Exam Cheat Sheet · Quick Reference
Mississippi Mechanical Contractor
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
100
Passing score
70%
Exam time
240 min
Administered by
PSI Services Contractor
Format
Reference materials allowed
Mississippi State Portion 100 questions
Boilers and Hydronics
6 Q · 6%
Boiler types and classifications (conventional vs. condensing, cast-iron vs. steel vs. copper-tube, wet-base vs. dry-base)Safety and relief valve requirements and discharge piping installationBoiler controls (aquastat, low-water cutoff, high-limit control, flow switch, pressure-reducing valve)Hydronic system piping configurations (series loop, one-pipe, two-pipe direct/reverse return, zoned systems)Expansion tank types, sizing, and installation requirementsHydronic piping materials, joints, and connection methods per IMC Chapter 12Air removal components and procedures (air vents, air scoops, air separators, bleeding and purging)Radiant hydronic systems (wet and dry underfloor installation methods, embedded piping requirements, insulation and thermal break)Full Text
Gas Systems
10 Q · 10%
Gas piping materials, fittings, and allowable pipe types (metallic pipe, copper tubing, CSST)Gas piping sizing methods and capacity tables (longest length, branch length, hybrid pressure methods)Gas piping pressure requirements and pressure testingLP-Gas container types, installation, and separation distances (NFPA 58)LP-Gas pressure relief devices and container appurtenancesVenting system types and appliance categories (Type B, B-W, single-wall, direct-vent, Category I–IV)Chimney and vent sizing, termination requirements, and clearancesCombustion air requirements for gas appliancesGAS PIPING INSTALLATIONS
Cleanouts
3 Q · 3%
Cleanout placement intervals for horizontal drains, building drains, and building sewersCleanout requirements at changes of horizontal direction greater than 45 degreesCleanout requirement at the building drain and building sewer junctionCleanout sizing relative to pipe size servedCleanout installation arrangement and direction of flow requirementRequired clearance distances at cleanout openingsCleanout access requirements and prohibition against concealed locationsCleanout plugs: approved materials, head types, and trim cover provisionsSanitary Drainage
Plumbing Fixtures and Equipment
5 Q · 5%
Minimum number and type of plumbing fixtures by occupancy classificationFixture installation requirements (clearances, spacing, and setting)Fixture materials, standards, and product approvalsFlushing devices for water closets and urinalsShower and bathtub valves, temperature limits, and anti-scald requirementsWater heater safety devices, relief valves, and installation requirementsAccessible plumbing fixtures and facilities (ICC A117.1 dimensional requirements)Grease interceptors, oil separators, and trap requirementsWater Supply and Distribution
Water Heaters
2 Q · 2%
Temperature and pressure relief valve requirements and discharge piping specificationsWater heater installation requirements including access, clearances, and attic installationsSafety devices including antisiphon devices, vacuum relief valves, and shutdown provisionsCold water supply valve and drain valve requirementsWater heater labeling, certification, and pressure marking of storage tanksCombination potable water heating and space heating systems with temperature limitingDrain pan requirements including materials, dimensions, and drain terminationUnfired hot water storage tank insulation requirementsCHIMNEYS AND VENTS
Piping, valves, controls
5 Q · 5%
Pipe and tubing materials selection and standards (steel, copper, plastic, brass, stainless steel)Pipe joining methods (threaded, welded, grooved, brazed, soldered, solvent-cemented, press-connect)Pipe sizing and gas piping capacity tables (longest length, branch length, and hybrid pressure methods)Pipe support, hanger spacing, and hanger rod sizing requirementsValve types and functions (isolation, pressure-reducing, backflow prevention, expansion, solenoid, pressure-relief)Refrigerant metering devices and expansion valves (TXV, AXV, EEV, float controls)Hydronic piping system components (fittings, flexible connectors, expansion compensators)Protection of piping against freezing, corrosion, and mechanical damageFull Text
Sheet Metal and Ducts
6 Q · 6%
Rectangular duct gauge selection and reinforcement schedules based on static pressure classTransverse joint types and longitudinal seam options for rectangular ductRound and oval duct wall thickness and seam types for positive and negative pressureDuct sealing requirements and seal classes by pressure classificationHanger and support systems for rectangular, round, and flat oval ductsFlexible duct installation, support, and connection requirementsDuct fittings, elbows, transitions, and branch connectionsDuct accessories including volume dampers, access doors, and duct liner installationRectangular Duct Construction
Water Supply
5 Q · 5%
Potable water protection and cross-connection control (backflow prevention devices, air gaps, vacuum breakers)Water service pipe materials, standards, and joint typesWater distribution system design criteria (flow rates, pressures, and fixture supply sizing)Water piping system sizing methods (segmented loss method, fixture unit loads, pressure calculations)Water pressure regulation and booster systems (pressure-reducing valves, booster pumps, tanks)Individual water supply sources (wells, pumps, disinfection requirements)Fire suppression water supply requirements (NFPA 13 water supply types, flow/pressure/duration demands)Underground private fire service main installation (pipe materials, burial depth, restraint, hydrostatic testing)Water Supply and Distribution
Hangers and Supports
4 Q · 4%
Maximum hanger spacing by pipe type and materialHanger rod sizing requirements by pipe diameterFastener types and installation requirements for concrete, steel, and woodTrapeze hanger design and section modulus selectionSupport of risers and riser clamp requirementsHVAC duct hanger sizing and upper attachment methods for rectangular and round ductPipe support spacing for water, gas, steam, and plastic pipe linesSeismic bracing and earthquake protection requirements for hangersFull Text
Drain, Waste, and Vent
5 Q · 5%
Drainage fixture unit (DFU) values and system sizingSlope and installation requirements for horizontal drainage pipingCleanout placement, sizing, and access requirementsTrap requirements, prohibited traps, and trap seal protectionVent types, sizing, and maximum developed lengthGrease interceptors, oil separators, and other interceptor requirementsIndirect waste connections, air gaps, and air breaksPipe materials, fittings, and joint methods for DWV systemsSanitary Drainage
Backflow
2 Q · 2%
Types of backflow (backsiphonage vs. backpressure) and hazard levels (high vs. low)Backflow preventer device selection per Table 608.1 (air gap, reduced pressure principle assembly, double check assembly, vacuum breakers, dual check valve)Specific application requirements for backflow protection (boilers, sprinkler systems, lawn irrigation, carbonated beverage dispensers, hose connections, humidifiers)Minimum air gap dimensions per Table 608.16.1Cross-connection control and prohibition of cross connections without approved protectionLocation, access, and installation requirements for backflow prevention assembliesInspection and testing of backflow prevention assemblies (annual inspections, test standards per Section 312.10)Water Supply and Distribution
Traps and Interceptors
4 Q · 4%
Fixture trap requirements: dimensions, seal depth, and prohibited trap typesTrap seal protection methods: primer valves, barrier devices, and fixture drain connectionsGrease interceptor requirements: where required, sizing, and installationAutomatic grease removal devices and hydromechanical grease interceptorsOil separators: where required, design requirements, and capacity calculationsSand, solids, and specialty interceptors: clothes washers, bottling plants, and slaughterhousesVenting, access, and maintenance of interceptors and separatorsTrap sizing relative to fixture drain size and drainage fixture unit valuesSanitary Drainage
Developed length
3 Q · 3%
Calculating total effective length of duct runs using fitting equivalent lengthsEquivalent length values for duct fittings by group (elbows, tees, takeoffs, transitions)Determining available static pressure and design friction rate for duct sizingEquivalent length of pipe fittings and valves for gas piping systemsDeveloped length calculations for pipe bends including setback and arc lengthEquivalent length allowances for fittings in water supply piping systemsLongest length method for gas piping system sizing using developed pipe lengthFull Text
Isometric Analysis
8 Q · 8%
Reading and interpreting pipe isometric drawingsCalculating offset dimensions including travel, run, and offset relationshipsDetermining setback and arc length for standard bend anglesRolling offset calculations using the Pythagorean theoremPipe fitting allowances and laying lengths in isometric layoutsLocating bend starting and ending points along pipe runsMulti-bend layouts including double offsets and 90-degree turnsFull Text
Safety
4 Q · 4%
Lockout/tagout (LOTO) procedures and requirementsPersonal protective equipment (PPE) selection and useFall protection systems (guardrails, safety nets, personal fall arrest)Fire protection, prevention, and extinguisher selectionElectrical safety and ground-fault circuit interrupter (GFCI) requirementsConfined space entry requirements and permit proceduresHazard communication, safety data sheets (SDS), and toxic/hazardous substance exposure limitsLadder and stairway safety requirementsSubpart D-Occupational Health and Environmental Controls
Joints and Connections
3 Q · 3%
Soldered and brazed joint procedures for copper tubing and pipeThreaded pipe joints and connection requirementsMechanical and grooved coupling connectionsSolvent-cement joints for plastic pipe (ABS, PVC, CPVC)Flared and swaged tubing connectionsPush-fit, press-connect, and compression-type fittingsHeat-fusion joints for polyethylene and polypropylene pipeDuct seams, joints, and connection requirements for metallic and nonmetallic duct systemsFull Text
General Plumbing Regulations
3 Q · 3%
Permit requirements and code administrationRequired inspections and testing proceduresPipe support, protection, and installation requirementsTrenching, excavation, and backfill requirementsMaterials identification, approval, and third-party certificationProhibited materials and connections to drainage and water supply systemsFlood hazard resistance requirements for plumbing systemsMinimum plumbing fixture counts by occupancy typeWater Supply and Distribution
Fire Sprinklers
4 Q · 4%
Sprinkler system types (wet pipe, dry pipe, preaction, deluge)Sprinkler location requirements and allowable omissionsSprinkler spacing, coverage areas, and deflector positioning by sprinkler typePiping installation requirements (materials, schedules, fittings, and joining methods)Pipe hangers, support spacing, and seismic bracing requirementsHydraulic calculations and working plan submittal requirementsStorage occupancy protection (commodity classification, CMDA, CMSA, ESFR, and in-rack sprinklers)System acceptance testing (hydrostatic tests, operational tests, and contractor certification)Protection of Rack Storage Using In-Rack Sprinklers
Refrigerants and Refrigeration
4 Q · 4%
Refrigerant classification (CFCs, HCFCs, HFCs, HFOs, HCs) and environmental impact (ODP, GWP)EPA regulations, Clean Air Act Section 608, and refrigerant handling certification requirementsRefrigerant cylinders: storage, disposable, and recovery cylinder types, DOT regulations, and safe handlingRefrigerant recovery, recycling, and reclaiming procedures and equipmentLeak detection methods and equipment (electronic, ultrasonic, fluorescent dye, halide torch)System evacuation procedures, vacuum pumps, and micron gauge useRefrigerant charging methods: checking charge by superheat and subcoolingPressure-temperature relationships and gauge manifold use in refrigeration serviceCommercial Metering Devices and Valves
Heating and Cooling Principles
4 Q · 4%
Heat transfer mechanisms (conduction, convection, radiation)Sensible heat and latent heat calculationsThermal resistance and insulation values (R-value, U-value, K-value, C-value)Heating and cooling load calculation methods (Manual J, Manual N)Compression refrigeration cycle operation (evaporation, condensation, compression, metering)Heat pump heating and cooling modes and efficiency ratings (COP, SEER, HSPF)Design temperatures and indoor/outdoor temperature differentialsGas laws and pressure-temperature relationships in refrigerant systemsHeating and Cooling Loads
Load Calculations
5 Q · 5%
Heat transfer principles and thermal properties (K-value, C-value, R-value, U-value)Outdoor and indoor design temperatures and conditions (Manual N Table 1A)Fenestration heating and cooling loads (HTM calculations, U-value, SHGC, PSF, CLF)Opaque panel heating and cooling loads (walls, roofs, ceilings, floors, CLTD method)Infiltration load calculations (air change method, tightness categories, door traffic)Internal loads (lighting, occupants, equipment heat gains)Duct system loads (conduction, leakage, insulation R-value, supply and return systems)Ventilation loads (engineered ventilation, sensible and latent heat equations, altitude correction)Table 7: Duct Load Factors
Testing, Adjusting and Balancing
1 Q · 1%
Airflow measurement instruments and methods (flow hood, anemometer, pitot tube, manometer)System air balancing procedures and damper adjustmentDuct leakage testing (blower door and duct blaster methods)Available static pressure and device pressure loss calculationsFriction rate and total effective length determinationAir velocity limits for ducts and supply outletsBlower performance data interpretation and operating pointFilter inspection and pressure drop measurementAir Distribution
HVAC Equipment
4 Q · 4%
Compression refrigeration cycle components (compressor, condenser, metering device, evaporator)Compressor types and classifications (reciprocating, rotary, scroll, screw, centrifugal; hermetic vs. semi-hermetic vs. open-drive)Gas-fired heating systems (furnace types, heat exchangers, combustion, burners, ignition systems, AFUE ratings)Air-source heat pumps (reversing valve operation, heating/cooling modes, efficiency ratings, defrost controls)Split and self-contained air-conditioning systems (central, ductless mini-split, VRF, PTAC, packaged units)Electric heating systems (resistance elements, electric furnaces, duct heaters, baseboard units)Hydronic heating systems (boilers, circulating pumps, expansion tanks, terminal units, hydronic controls)Oil-fired heating systems (fuel oil properties, oil burner components, combustion efficiency testing)Commercial Metering Devices and Valves
Key Distinctions
K-value (thermal conductivity)vsC-value (thermal conductance)
K-value is heat passing through one square foot of material one inch thick in one 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)ACR tubing sizingvsCopper water tubing sizing
ACR tubing is designated by its actual outside diameter, whereas copper water tubing nominal size equals the outside diameter minus 1/8″.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 5 - Working with Tubing and Piping, Page 71 (Section 5.1.1) (20th ed.: Chapter 8, Section 8.1.1)Full-open valve locations (IPC 606.1)vsShutoff valve locations (IPC 606.2)
Full-open valves are required on the water supply pipe to every water heater, the discharge side of every water meter and the service pipe at the entrance to the structure, while supplies to fixtures, sillcocks and appliances need only shutoff valves — and in one- and two-family residential occupancies bathtubs and showers are exempt from individual fixture shutoff valves.
IPC 2021 Sections 606.1 and 606.2Subcooling method of chargingvsSuperheat method of charging
The subcooling method is used to check the charge on systems with thermostatic expansion valves, while the superheat method is used on systems with fixed-orifice devices such as capillary tubes.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 12 - Working with Refrigerants, Page 221 (20th ed.: Page 236)Low-temperature refrigeration TXV superheatvsMedium-temperature refrigeration TXV superheat
Low-temperature refrigeration superheat is generally set at 3°F to 4°F, while medium-temperature refrigeration is generally set at 6°F to 8°F.
Modern Refrigeration and Air Conditioning (22nd ed.), Page 1249 — Adjusting Thermostatic Expansion Valves (TXVs) sectionHot-surface igniter (glow coil)vsFlame rod
In an HSI system the glow coil only ignites the burner and cannot sense flame; a separate flame rod must send a microamp rectification signal to the module, and a dirty or oxidized rod causes shutdown seconds after the gas valve opens.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 33 - Gas-Fired Heating Systems, Page 765 (20th ed.: Pages 1126-1128 (Sections 41.5.4 and 41.6.1))Steel globe and angle valves, butt welding endsvsGate valves and swing check valves, butt welding ends
Globe and angle valve laying lengths use the designation '2 × A,' while gate and swing check valves with butt welding ends use 'Dimension A.'
Pipefitter's Handbook (3rd ed.), Chapter 1 (Full Text), Laying Lengths of Steel Globe and Angle Valves With Butt Welding EndsGlobe valve damaged disc-stem jointvsGlobe valve seat damage / deposits under the disc
A damaged disc-stem joint is remedied by replacing the disc or stem, seat damage requires replacing the valve, and deposits under the disc require washing or cleaning.
Pipefitter's Handbook (3rd ed.), Chapter 1 (Full Text), Trouble Shooting Methods — Globe Valves, Page 5-10Infiltration in heating load calculationsvsInfiltration in cooling load calculations
Heating loads count only the sensible heat loss from infiltration, while cooling loads must include both sensible and latent heat gains.
Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 32 - Heating and Cooling Loads, Page 733 (20th ed.: Page 1004)Oil separator water seal (IPC 1003.4.2.1)vsSand interceptor water seal (IPC 1003.5)
A designed (not listed and labeled) oil separator outlet opening needs a water seal of not less than 18 inches, while the 6-inch water seal applies to sand interceptors.
IPC 2021 Section 1003.4.2.1CPVC solvent cement — primer required (two-step)vsCPVC solvent cement — one-step permitted
Where a primer is required the solvent cement must be orange in color conforming to ASTM F493; where a one-step cement is allowed it must be yellow in color conforming to ASTM F493 (PVC joints, by contrast, use purple primer to ASTM F656 with cement to ASTM D2564).
IPC 2021 Section 605.14.2Ventilated thimble — residential and low-heat appliancesvsVentilated thimble — listed draft-hood/Type B and unlisted draft-hood appliances
For residential and low-heat appliances the thimble shall be not less than 12 inches larger in diameter than the metal pipe, while listed appliances with draft hoods and appliances listed for Type B vents use 4 inches (2 inches where there is a run of at least 6 feet of pipe in the open) and unlisted appliances having draft hoods use 6 inches.
IFGC 2021 Section 503.7.7Short sweep fittingvsSanitary tee
Per Table 706.3, a short sweep is permitted for horizontal-to-horizontal changes of direction only for a 2-inch or smaller fixture drain (footnote a); a long sweep is permitted for horizontal-to-horizontal without that limitation, while a sanitary tee is permitted only for horizontal-to-vertical changes.
IPC 2021 Table 706.3Copper tubing Types K and L for building sewervsCopper tubing Types M and DWV
Building sewer copper or copper-alloy tubing is limited to Type K or L; Types M and DWV are not permitted for building sewer applications (and galvanized steel is not an approved building sewer material at all).
IPC 2021 Table 702.3Key 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, years of experience or certificate is required.
Lead-free solder (IPC 605.12.3) IPC 2021 Section 605.12.3
Solder and flux used to join water supply piping having a chemical composition equal to or less than 0.2-percent lead.
Cleanout (IPC 708.1.1 and 708.1.4) IPC 2021 Section 708.1.1
An access fitting required in horizontal drainage piping in buildings at intervals of not more than 100 feet and at any change of horizontal direction greater than 45 degrees.
Water-hammer arrestor (IPC 604.9) IPC 2021 Section 604.9
A device conforming to ASSE 1010 and installed per manufacturer's instructions that shall be installed where quick-closing valves are utilized.
Antisiphon fill valve (IRC P2902.4.1) IRC Section P2902.4.1
A flush tank fill valve conforming to ASSE 1002/ASME A112.1002/CSA B125.12 or CSA B125.3 whose critical level is located not less than 1 inch above the top of the flush tank overflow pipe.
Hose connection backflow protection (IRC P2902.4.3) IRC Section P2902.4.3
Sillcocks, hose bibbs, wall hydrants and other hose-connection openings must be protected by an atmospheric- or pressure-type vacuum breaker, a pressure vacuum-breaker assembly, or a permanently attached hose connection vacuum breaker.
Prohibited joints (IPC 605.9) IPC 2021 Section 605.9
Joints not permitted in a water supply system, including cement or concrete joints, joints made with fittings not approved for the installation, solvent-cement joints between different types of plastic pipe, and saddle-type fittings.
Nonpotable water distribution piping identification (IPC 608.9.2) IPC 2021 Section 608.9.2
Piping that must be purple in color and embossed, integrally stamped or marked 'CAUTION: NONPOTABLE WATER – DO NOT DRINK,' or installed with purple identification tape or wrap.
Backflow assembly field testing (IPC 312.10.2) IPC 2021 Section 312.10.2
Reduced pressure principle, double check, pressure vacuum breaker, reduced pressure detector fire protection, double check detector fire protection, spill-resistant vacuum breaker assemblies and hose connection backflow preventers must be tested at installation, immediately after repairs or relocation, and at least annually, using test gauges complying with ASSE 1064.
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.
PTC relay (positive temperature coefficient thermistor) Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 17 - Electrical Control Systems, Page 346 (Section 17.5) (20th ed.: Chapter 16, Section 16.3.5 — Positive Temperature Coefficient Relays)
A starting device wired in parallel with the run capacitor whose resistance is low (3 Ω to 12 Ω) when cool so start current takes that path, then rises to 10 kΩ–20 kΩ as it heats, forcing current through the run capacitor.
Current relay terminals L, M, S Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 17 - Electrical Control Systems, Page 339 (Section 17.4.5) (20th ed.: Chapter 16, Section 16.3.5 — Current Relays)
On a current relay, L is line voltage, M is the main (run) winding and S is the start winding; a normally open contact lies between L and S and the relay coil between L and M.
Category IV gas furnace Modern Refrigeration and Air Conditioning (22nd ed.), Chapter 33 - Gas-Fired Heating Systems, Page 776 (20th ed.: Page 1134)
A positive-pressure venting, condensing furnace with flue gas temperature less than 140°F (below its dew point), using secondary heat exchangers and PVC vent piping.
Pilot-operated reversing valve Modern Refrigeration and Air Conditioning, Ch. 36, Section 36.4.4
A four-way valve in which the solenoid shifts a small pilot valve that directs high-pressure discharge vapor to one end of the main piston and low-pressure suction vapor to the other, so high-pressure vapor from the compressor actually moves the piston.
Evaporator pressure regulator (EPR) Modern Refrigeration and Air Conditioning (22nd ed.), Pages 1224 to 1225
A valve that holds an evaporator at a preset pressure/temperature; if it has not been readjusted and the box runs too cold with freezing product, the needle is probably leaking (a clogged screen or stuck-shut valve instead stops flow and raises conditions).
Liquid line pressure differential valve (reverse cycle hot-gas defrost) Modern Refrigeration and Air Conditioning (22nd ed.), Page 1058
A valve that creates a 20–40 psi drop in the liquid line during defrost of one evaporator so both defrost flow and normal operating flow can occur, forcing liquid from the defrosting evaporator through the other evaporators' liquid line solenoids.
Air loading factor (ALF) for duct table selection (Manual N 25-8) ACCA Manual N (5th ed.), Section 25-8 — Duct Loads for Air Handler Two, Page 225
The best-match Cfm/SqFt value used to pick one duct load factor table applied to every hour-of-day run for an air handler — in the case study the defaults ranged 0.31 to 1.10 and 1.00 was selected (table 7TC-1.0), disregarding the 5 AM value.
First aid kit inspection (29 CFR 1926.50) OSHA 1926 CFR 29, Page 33
Kit contents must be checked by the employer before being sent out on each job and at least weekly on each job so expended items are replaced.
Formulas to Know
Static pressure loss or gain due to elevation (head)
psi = 0.433 psi per foot × feet of elevation change; example: 14 ft × 0.433 = 6.06 psi, subtracted from available pressure when the fixture is above the sourceEquivalent length allowance for fittings and valves (segmented loss method)
Add 50% of the developed length: equivalent length = developed length × 1.5; example: 225 ft × 1.5 = 338 ftMaximum developed length for water piping (IPC E104.1, Item 3)
Maximum developed length = actual pipe length from source of supply to most remote fixture (including hot or cold branches) × 1.2Total equivalent length of a duct run
TEL = sum of straight measured lengths + elbow equivalent lengths; example: 28 + 22 + 15 = 65 ft straight, plus 2 elbows × 25 ft = 50 ft, TEL = 115 ftDuct friction loss for a run
Friction loss = (total equivalent length ÷ 100) × friction rate; example: (115/100) × 0.10 in. WC = 0.115 in. WCOutdoor-air percentage of mixed air (and mixed-air balance)
OA% = outdoor CFM ÷ total mixed CFM; mixed CFM = sum of entering streams; example: 1,000 ÷ 4,000 = 25%, with 3,000 return + 1,000 outdoor = 4,000 CFM mixedHeating load of a surface (HTM method, Q = U × A × TD)
HTM = U × HTD, then load = area × HTM; examples: wall 0.07 × 50 = 3.50 HTM × 136 sq ft = 476 Btuh; glass 0.30 × 50 = 15.00 × 60 sq ft = 900 Btuh; ceiling 0.03 × 50 = 1.50 × 140 sq ft = 210 Btuh; door 0.50 × 70 = 35.00 × 20 sq ft = 700 Btuh; net wall area = gross wall − window and door openingsInfiltration airflow and infiltration heating load
CFM = air changes per hour × room volume ÷ 60 (example: 0.6 × (16 × 14 × 10 = 2,240 cu ft) ÷ 60 = 22.4 cfm); load = 1.1 × ACF × CFM × HTD (example: 1.1 × 1.00 × 22.4 × 70 = 1,725 Btuh); sensible infiltration may also be computed as Qs = 0.018 × CFH × TDScaling a total heating load for a colder design temperature
Every component is proportional to HTD, so total scales by new HTD ÷ old HTD; example: 4,820 Btuh × (80 ÷ 70 = 1.1429) = 5,509 BtuhSlab-on-grade additional heat loss (0°F outdoor design temperature)
Slab loss = building perimeter length (ft) × 18 Btu/hr per linear foot, added to the above-ground heat losses (based on perimeter, not slab area)Thermal expansion of steel pipe
Expansion = (table value at operating temp − table value at installation temp) per 100 ft × (line length ÷ 100); example: (3.245 at 400°F − 0.449 at 60°F) = 2.796 in./100 ft × 5.0 = 13.98 in. for 500 ftPipe bend setback and crossover bend dimensions
150° bend: Setback = Radius × 3.73 (135° uses 2.41, 120° uses 1.732, and 3.14 is the length-of-bend multiplier for 180°); 240° crossover = four 60° bends with Offset = Radius, Run = Radius × 3.4640, Length of Pipe in Bend = Radius × 4.1887