §301.15 Wind resistance: Mechanical equipment, appliances and supports that are exposed to wind shall be designed and installed to resist the wind pressures determined in accordance with the IBC.

Key Concepts

Psychrometrics. On a psychrometric chart, a sensible heat change is shown by horizontal movement — no moisture is added. Wet bulb temperature represents a combination of heat and moisture content (enthalpy and moisture).

Sensible vs. latent loads (Manual J, Section 1-1). Sensible loads affect the dry-bulb temperature in the occupied space; latent loads affect the amount of airborne moisture (wet-bulb temperature and relative humidity). Specifically: - Fenestration and opaque structural panels produce sensible heating and cooling loads. - Infiltration produces sensible and latent heating loads, and sensible and latent cooling loads. - Occupants and appliances produce sensible and latent cooling loads.

Key Concepts

Chemical classification (Ch. 10, Section 10.3.1). Methane-, ethane-, and propane-based refrigerants contain carbon as their main component, making them organic compounds. Ammonia is not classified as an organic compound, so it is categorized as an inorganic refrigerant. Cyclic organic refrigerants have double bonds in the molecule.

ODP (Ch. 10, Section 10.2.2). Ozone depletion potential is a numeric scale with a minimum value of zero, comparing a chemical's destructive potential to R-11, which is 1 on the ODP scale. "CFC refrigerants receive a high ODP rating. HCFC refrigerants possess a low ODP. HFCs, HCs, and inorganic refrigerants have no ODP."

Mississippi Residential HVAC · 6 questions (8% of exam)

Key Concepts

Material property definitions (Modern Refrigeration & A/C, 22nd ed., Ch. 32, p. 717–718) - K-value (thermal conductivity): heat passing through one square foot of material one inch thick, in one hour, with a 1°F difference between the two sides. Units: Btu·in/hr·ft²·°F. - C-value (thermal conductance): same idea, but based on the material's actual measured thickness, not a standardized 1 inch. - U-value (thermal transmittance): like C-value but includes an adjustment for boundary air films on the inside and outside surfaces. U-values assume a 15-mph wind outside and a 15-fps draft inside, making them generally more accurate than K- or C-values. U = 1 ÷ total R (including both…

Key Concepts

Anemometers (Ch. 28 — Air Movement and Measurement, p. 627; 20th ed. p. 707) - A hot-wire anemometer has a small wire element often made of tungsten. Current is passed through the element to heat it; as the element's temperature increases, so does its resistance. Airflow passing over the element cools the wire and reduces its resistance. By measuring the current draw through the element, the instrument calculates air velocity. - A vane anemometer is placed with the propeller in the airstream at a right angle to the airflow, allowed to reach a constant speed (after about one minute), then the registering mechanism is tripped. - A swinging-vane anemometer is entirely mechanical (no spark risk), but is…

Key Concepts

Cut-in, cut-out, range, differential - Cut-in is the condition value at which a system begins operation; cut-out is the value at which it ceases operation (22nd ed., Ch. 17, Sec. 17.2; 20th ed. Ch. 16, Sec. 16.2). - Range adjustment raises or lowers both the cut-in and cut-out values by an equal amount; the differential does not change. On a low-pressure motor control, turning the screw in (clockwise) raises both; turning it out (counterclockwise) lowers both (22nd ed., p. 333, Sec. 17.4.4; 20th ed. Ch. 16, Sec. 16.3.4). - Differential adjustment on a low-pressure motor control affects only the cut-out. Clockwise (in) raises the cut-out by increasing spring tension; counterclockwise lowers it. The spring has no effect…

Key Concepts

Compressors - Open-drive (external-drive) compressors require a shaft seal where the shaft passes through the case; refrigerant cannot leak out and air cannot leak in. Hermetic designs eliminate this concern (Modern Refrigeration and Air Conditioning, 22nd ed., Ch. 43, p. 1020). Open-drive units are joined to the motor by direct drive (coupling, same speed as motor) or belt drive (Ch. 43, pp. 1020–1021). - Scroll: vapor pockets shrink as they move toward the center; the high-pressure vapor discharges out of the center port (Ch. 9, p. 141). Capacity modulation may be by a bypass port partway between the outer edge and center of the fixed scroll that returns partially compressed vapor to the compressor inlet, by a…

Key Concepts

Combustion and flame appearance (gas) - A yellow flame tip indicates insufficient primary air and incomplete combustion — unburned carbon particles produce the yellow color. Orange outer edges come from dust drawn in with primary air and are not a sign of lack of primary air. Too much primary air makes the flame noisy and lift above the burner. (Modern Refrigeration and AC, 22nd ed., Ch. 33, Page 758, Section 33.2.2) - Ultimate CO₂ is about 11.9% for natural gas and 13.9% for LP gas, but with excess air dilution, flue CO₂ should be near 7.6% (natural gas) and 8.9% (LP). (Ch. 33, Page 759) - 1% decrease in efficiency for every 40°F increase in stack temperature;…

Key Concepts

Scope split. IFGC Chapter 5 governs venting of gas-fired appliances; appliances burning other fuels are regulated by the IMC (IFGC 501.1; IMC 801.1). Every gas appliance must discharge products of combustion outdoors except those exempted by IFGC 501.8.

Appliances not required to be vented (IFGC 501.8) include ranges, Type 1 clothes dryers, refrigerators, counter appliances, room heaters listed for unvented use, direct-fired makeup air heaters, other appliances listed for unvented use without flue collars, and specialized limited-input appliances. Where appliances in Items 5 through 11 are installed so the aggregate input exceeds 20 Btu per hour per cubic foot of room volume, one or more must be vented so the remaining unvented aggregate does not exceed 20…

Key Concepts

Scope and which code applies. Combustion and dilution air for gas-fired appliances is governed by IFGC Section 304; appliances operating with fuels other than fuel gas are regulated by the International Mechanical Code (2018 IFGC, Section 301.1.1). IMC Chapter 7 confirms this: solid fuel-burning appliances follow the manufacturer's installation instructions, oil-fired appliances follow NFPA 31, and gas-fired appliances follow the IFGC (IMC 2021, Section 701.1).

General requirement. Air for combustion, ventilation and dilution shall be provided by one of the methods in Sections 304.5 through 304.9. Where Section 304.5 is not met, outdoor air shall be introduced per Sections 304.6 through 304.9. Direct-vent appliances, gas appliances other than natural draft design, vented gas appliances not designated Category…

Key Concepts

Pressure class & sealing (SMACNA HVAC-DCS, 4th ed.) - When the designer does not designate a pressure class, the basis of compliance is 2 in. wg (500 Pa) for all ducts between the supply fan and variable volume control boxes, and 1 in. wg (250 Pa) for all other ducts (Ch. 1, Sec. 1.4.1). - Seal Class B = all transverse joints and longitudinal seams only; Seal Class A adds duct wall penetrations; Seal Class C covers transverse joints only (Table 1-1). - NFPA Standard 90A requires adhesives to have a flame spread rating not over 25 and a smoke developed rating not over 50 (Sec. 1.4.14). - Seal ductwork with metal tape or mastic sealant; never…

Key Concepts

Exhaust discharge locations (IMC 2021, Section 501.3.1) - For all environmental air exhaust: 3 feet from property lines; 3 feet from operable openings into buildings for all occupancies other than Group U; and not less than 10 feet from mechanical air intakes. Such exhaust is not considered hazardous or noxious (Item 3). - For other product-conveying outlets: 10 feet from property lines; 3 feet from exterior walls and roofs; 10 feet from operable openings into buildings; 10 feet above adjoining grade (Item 2). - For ducts conveying explosive or flammable vapors, fumes or dusts: 30 feet from property lines; 10 feet from operable openings into buildings; 6 feet from exterior walls and roofs; 30 feet from combustible…

Topic: Safety

Key Concepts

Fire protection on construction sites (OSHA 1926.150) - A fire extinguisher rated not less than 2A shall be provided for each 3,000 square feet of protected building area, or major fraction thereof; travel distance from any point of the protected area to the nearest extinguisher shall not exceed 100 feet — 1926.150(c)(1)(i). - One or more extinguishers rated not less than 2A shall be provided on each floor; in multistory buildings at least one shall be located adjacent to a stairway — 1926.150(c)(1)(iv). - An extinguisher rated not less than 10B shall be provided within 50 feet of wherever more than 5 gallons of flammable or combustible liquids or 5 pounds of flammable gas…

Key Concepts

Scope. The code covers piping systems for natural gas at 125 psig or less and LP-gas at 20 psig or less (101.2.2), extending from the point of delivery to the outlet of the appliance shutoff valves.

Materials (403). Cast-iron pipe shall not be used (403.4.1). Steel, stainless steel and wrought-iron pipe shall be not lighter than Schedule 10, meeting ASME B36.10M and ASTM A53/A53M, A106 or A312 (403.4.2). Copper tubing shall comply with Type K or L of ASTM B88 or ASTM B280; copper and copper alloy shall not be used where the gas contains more than an average of 0.3 grains of hydrogen sulfide per 100 standard cubic feet (403.5.3, 403.4.3). CSST shall be listed to…

Key Concepts

Scope and LP storage - LP-gas storage systems shall be designed and installed in accordance with the International Fire Code and NFPA 58 (Section 401.2). Chapter 4 governs piping from the point of delivery to the appliance connections (Section 401.1). - LP-gas systems designed to operate below -5°F (-21°C), or with butane or a propane-butane mix, shall be designed to either accommodate liquid LP-gas or prevent LP-gas vapor from condensing into a liquid (Section 402.7.1).

Materials and joints - Cast-iron pipe shall not be used (403.4.1). Steel, stainless steel and wrought-iron pipe shall be not lighter than Schedule 10 (403.4.2). Aluminum-alloy pipe and tubing shall not be used in exterior locations or underground (403.4.4, 403.5.4). - Copper…

Key Concepts

Fuel oil grades and properties (Residential Oil Burner Reference, Ch. 2, Table 2-1) - Minimum flash point for #2 fuel oil is 100°F; #1 is 100°F, #4 is 130°F, #6 is 140°F. Flash point is the temperature at which vapors escaping the oil surface can momentarily be ignited by an external flame. - Fire point is approximately 20 degrees higher than the flash point. - Pour point of #2 oil is approximately 20°F (#1 kerosene ≈ 0°F). At the pour point wax crystals form and clog filters and oil lines. - API degree range for #2 fuel oil is 30 to 38 degrees. As API degrees decrease, viscosity, flash point and heat content increase; as API degrees…

Key Concepts

Gas piping materials (IFGC Section 403) - Cast-iron pipe shall not be used (403.4.1). - Steel, stainless steel and wrought-iron pipe shall be not lighter than Schedule 10 and shall comply with the dimensional standards of ASME B36.10M and one of: ASTM A53/A53M, ASTM A106, ASTM A312 (403.4.2). - Copper and copper alloy pipe shall not be used if the gas contains more than an average of 0.3 grains of hydrogen sulfide per 100 standard cubic feet (0.7 mg per 100 L) (403.4.3). The same limit applies to copper/copper-alloy tubing (403.5.3). - Copper tubing shall comply with Standard Type K or L of ASTM B88 or ASTM B280 (403.5.3). - CSST shall be listed to ANSI LC…