Mississippi HVAC

Key Concepts

Tubing, bending and brazing (Modern Refrigeration and Air Conditioning, 22nd ed., Ch. 5) - Steel pipe is not generally used to circulate refrigerant; the major exception is ammonia refrigeration systems, because ammonia may chemically react with copper tubing. Steel pipe is primarily used for gas lines or water lines (p. 74; 20th ed. p. 142). - Minimum radius for a 90° tubing bend is five times the diameter of the tubing (1/2″ tubing → 2-1/2″ minimum radius) (p. 76; 20th ed. p. 145). - All air must be removed from tubing being brazed by purging with nitrogen or carbon dioxide; nitrogen is set to flow through the inside of the brazing joint at low pressure, **1 psi…

Mississippi HVAC exam · 1 question (1% of exam)

Key Concepts

Duct insulation R-value from installed thickness (IMC 2021, Section 604.7, Item 2, Page 6-6) - For duct wrap, installed thickness is assumed to be 75 percent of nominal thickness (25 percent compression) when determining R-value. - Duct board, duct liner and factory-made rigid ducts use the full nominal thickness. - IECC principle (Manual D, A10-2): "duct insulation thermal performance shall be determined by the installed wall thickness divided by the thermal conductivity of the insulation at that installed thickness."

Key Concepts

Basic requirement (SMACNA Ch. 5, Section 5.1.1). Rigid round, rectangular and flat oval metal ducts shall be installed with the support systems indicated in Tables 5-1 to 5-3 and Figures 5-1 to 5-10, and 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.

Exam weight: 1 question, 1% — but the supporting duct-system rules in IMC Chapter 6 overlap several other topics.

Key Concepts

Noise sources and paths (ACCA Manual D, Appendix 13, p. 185). Noise is produced by the blower and can be generated by air-side components, dampers, fittings, supply outlets and return grilles. Noise is generated when airflow velocities are too high, and by equipment vibrations. It propagates through the duct system, is transmitted through or radiated from duct walls, and travels through structural components.

Control methods (A13, p. 185). Noise is controlled by observing air velocity limits, using aerodynamic fittings, using duct liner or duct board, avoiding line-of-sight connections between a noise source and a supply outlet or return…

Key Concepts

Heat transfer and heat loads. Heat always flows from hot to cold. Heat loss is the transfer of heat from inside to outside by means of conduction, convection, and radiation through all surfaces of a house; heat gain is the increase of heat within a space from solar radiation and from sources such as lights, equipment, or people (Section 32.1.1). Every heat load is either a sensible heat load (temperature change) or a latent heat load (moisture evaporating or condensing) (Section 32.1.2).

High side vs. low side. The evaporator operates at low-side pressure — low-pressure liquid refrigerant boils there as it absorbs heat, and the compressor draws that low-pressure vapor in. The discharge line, condenser, and liquid…

Study brief (6 questions, 8% of exam)

Key Concepts

Checking charge (Ch. 12, Section 12.1) - The subcooling method is used on systems with a thermostatic expansion valve; the superheat method is used on systems with fixed orifice devices such as capillary tubes (Section 12.1.1, p. 221). - Subcooling procedure: run the system 10–15 minutes with proper airflow, take a head (high-side) pressure reading, convert it with a P/T chart, measure liquid line temperature near the metering device, then subtract (Section 12.1.1, pp. 221–223). - Superheat procedure requires four measurements: ambient dry-bulb at the condenser, return-air wet-bulb just before the evaporator, evaporator refrigerant pressure, and suction line service valve temperature; actual superheat is compared to a target superheat from…

Key Concepts

• ACCA publications: Manual J — Residential Load Calculation; Manual N — Commercial Load Calculation (Modern Refrigeration and Air Conditioning, 22nd ed., Ch. 32, p. 715).
• K-value (thermal conductivity): heat through one square foot of material one inch thick, in one hour, per 1°F difference; units Btu·in/hr·ft²·°F (MRAC Ch. 32, p. 717).
• C-value (thermal conductance): same basis but uses the material's actual measured thickness, with no boundary-air-film adjustment (MRAC pp. 717–718).
• U-value (thermal transmittance): like C-value but includes boundary air films on inside and outside surfaces; assumes a 15-mph wind outside and a 15-fps (1/6-mph) draft inside, making it generally more accurate than K or C (MRAC pp. 718–719).
• **Outdoor design temperatures (ASHRAE,…

Key Concepts

• Duct leakage test pressure. After all registers and grilles have been sealed, the ducts are pressurized to +25 Pa WRT the conditioned space; the airflow needed to maintain that pressure is called cfm25. (Modern Refrigeration and Air Conditioning, 22nd ed., Ch. 31 — Ventilation System Service, p. 703)
• Maximum duct leakage (Code Alert, Ch. 31). The IECC specifies 4 cfm per 100 ft² of conditioned space when ducts are pressurized to 25 Pa. If the ductwork is tested before the air handler is installed, the maximum is 3 cfm per 100 ft². These are two separate limits for two different cases. Duct testing is not required when the air handler and ductwork are contained within…

Mississippi HVAC · 3 questions (4% of exam)

Key Concepts

Control system fundamentals (cut-in, cut-out, range, differential) - Differential is the number of units between the cut-out value and the cut-in value; always subtract the lower value from the higher (Ch. 17, §17.2.2, Page 325). - Range adjustment lowers both the cut-in and the cut-out an equal distance when the screw is turned out (counterclockwise) and raises both when turned in (clockwise). Differential adjustment affects only the cut-out: turning the screw in (clockwise) increases spring tension and raises the cut-out; turning it counterclockwise lowers the cut-out. The spring has no effect on the cut-in setting (§17.4.4, Page 333). - Increasing a control's differential leads to a longer run time…

Key Concepts

High side / low side dividers. The compressor and the metering device are the two components separating the low side from the high side. The compressor draws low-pressure vapor from the evaporator and pushes high-pressure vapor into the high side; the metering device divides the high side from the low side at the other end. (22nd ed., Ch. 8, p. 128; 20th ed. pp. 91–93)

Comfort cooling temperatures. Most comfort cooling systems have a refrigerant evaporating temperature of 40°F to 50°F (4°C to 10°C) and maintain the high-temperature refrigeration range of 55°F (13°C) to 90°F (32°C). (22nd ed., Ch. 22, p. 456)

Key Concepts

Combustion and flame appearance (Ch. 33, Section 33.2.2, p. 758) - A proper gas flame has a blue inner core surrounded by light blue. A yellow flame tip = insufficient primary air, i.e., unburned carbon particles from incomplete combustion. - Orange outer edges come from dust particles drawn in with primary air — not a sign of lack of primary air. - Too much primary air makes the flame noisy and jump around above the burner. - Primary air is mixed with fuel before ignition; secondary air surrounds the flame's outer area.

Key Concepts

Venting required; exempt appliances. Every appliance shall discharge the products of combustion to the outdoors, except for appliances exempted by IFGC Section 501.8 (IFGC 501.2). Section 501.8 lists ranges, built-in cooking units listed for optional venting, hot plates and laundry stoves, Type 1 clothes dryers, a single booster-type instantaneous water heater (as described), 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 appliances of limited input. Where appliances listed in Items 5 through 11 are installed so the aggregate input rating exceeds 20 Btu per hour per cubic foot (207 W per m³) of room volume, one or more shall be provided…

Mississippi HVAC Exam · 2 questions (2% of exam)

Key Concepts

Combustion basics. Combustion is rapid oxidation in which fuel and oxygen produce heat and light. The three basic elements necessary are fuel, heat, and oxygen (MRAC 22nd ed., Ch. 33, Figure 33-11). Stoichiometric (perfect) combustion: CH₄ + 2 O₂ = CO₂ + 2 H₂O + energy — an oxygen-to-methane ratio of 2:1. Because air is only about 21% oxygen and about 78% nitrogen, the air-to-methane ratio supplied to a burner must be roughly 10:1 (Ch. 33, Combustion Air section).

Key Concepts

Scope and pressure limits - The code covers piping for natural gas at an operating pressure of 125 psig or less and LP-gas at 20 psig or less, except as provided in Section 402.7 (IFGC 2021, 101.2.2). - Temporary LP-gas piping for buildings under construction or renovation that will not become part of the permanent system is outside the scope of the code (101.2.4, item 13). - Maximum operating pressure inside buildings shall not exceed 5 psig, except where one of the eight listed conditions is met (welded/brazed joints, listed ANSI LC-4/CSA 6.32 fittings, ventilated chase, certain occupancy uses, temporary construction piping, agricultural use, NFPA 58 LP-gas over 20 psi) (402.7).

5 questions · 6% of exam

Key Concepts

Operating pressure limits - Maximum operating pressure for piping systems inside buildings: 5 psig, except where a listed condition is met — welded or brazed joints, fittings listed to ANSI LC-4/CSA 6.32, flanged connections made by welding/brazing, ventilated chase, spaces used exclusively for industrial processing or heating, research, warehousing, or boiler/mechanical rooms, temporary construction piping, agricultural piping, or an LP-gas system operating above 20 psi that complies with NFPA 58 (IFGC 402.7). - Plastic pipe shall be installed outdoors underground only, not within or under any building or slab, and shall not be operated above 100 psig for natural gas or 30 psig for LP-gas (IFGC 404.17.1). Plastic pipe in undiluted…

Key Concepts

Manual D sizing chain. Pressures are expressed in inches water column because 27.7 IWC = 1.0 psi (and 1.0 IWC = 249 Pa) — Manual D Section 1-1. Available static pressure = blower-table external static pressure minus the pressure drops of air-side components in the critical path (Sections 1-10/1-11). Duct sizes are based on the net pressure available, not the blower's total external pressure.

Device pressure losses. Manual D Section 6-5 (Steps 2 and 3) gives a default of 0.03 IWC each for a supply outlet, a return grille, and an open balancing damper. Components already discounted in the blower table (e.g., a wet coil and standard filter noted in the footnotes) are not subtracted again. MRAC…

Key Concepts

Conventional vs. condensing boilers. Conventional boilers operate at water temperatures above 140°F (60°C). If water drops below 140°F (60°C), it absorbs so much heat that combustion gases condense back to liquid and become highly corrosive, reducing boiler life and damaging the flue. A system with a conventional (noncondensing) boiler must include thermostatic controls keeping return water above 140°F (60°C). Condensing boilers are built to condense combustion gases and drain the condensate, use secondary heat exchangers primarily made of stainless steel, are over 90% efficient, may be gas- or oil-fired, and can vent through a wall, roof, or unused chimney. (MRAC 22nd ed., Ch. 38, Page 890; 20th ed. Page 1036)

Mississippi HVAC | Topic weight: 1 question (~1%)

Key Concepts

Machinery room ventilation (IMC 2021 Section 1105.6) - Machinery rooms shall be mechanically ventilated to the outdoors (1105.6). The exception applies only where a refrigerating system is located outdoors more than 20 feet from any building opening and is enclosed by a penthouse, lean-to or other open structure; then natural or mechanical ventilation is allowed, sized by Equation 11-1. - Exhaust shall discharge to the outdoors not less than 20 feet from a property line or openings into buildings (1105.6.1). - Indoor exhaust intake openings go where leakage is likely to concentrate, based on the refrigerant's relative density to air (1105.6.1.1). - Makeup air must be provided; supply and…

Key Concepts

Evaporative cooling principle. When warm, dry air moves rapidly over water at the same temperature, some water evaporates; the evaporating molecules absorb heat from the surrounding molecules, dropping the temperature of the remaining liquid. Used this way to cool a space or objects, it is called evaporative cooling. It is effective and practical in dry climates (Modern Refrigeration and Air Conditioning, 22nd ed., Section 27.4, Page 604).

Direct evaporative cooler. Outside air filters through a water-saturated evaporative medium (wetted aspen pads), is cooled by evaporation, and is circulated by a blower; supply air is 15°F to 30°F (8°C to 17°C) cooler than outdoor air (Figure 27-17, Page 604).

Key Concepts

Fire protection — extinguisher coverage (Subpart F). A fire extinguisher rated not less than 2A shall be provided for each 3,000 square feet of the protected building area, or major fraction thereof, and 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 adjacent to a stairway — §1926.150(c)(1)(iv). Substitutes: one 55-gallon open drum of water with two fire pails — §1926.150(c)(1)(ii); or a ½-inch diameter garden-type hose line not to exceed 100 feet, with a nozzle, discharging a minimum of 5 gallons per…