Exam Cheat Sheet · Quick Reference
Mississippi HVAC
Mississippi · PSI Services Contractor
Verified, not estimated. Every figure below is drawn from the official exam structure we maintain — question counts, passing standard and topic weighting. Practice questions are grounded in the source law with statute citations. We omit any figure we can't verify rather than guess at it.
Total questions
80
Passing score
70%
Exam time
240 min
Administered by
PSI Services Contractor
Format
Reference materials allowed
Mississippi State Portion 80 questions
Piping
4 Q · 5%
Gas piping materials, specifications, and approved pipe types (steel, copper, CSST, PE, polyamide)Gas piping sizing methods — longest length, branch length, and hybrid pressure approachesAllowable pressure drop and maximum operating pressure for gas piping systemsPipe joint types and connection methods (threaded, brazed, welded, press-connect, flared)Hydronic piping materials, fittings, and joint standards (IMC Chapter 12)LP-Gas piping service limitations, pressure ratings, and installation requirements (NFPA 58)Piping support, protection, and spacing requirementsFuel oil piping materials, joints, and system installation requirements (IMC Chapter 13)Full Text
Insulation
1 Q · 1%
R-values of common building materials (batt, board, loose fill, spray-on insulation types)Duct wall insulation R-value requirements and correction factors (R-2 through R-8)Effect of insulation R-value on heating and sensible cooling duct loadsOpaque panel U-values and HTM calculation for walls, ceilings, roofs, and floorsVapor retarders for externally insulated ducts in humid climatesDuct insulation fire and smoke ratings (flame spread and smoke-developed index requirements per IMC Section 604)Double-wall duct insulation construction and liner installation standardsCompression correction factors for batt and blanket insulationChapter 6 Duct Systems
Hangers and Supports
3 Q · 4%
Hanger spacing requirements for horizontal ducts (maximum intervals and proximity to elbows and branch intersections)Minimum hanger sizes for rectangular duct by half-perimeter and support spacingMinimum hanger sizes for round duct by diameter and support spacingUpper attachment methods and allowable load limits (safety factor of 4 relative to failure load)Lower attachment methods for connecting hangers to ductTrapeze hanger selection and load calculationRiser support methods and spacing intervals for vertical ductsEffects of flexible duct sag and compression on pressure drop and equivalent lengthChapter 6 Duct Systems
Sound, Vibration and Seismic Control
1 Q · 1%
Noise sources in HVAC systems (blower, fittings, dampers, outlets)Air velocity limits for noise control in duct runs and outletsDuct geometry design to acoustically decouple noise sources from occupied spacesVibration isolation methods for equipment and ductworkCrosstalk prevention through duct layout and line-of-sight avoidanceNoise attenuation using duct liner, duct board, and sound trapsNoise Criteria (NC) values for residential and multi-family occupanciesVibration isolation connectors and flexible duct connections per IMC Chapter 6Chapter 6 Duct Systems
Heating and Cooling Principles
3 Q · 4%
Heat transfer mechanisms (conduction, convection, radiation)Sensible heat and latent heat distinctionsCompression refrigeration cycle components and operationThermal resistance values (K-value, C-value, R-value, U-value)Indoor and outdoor design temperatures for load calculationsManual J residential and Manual N commercial load calculation methodsHeat pump heating and cooling modes and efficiency ratingsRefrigerant classification, environmental impact, and pressure-temperature relationshipsHeating and Cooling Loads
Refrigerants and Refrigeration
6 Q · 8%
Refrigerant classification by chemical type (CFC, HCFC, HFC, HFO, HC)Environmental regulations and refrigerant phasedown (Montreal Protocol, Clean Air Act, AIM Act, Kigali Amendment)Refrigerant identification, numbering system, and safety classifications (ASHRAE 34, ODP, GWP)Refrigerant cylinders: types, color coding, DOT regulations, and safe handlingRefrigerant recovery, recycling, and reclamation procedures and equipmentSystem evacuation procedures and vacuum measurementChecking refrigerant charge by superheat and subcooling methodsIMC Chapter 11 refrigeration system classification, occupancy requirements, and machinery room provisionsCommercial Metering Devices and Valves
Load Calculations
10 Q · 12%
Heating and cooling design temperature selection (indoor and outdoor design conditions)Thermal resistance and transmittance values (R-value, U-value, K-value, C-value) for building componentsFenestration heating and cooling load calculations using HTM valuesOpaque panel heating and cooling load calculations (walls, ceilings, floors, doors)Infiltration load estimation methods (ACH method, component leakage area method)Duct system load calculations (heat loss, sensible and latent gain factors, leakage and insulation effects)Internal loads (occupants, appliances, lighting) for cooling load estimatesEngineered ventilation sensible and latent load calculationsHeating and Cooling Loads
Testing, Adjusting and Balancing
1 Q · 1%
Air-side balancing procedures and damper adjustmentAirflow measurement instruments and methods (flow hood, anemometer, pitot tube, velometer)Blower door testing for building envelope airtightness and duct leakageDuct leakage testing using duct blaster pressurizationSystem operating point determination using blower curves and system resistance curvesAvailable static pressure calculation and friction rate design valuesAir velocity limits for ducts, grilles, registers, and diffusersDuct system noise problems and draft identification and correctionDuct Sizing Calculations
Controls
3 Q · 4%
Thermostat types, sensing elements, and operational principlesMotor starting relays (current, potential, PTC, and solid-state)Pressure motor controls (high-pressure, low-pressure, oil pressure, and safety controls)Control system fundamentals (range adjustment, differential adjustment, cut-in and cut-out settings)Direct digital control (DDC) systems and building automation systems (BAS)Zoned system controls (dampers, zone control panels, bypass dampers, and freezestats)Heat pump controls (reversing valve operation, defrost controls, and thermostat wiring terminals)Electronic expansion valves (EEVs) and solenoid valve control applicationsCommercial Metering Devices and Valves
A/C and Heat Pump Equipment
4 Q · 5%
Split system configurations (central, mini-split, multi-split, VRF)Heat pump operating principles and reversing valve operationCompressor types and protection devices (reciprocating, rotary, scroll)Metering device types and selection (capillary tube, fixed orifice, TXV, biflow valves)Packaged and self-contained equipment types (room AC, PTAC, PTHP, rooftop units)Geothermal and water-source heat pump system types and componentsHeat pump efficiency ratings and defrost controls (SEER2, HSPF2, reverse cycle defrost, auxiliary heat)Commercial system types (chillers, VRF, cooling towers, compressor capacity control)Residential Split Air-Conditioning Systems
Furnaces and Heaters
4 Q · 5%
Gas furnace sequence of operation and heat exchanger types (primary vs. secondary, condensing vs. noncondensing)Gas burner types and ignition systems (atmospheric, power, standing-pilot, intermittent-pilot, DSI, HSI)Gas furnace safety and operational controls (high-limit switch, rollout switch, pressure switch, ignition control module lockout functions)Combustion principles and efficiency testing (CO2, O2, CO content, stack temperature, gas pressure measurement)Oil furnace components and operation (gun burner, fuel unit, primary control unit, cad cell relay, stack relay)Electric heating systems and controls (resistance elements, sequencers, duct heaters, baseboard units, safety cutoffs)Heat pump heating operation, defrost controls, and auxiliary heatVenting categories, vent types, and installation requirements for gas-fired appliances (Type B, direct-vent, Category I–IV, chimney sizing)Gas-Fired Heating Systems
Chimneys, Flues and Vents
3 Q · 4%
Vent system types and appliance matching (Type B, BW, L, single-wall metal pipe, special gas vent)Chimney and vent termination height and location requirementsSizing of Category I venting systems using code tables (single and multiple appliances)Vent connector installation requirements (slope, length, clearances, materials, joints)Masonry chimney requirements (lining, cleanouts, inspection, reuse with gas appliances)Factory-built chimney listing, installation, and support requirementsDraft hoods, draft controls, and mechanical draft systemsDirect-vent and Category II/III/IV appliance venting requirementsGas-Fired Heating Systems
Combustion Air
2 Q · 2%
Indoor combustion air requirements and volume calculationsOutdoor combustion air methods and opening sizingCombustion air duct requirements and installationPrimary air, secondary air, and excess air in gas burnersMakeup air units and negative pressure preventionDampered openings and motorized louver interlocksMechanical combustion air supply systems and appliance interlocksProtection from corrosive or flammable fumes in combustion air supplyGas-Fired Heating Systems
Fuel Gas Systems
9 Q · 11%
Gas piping materials, fittings, and allowable pipe types (steel, copper, CSST, PE)Gas pipe sizing methods and capacity tables (longest length, branch length, hybrid pressure)Gas piping pressure requirements and maximum operating pressure limitsGas piping installation requirements (support, identification, testing, purging, and leak detection)Venting system types and appliance category matching (Type B, BW, L, direct-vent, single-wall metal)Chimney and vent sizing, termination heights, and clearance requirementsLP-Gas container types, installation, separation distances, and location requirements (NFPA 58)Combustion air requirements for gas-fired appliances (indoor, outdoor, and combined methods)GAS PIPING INSTALLATIONS
LP Gas Systems
5 Q · 6%
Container location and separation distance requirementsPressure regulator types, selection, and installationLP-Gas piping materials, joints, and installation requirementsPiping system sizing methods and tables for undiluted propaneContainer appurtenances including pressure relief valves, excess-flow valves, and liquid level gaugesLP-Gas transfer operations and safety proceduresCylinder storage requirements inside and outside buildingsCorrosion protection for underground and aboveground LP-Gas systemsGAS PIPING INSTALLATIONS
Ducts
10 Q · 12%
Duct sizing calculations using ACCA Manual D (friction rate, available static pressure, total effective length)Rectangular duct construction standards (gauge selection, reinforcement schedules, pressure classification)Round, oval, and flexible duct construction standards (gauge, seams, joints, negative pressure requirements)Duct leakage, sealing requirements, and testing methodsDuct insulation requirements and efficiency (R-values, conduction losses, vapor retarders)Duct system types and layout (radial, extended plenum, reducing trunk, trunk-and-branch)Duct hangers, supports, and installation requirementsFitting equivalent lengths and their effect on duct system designRectangular Duct Construction
Boilers and Hydronics
4 Q · 5%
Boiler types and classifications (conventional vs. condensing, cast-iron vs. steel vs. copper-tube, wet-base vs. dry-base)Boiler safety devices and controls (pressure-relief valves, low-water cutoff, aquastat, high-limit control, flow switch)Hydronic system piping configurations (series loop, one-pipe, two-pipe direct return, two-pipe reverse return, zoned systems)Expansion tanks and air-removal components (bladder/diaphragm tanks, air scoops, air separators, air vents)Hydronic system valves (mixing valves, balancing valves, pressure-reducing valves, backflow preventers, flow-control valves, zone valves)Hydronic piping materials, joints, and installation requirements per IMC Chapter 12Radiant hydronic systems (wet and dry underfloor installation methods, PEX tubing, manifolds, embedded piping code requirements)Boiler water treatment, system start-up, balancing, and troubleshooting (water quality, glycol, cavitation, circulation problems)Full Text
Machine Room
1 Q · 1%
Machinery room design and construction requirements (IMC Section 1105)Machinery room ventilation requirements: normal and emergency ratesRefrigerant detectors and emergency pressure control systemsFuel-burning appliances and open flame restrictions in machinery roomsMeans of egress requirements for machinery roomsSpecial requirements for flammable refrigerant machinery rooms (Groups A2, A3, B2, B3)Remote control and emergency shutoff systems for machinery room equipmentLP-Gas facility construction, ventilation, and heating requirements (NFPA 58 Chapter 10)Chapter 11 Refrigeration
Evaporative Cooling
2 Q · 2%
Basic principles of evaporative cooling and psychrometrics (dry-bulb, wet-bulb, relative humidity relationships)Direct evaporative cooler components and operation (wetted media/excelsior pads, blower, water supply, pump)Wet roof cooling methods (roof ponds and roof mist cooling systems)Indoor/greenhouse evaporative cooling applicationsIMC Section 928 installation requirements for evaporative cooling equipment (level platform, water supply, wall/roof flashing, air intake location)Cooling towers and evaporative condensers installation requirements (location, drift eliminators, drainage, water supply per IMC Section 908)Climate suitability and limitations of evaporative cooling (hot, dry vs. humid climates)Absorption and Evaporative Cooling Systems
Safety
4 Q · 5%
Lockout/tagout (LOTO) procedures and requirementsPersonal protective equipment (PPE) selection and useElectrical hazards and GFCI protection on construction sitesFire hazards, fire extinguisher classes, and PASS techniqueFall protection systems (guardrails, personal fall arrest, safety nets)Ladder and stairway safety requirementsRefrigerant and pressure vessel hazardsHazard communication and Safety Data Sheets (SDS)Subpart D-Occupational Health and Environmental Controls
Key Distinctions
K-value (thermal conductivity)vsC-value (thermal conductance)
K-value is the heat passing through one square foot of material one inch thick per hour per 1°F difference (Btu·in/hr·ft²·°F), while C-value is based on the material's actual measured thickness rather than a standardized 1-inch thickness.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 32 - Heating and Cooling Loads, Page 717 (20th ed.: Page 986)C-valuevsU-value
C-value is heat flow per square foot per hour per 1°F for the component's measured thickness with no adjustment for boundary air films, while U-value is similar but adds the boundary air films.
Modern Refrigeration and Air Conditioning (22nd ed.), Pages 717 to 718K-values and C-values (lab-measured)vsU-value (field-assumed air motion)
K- and C-values are measured in a laboratory with perfectly still air, while the U-value assumes a 15-mph wind on the outside surface and a 15-fps (1/6-mph) draft on the inside surface, making U-values generally more accurate.
Modern Refrigeration and Air Conditioning (22nd ed.), Page 718Category I gas furnacevsCategory II gas furnace
Category I is negative-pressure/noncondensing and uses Class B vent, while Category II is negative-pressure/condensing and requires special vent material specified by the manufacturer.
Modern Refrigeration and Air Conditioning (22nd ed.), Page 776Category III gas furnacevsCategory IV gas furnace
Category III is positive-pressure/noncondensing and is typically vented with stainless steel pipe, while Category IV is positive-pressure/condensing (flue gas less than 140°F, below its dew point) and uses PVC vent piping.
Modern Refrigeration and Air Conditioning (22nd ed.), Page 776Subcooling charging methodvsSuperheat charging method
The subcooling method is generally used to check charge on systems with thermostatic expansion valves, while the superheat method is used on systems with fixed orifice metering devices such as capillary tubes.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 12 - Working with Refrigerants, Page 221 (20th ed.: Page 236)Shell-and-coil water-cooled condenservsShell-and-tube water-cooled condenser
The coiled water tubing of a shell-and-coil condenser cannot be accessed for mechanical cleaning and must be cleaned with chemicals, while shell-and-tube condensers have removable end caps that allow mechanical cleaning of the straight tubes.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 44 - Commercial Evaporators, Condensers, and Other Heat Exchangers, Page 1069 (Section 44.2.2) (20th ed.: Page 537)Three-phase motor startingvsSingle-phase motor starting
A three-phase supply is already phase-split, so the motor develops starting torque from its rotating magnetic field without start windings or capacitors, which single-phase motors need for phase splitting.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 16 - Electric Motors, Page 308 (Section 16.2.2) (20th ed.: Chapter 15, Section 15.2.2)Duct leakage test before the air handler is installedvsDuct leakage test with the air handler installed
Per the IECC, maximum duct leakage is 3 cfm per 100 ft² of conditioned space when tested before the air handler is installed, and 4 cfm per 100 ft² when tested with the air handler installed.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 31 - Ventilation System Service, Page 703 (Section 31.1.5) (20th ed.: Page 793)cfm50 (blower door test)vscfm25 (duct leakage test)
cfm50 is the airflow through the blower door fan with indoor pressure held at −50 Pa WRT outdoors (used to find ACH50), while cfm25 is the airflow needed to hold sealed ductwork at +25 Pa WRT the conditioned space.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 31 - Ventilation System Service, Page 702 (20th ed.: Page 791)Radial duct systemvsPerimeter loop duct system
A radial system runs individual branch ducts directly from the central plenum to each outlet with no trunk duct, while a perimeter loop system is used with a downflow furnace in homes without basements, with branch ducts in the slab feeding a perimeter duct and diffusers at intervals.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 30 - Air Distribution, Page 672 (20th ed.: Page 755)Infiltration in heating load calculationsvsInfiltration in cooling load calculations
Heating loads consider only the sensible heat loss due to infiltration, while cooling loads must include both sensible and latent heat gains from infiltration.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 32 - Heating and Cooling Loads, Page 733 (20th ed.: Page 1004)Lead action level (29 CFR 1926.62(b))vsLead permissible exposure limit (29 CFR 1926.62(c)(1))
The action level is an airborne lead exposure of 30 µg/m³ as an 8-hour TWA without regard to respirator use, while the PEL is the 50 µg/m³ figure.
OSHA 29 CFR 1926, §1926.62(b)AHRI Standard 700vsAHRI Standard 740
AHRI 700 is the purity standard reclaimed refrigerant must meet before it can be sold, while AHRI 740 relates to recovery/recycling machine certification.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 11 - Equipment and Instruments for Refrigerant Handling and Service, Page 212 (20th ed.: Page 225)Key Terms
Competent person (29 CFR 1926.32(f)) OSHA 1926 CFR 29, Page 31
One capable of identifying existing and predictable hazards in the surroundings or working conditions that are unsanitary, hazardous, or dangerous to employees, and who has authorization to take prompt corrective measures to eliminate them (no specific degree, experience, or certificate required).
Lead action level (29 CFR 1926.62(b)) OSHA 29 CFR 1926, §1926.62(b)
Employee exposure, without regard to respirator use, to an airborne lead concentration of 30 micrograms per cubic meter averaged over 8 hours, which triggers exposure monitoring, medical surveillance, and training.
Construction site fire extinguisher requirement (OSHA 1926.150(c)(1)(i)) OSHA 1926.150(c)(1)(i)
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.
Construction fire alarm system (OSHA 1926.150(e)) OSHA 1926.150(e)(1) and (e)(2)
An alarm system such as a telephone system or siren that the employer must establish so employees on site and the local fire department can be alerted for an emergency, with the alarm code and reporting instructions conspicuously posted at phones and employee entrances.
First aid kit inspection (29 CFR 1926.50) OSHA 1926 CFR 29, Page 33
The employer must check first aid kit contents before the kit is sent out on each job and at least weekly on each job to ensure expended items are replaced.
Low-heat appliance / residential appliance (IFGC 2021 §202) IFGC 2021 Section 202, Appliance Type definitions
Any appliance in which the products of combustion at the point of entrance to the flue under normal operating conditions have a temperature of 1,000°F (538°C) or less.
Manifold appliance shutoff valve (IFGC 2021 §409.5.3) IFGC 2021 to 4-48 (Section 409.5.3)
An appliance shutoff valve may be installed at a manifold provided it is located within 50 feet of the appliance served and is readily accessible and permanently identified; the 6-foot limit applies to a valve in the same room as the appliance (§409.5.1).
Spray polyurethane foam duct insulation (IMC 2021 §604.3 Exception) IMC 2021 Section 604.3 Exception
Foam 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 when tested to ASTM E84 or UL 723, a relaxation from the 50 smoke-developed limit for other coverings and linings.
Refrigerant distributor Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 45 - Commercial Metering Devices and Valves, Page 1088 (Section 45.1.4) (20th ed.: Pages 490-491)
A device at the metering device outlet that splits refrigerant flow into several shorter parallel evaporator circuits, reducing pressure drop across the evaporator and giving more consistent coil temperature.
Externally equalized TXV Modern Refrigeration and Air Conditioning (22nd ed.), Page 1083
A thermostatic expansion valve with an equalizer line that delivers suction-line pressure at the sensing bulb to the underside of the diaphragm; it should be used when the pressure drop between evaporator inlet and outlet is more than 4 psi (28 kPa).
Potential relay Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 17 - Electrical Control Systems, Page 339 ("Potential Relays") (20th ed.: Chapter 16, Section 16.3.5 — Potential Relays)
A start relay that energizes on the increasing counter electromotive force (cemf) induced in the motor start winding as the motor approaches rated speed (pickup at about three-fourths of normal speed), opening its normally closed contacts to drop out the start capacitor.
Internal bimetal overload protector Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 17 - Electrical Control Systems, Page 348 (Section 17.5.3) (20th ed.: Chapter 16, Section 16.4.3)
A protection device located inside the compressor on or in the motor windings whose bimetal disc bends to open the circuit on excess current or heat and recloses when the motor cools, producing cycling on and off.
High-pressure motor control Modern Refrigeration and Air Conditioning (22nd ed.), Page 334
A safety device—not a normal operating control—on the compressor discharge usually set to cut out at 20% above normal head pressure to protect the compressor from excessive head pressure, overheating, and high current draw.
Low-pressure control differential adjustment Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 17 - Electrical Control Systems, Page 333 (Section 17.4.4) (20th ed.: Chapter 16, Section 16.3.4 — Low-Pressure Motor Controls)
An adjustment that affects only the cut-out setting; turning the screw clockwise (in) increases spring tension and raises the cut-out pressure, leaving the cut-in setting unchanged.
Sequencer (electric furnace) Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 35 - Electric Heating Systems, Page 834 (20th ed.: Pages 1196 and 1205)
A device that closes and opens contacts on a time delay to energize heating elements one after another, typically about 30 seconds apart, minimizing the furnace's initial current draw.
Ladder diagram Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 18 - Electrical Troubleshooting Fundamentals, Page 368 (Section 18.2.1) (20th ed.: Chapter 16, Section 16.1.2)
A wiring diagram in which the two vertical lines are the power supply and each horizontal rung is an individual circuit containing a load and any controls such as switches or relays.
Heat transfer multiplier (HTM) – Manual J Modern Refrigeration and Air Conditioning (22nd ed.), Pages 722 and 737
A building component's U-value multiplied by the design temperature difference (for heating, indoor design temperature minus outdoor design temperature), then multiplied by the reference area to obtain Btu/hr.
Panned joist space Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 30 - Air Distribution, Page 672 (20th ed.: Page 755)
A common basement return-air branch duct method that uses the joists and floorboards as three sides of the duct.
Small duct, high-velocity (SDHV) system Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 30 - Air Distribution, Page 666 (20th ed.: Page 748)
An air distribution system using small-diameter ducts that deliver air at high velocity, with supply air going to the inside perimeter of the building and return air originating from the center.
Splitter in a divided-flow branch (SMACNA Fig. 4-5) SMACNA HVAC Duct Construction Standards (3rd ed.), Page 4.7 (Figure 4-5)
A stationary splitter shown in a divided-flow branch fitting whose length should be 1.5 W (or 1.5 D3); volume control should still be by branch dampers, not the splitter.
Formulas to Know
Heat transfer multiplier and component load (Manual J heating)
HTM = U × HTD (design temperature difference); Load (Btuh) = HTM × area. Examples: wall HTM = 0.07 × 50 = 3.50, 136 sq ft × 3.50 = 476 Btuh; wall HTM = 0.07 × 75 = 5.25, 208 sq ft × 5.25 = 1,092 Btuh; windows 60 sq ft × (0.30 × 50 = 15.00) = 900 Btuh; ceiling 140 sq ft × (0.03 × 50 = 1.50) = 210 Btuh; door 20 sq ft × (0.50 × 70 = 35.00) = 700 Btuh.Net exposed wall area and room total heating load
Net wall = (exposed wall length × height) − (glass area + door area); e.g., 24 ft × 9 ft = 216 − (2×30 + 20) = 136 sq ft; 32 ft × 9 ft = 288 − (3×20 + 20) = 208 sq ft. Total room load = wall + glass + door + ceiling + infiltration (e.g., 770 + 1,155 + 700 + 470 + 1,725 = 4,820 Btuh; 1,008 + 504 + 432 + 1,014 = 2,958 Btuh).Net wall area of a whole building (textbook method)
Perimeter × ceiling height − (window area + door area); example: (40+50+40+50) = 180 ft perimeter × 8 ft = 1,440 ft²; 1,440 − (80 + 42) = 1,318 ft².Infiltration airflow and infiltration heating load (Manual J)
CFM = air changes per hour × room volume ÷ 60 (example: 0.6 × (16×14×10 = 2,240 cu ft) ÷ 60 = 22.4 cfm); Infiltration load = 1.1 × ACF × CFM × HTD (example: 1.1 × 1.00 × 22.4 × 70 = 1,725 Btuh).Sensible heat loss due to infiltration (heating loads)
Qs = 0.018 × CFH × TD (dry-bulb temperature difference); heating loads consider sensible infiltration only.Latent heat gain due to infiltration
Q = 0.01133 × CFH × ΔW, where ΔW is the outdoor-to-indoor moisture difference expressed in grains per pound of air.Slab-on-grade and basement heat loss
Slab: building perimeter length (ft) × 18 Btu/hr per linear foot (for a 0°F outdoor design temperature), added to above-ground losses. Basement used as living space: U-value (usually about 0.10 for concrete floors and walls) × temperature difference between basement and ground × floor and wall area.Duct equivalent length and friction loss
Straight measured length = sum of straight segments (28 + 22 + 15 = 65 ft, elbows not counted); Total equivalent length = straight length + elbow allowances (65 + 2 × 25 = 115 ft); Friction loss = (total equivalent length ÷ 100) × friction rate = (115/100) × 0.10 in. WC = 0.115 in. WC.Coefficient of performance from a pressure-enthalpy diagram
COP = refrigeration effect ÷ heat of compression; example: (179 − 116 = 63 Btu/lb) ÷ (198 − 179 = 19 Btu/lb) = 3.32:1.Btu/hr to kilowatts for electric heat sizing and minimum circuit ampacity
kW = Btu/hr ÷ 3,413 (example: 50,000 ÷ 3,413 = 14.65 kW minimum capacity). MCA = 1.25 × largest motor FLA/RLA + sum of other loads (example: 1.25 × 24.0 + 2.1 = 32.1 A).