mep-rules.json

{
  "version": "2026-08-13",
  "disclaimer": "Drafting aid, not engineering. Typical/approximate values — verify with the local authority having jurisdiction.",
  "basis": "2021 International Residential Code (IRC) as published (consulted via the Pennsylvania adoption on UpCodes), plus flagged industry rules of thumb. Plumbing = IRC Part VII (chapters 25-33); mechanical = IRC Part V (chapters 12-24). UPC-jurisdiction deltas noted where they matter for geometry.",
  "sources": [
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/30/sanitary-drainage",
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/31/vents",
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/32/traps",
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/29/water-supply-and-distribution",
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/27/plumbing-fixtures",
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/15/exhaust-systems",
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/14/heating-and-cooling-equipment-and-appliances",
    "https://up.codes/viewer/pennsylvania/irc-2021/chapter/13/general-mechanical-system-requirements",
    "https://hqplumbingandair.com/resources/code-library/upc-table-1002-2-trap-arm-length",
    "https://washingtonplumbers.net/glossary/wet-wall/",
    "https://horow.com/blogs/guide/bathroom-rough-in-dimensions-plumb-guide-for-bathroom-plumbing",
    "https://basc.pnnl.gov/resource-guides/air-conditioning",
    "https://www.energystar.gov/products/room_air_conditioners",
    "https://hvacprocalculator.com/guides/cfm-per-ton",
    "https://mepacademy.com/how-many-cfm-per-ton",
    "https://www.hvacrschool.com/manual-d-speedsheet-walkthrough/",
    "https://aircondlounge.com/mini-split-placement-best-location-for-mini-splits/",
    "https://www.inchcalculator.com/calculate-btus-to-heat-home/"
  ],
  "plumbing": {
    "wetWall": {
      "nominalStud": "2x6",
      "actualCavityDepthIn": 5.5,
      "note": "Industry practice, not a code table: a 3-in DWV stack needs ~3.5 in of clearance at hubs/fittings, which does not fit a 2x4 wall (3.5 in actual). Frame plumbing walls 2x6 (5.5 in) or double 2x4. Stud boring limits (max ~40% bearing / 60% non-bearing) also push large pipes into deeper walls.",
      "shieldPlateRequiredWithinInOfStudFace": 1.5,
      "shieldPlateNote": "IRC P2603.2.1: steel shield plates where pipe is closer than 1.5 in to the stud/plate face."
    },
    "dwv": {
      "toiletDrainIn": 3,
      "showerDrainIn": 2,
      "tubDrainIn": 1.5,
      "lavDrainIn": 1.25,
      "kitchenSinkDrainIn": 1.5,
      "laundryTubDrainIn": 1.5,
      "clothesWasherStandpipeIn": 2,
      "dishwasherTrapIn": 1.5,
      "showerDrainNote": "IRC Table P3201.7: 1.5 in allowed for showers <= 5.7 gpm; 2 in required for 5.7-12.3 gpm. 2 in is the safe modeling default.",
      "buildingDrainIn": 3,
      "buildingDrainNote": "3 in is the minimum where any water closet discharges (Table P3005.4.1/.2); 4 in typical for larger homes or long DFU accumulations.",
      "ventStackIn": 3,
      "ventStackNote": "Every building drain needs at least one vent pipe to outdoors (P3102.1). Vents min 1.25 in and >= half the drain served (P3113.1); where 97.5% outside design temp <= 0F, roof penetrations must be >= 3 in (P3103.2), so 3 in is the standard main-stack model.",
      "minVentIn": 1.25,
      "ventMinFractionOfDrainServed": 0.5,
      "ventIncreaseOneSizeOverDevelopedFt": 40,
      "roofVentTerminationAboveRoofIn": 6,
      "coldClimateRoofVentMinIn": 3,
      "maxTrapArmFtBySize": { "1.25": 5, "1.5": 6, "2": 8, "3": 12, "4": 16 },
      "maxTrapArmFtBySizeUpc": { "1.25": 2.5, "1.5": 3.5, "2": 5, "3": 6, "4": 10 },
      "trapArmNote": "IRC Table P3105.1 (trap weir to vent fitting, at the slopes below). UPC Table 1002.2 is much stricter — use the UPC map in CA/WA/NV and other IAPMO states. Water closets (self-siphoning) are exempt from the distance limit in the IRC.",
      "slopeInPerFtBySize": { "1.25": 0.25, "1.5": 0.25, "2": 0.25, "2.5": 0.25, "3": 0.125, "4": 0.125 },
      "slopeNote": "IRC P3005.3: 1/4 in/ft for 2.5 in and smaller, 1/8 in/ft minimum for 3 in and larger (1/4 in/ft still preferred in practice for 3 in).",
      "trapSealMinIn": 2,
      "trapSealMaxIn": 4,
      "dfuByFixture": {
        "lavatory": 1,
        "kitchenSink": 2,
        "shower": 2,
        "bathtub": 2,
        "waterCloset1_6gpf": 3,
        "clothesWasherStandpipe": 2,
        "fullBathGroup": 5,
        "halfBathGroup": 4
      },
      "dfuNote": "IRC Table P3004.1 (water closets > 1.6 gpf count 4 DFU).",
      "maxDfuHorizontalBranchBySize": { "1.5": 3, "2": 6, "2.5": 12, "3": 20, "4": 160 },
      "maxDfuStackBySize": { "1.5": 4, "2": 10, "2.5": 20, "3": 48, "4": 240 },
      "maxDfuBuildingDrainBySizeAtQuarterInSlope": { "2": 21, "3": 42, "4": 216 },
      "maxDfuBuildingDrainBySizeAtEighthInSlope": { "3": 36, "4": 180 },
      "branchSizingNote": "IRC Tables P3005.4.1 and P3005.4.2. No water closet on pipe smaller than 3 in."
    },
    "supply": {
      "mainIn": 0.75,
      "mainNote": "IRC P2903.7: water service pipe minimum 3/4 in. 1 in common for 3+ bath homes (approximate practice).",
      "branchIn": 0.5,
      "branchNote": "Industry practice: 3/4 in trunk with 1/2 in branch runouts to individual fixtures; final sizing per IRC Table P2903.6 / Appendix AP.",
      "staticPressureMaxPsi": 80,
      "pressureNote": "IRC P2903.3.2: PRV required above 80 psi static. Minimum is functional (must meet Table P2903.1 fixture flow pressures); 40-60 psi is the usual design target (approximate).",
      "maxVelocityFpsPlastic": 12,
      "maxVelocityFpsMetallic": 8,
      "velocityNote": "IRC Table P2903.8.1 (manifold systems); commonly applied to all distribution piping.",
      "fixtureDesignFlow": {
        "lavatory": { "gpm": 0.8, "atPsi": 8 },
        "kitchenSink": { "gpm": 1.75, "atPsi": 8 },
        "shower": { "gpm": 2.5, "atPsi": 20 },
        "bathtub": { "gpm": 4, "atPsi": 20 },
        "waterClosetTank": { "gpm": 3, "atPsi": 20 },
        "hoseBibb": { "gpm": 5, "atPsi": 8 }
      },
      "fixtureDesignFlowNote": "IRC Table P2903.1 (flow rate at outlet / minimum flow pressure).",
      "waterHeaterClearanceIn": 30,
      "waterHeater": {
        "serviceSpaceDeepIn": 30,
        "serviceSpaceWideIn": 30,
        "garageIgnitionElevationIn": 18,
        "atticPassagewayMaxFt": 20,
        "atticPassagewayMinHighIn": 30,
        "atticPassagewayMinWideIn": 22,
        "accessDoorMinWideIn": 24,
        "note": "IRC M1305.1 (30x30 in level working space at control side), M1307.3 (ignition source 18 in above garage floor), M1305.1.2 (attic access). Combustion clearances per appliance listing.",
        "tpDischargeMaxAboveFloorIn": 6,
        "tpNote": "IRC P2803.1 requires a temperature-and-pressure relief valve on every storage water heater; P2803.6.1 pipes the discharge FULL SIZE, never trapped or threaded, terminating not more than 6 in above the floor / waste receptor / drain pan it serves.",
        "panMinDepthIn": 1.5,
        "panNote": "IRC P2801.6: a storage-tank water heater installed where leakage causes damage sits in a pan not less than 1.5 in deep, drained by a 3/4 in minimum pipe to an approved location (P2801.6.1/.2). Modeled 2 in deep.",
        "seismicStraps": {
          "count": 2,
          "zones": "upper third + lower third of the tank's vertical dimension",
          "lowerAboveControlsIn": 4,
          "note": "IRC P2801.8 (Seismic Design Categories D0-D2 — CA/WA/NV style amendments): strap the tank against horizontal displacement at points within the upper one-third AND lower one-third of its height; the lower strap at least 4 in above the controls. Not required in low-seismic jurisdictions."
        }
      }
    },
    "fixtureRoughIn": {
      "toiletCenterFromWallIn": 12,
      "toiletCenterFromWallNote": "Rough-in measured to FINISHED wall; 10 in and 14 in variants exist (horow.com rough-in guide).",
      "toiletSideClearanceIn": 15,
      "toiletFrontClearanceIn": 21,
      "adjacentFixtureCenterToCenterIn": 30,
      "clearanceNote": "IRC P2705.1: WC/lav/bidet centerline >= 15 in to any side wall/partition/vanity, >= 30 in center-to-center between fixtures, >= 21 in clear in front.",
      "toiletSupplyHeightIn": 7,
      "toiletSupplyOffsetFromCenterIn": 6,
      "lavDrainHeightIn": 19,
      "lavDrainHeightNote": "18-20 in above finished floor typical.",
      "lavSupplyHeightIn": 21,
      "lavSupplyHeightNote": "2-3 in above the drain centerline, ~4 in each side of center (8 in spread).",
      "lavSupplySpreadIn": 8,
      "lavRimHeightIn": 31,
      "lavRimHeightNote": "Industry practice 30-32 in for standard vanities; 'comfort height' up to 36 in. Not a code number.",
      "showerValveHeightIn": 44,
      "showerValveHeightNote": "Practice varies 38-48 in above finished floor (consulted guide cites ~48 in); 44 in is a common drafting default. Not code-mandated.",
      "tubValveHeightIn": 30,
      "tubValveHeightNote": "28-36 in typical for tub or tub/shower valve.",
      "showerHeadHeightIn": 80,
      "showerHeadHeightNote": "76-84 in typical; ~80 in standard.",
      "tubSpoutAboveRimIn": 5,
      "showerMinInteriorAreaSqIn": 900,
      "showerMinDimensionIn": 30,
      "showerMinNote": "IRC P2708.1: >= 900 sq in interior cross-section, >= 30 in minimum dimension, maintained to 70 in above the drain."
    }
  },
  "hvac": {
    "sizingRuleOfThumb": {
      "method": "ACCA Manual J (loads) + Manual S (equipment selection) — required by IRC M1401.3; everything below is a first-pass estimate only",
      "coolingSqftPerTon": 500,
      "coolingSqftPerTonNote": "Climate/vintage-dependent, roughly 400-700 sqft/ton for average existing homes; ENERGY STAR room-AC table works out to ~500-600 sqft/ton; well-insulated low-load homes exceed 1,000 sqft/ton (PNNL BASC). Manual J is the real method.",
      "coolingBtuPerSqft": 20,
      "coolingBtuPerSqftNote": "ENERGY STAR baseline ~20-24 Btu/h per sqft at 8-ft ceilings.",
      "heatingBtuPerSqft": { "warm": 35, "moderate": 42, "cold": 55 },
      "heatingBtuPerSqftNote": "Rule of thumb by US climate zone: hot 30-35, warm 35-40, moderate 40-45, cool 45-50, cold 50-60 Btu/h/sqft, +/-10% for insulation quality (inchcalculator.com). Manual J governs.",
      "equipmentSelectionBandPctOfLoad": { "min": 95, "max": 115 },
      "equipmentSelectionNote": "Manual S / PNNL BASC: OEM capacity at design conditions should be 95-115% of design load; do not oversize cooling (humidity control). Furnaces tolerate up to ~150% oversize."
    },
    "miniSplit": {
      "btuPerIndoorUnitTypical": [6000, 9000, 12000, 18000, 24000],
      "btuPerIndoorUnitNote": "Standard wall-cassette capacity steps across manufacturers (Mitsubishi, Fujitsu, Cooper&Hunter et al). 9k-12k covers most bedrooms; 12k-18k living rooms.",
      "mountHeightIn": 84,
      "mountHeightNote": "Wall units: ~6-8 ft above floor (84 in = 7 ft default), >= 3 in below ceiling (8+ in is better), >= 2 in side clearance (aircondlounge.com). Manufacturer install manual governs.",
      "ceilingClearanceMinIn": 3,
      "sideClearanceMinIn": 2,
      "placementNote": "Prefer a high interior position on an exterior wall (short refrigerant lineset) that does not blow directly on occupants; avoid enclosed/recessed pockets that starve return air.",
      "roomBtuBySqftEnergyStar": [
        { "maxSqft": 150, "btuPerHr": 5000 },
        { "maxSqft": 250, "btuPerHr": 6000 },
        { "maxSqft": 300, "btuPerHr": 7000 },
        { "maxSqft": 350, "btuPerHr": 8000 },
        { "maxSqft": 400, "btuPerHr": 9000 },
        { "maxSqft": 450, "btuPerHr": 10000 },
        { "maxSqft": 550, "btuPerHr": 12000 },
        { "maxSqft": 700, "btuPerHr": 14000 },
        { "maxSqft": 1000, "btuPerHr": 18000 },
        { "maxSqft": 1200, "btuPerHr": 21000 },
        { "maxSqft": 1400, "btuPerHr": 23000 },
        { "maxSqft": 1500, "btuPerHr": 24000 },
        { "maxSqft": 2000, "btuPerHr": 30000 },
        { "maxSqft": 2500, "btuPerHr": 34000 }
      ],
      "roomBtuNote": "ENERGY STAR room-AC sizing table (8-ft ceilings); usable as per-room mini-split head first guess before Manual J."
    },
    "attic": {
      "trunkAboveWallTopM": 0.3,
      "ceilingJoistDepthM": 0.24,
      "topPlateBandM": 0.09,
      "basis": "IRC R602.6 / R602.6.1: top plates may only be drilled/notched within strict limits (a plate bored more than 50% of its width requires a 0.054-in / 16 ga metal tie strapped across) — a supply duct never fits those limits, so ducts do NOT pass through top plates. IRC M1601 (duct installation) + residential practice: the supply trunk runs in the attic above the ceiling joists (~0.24 m deep 2x10s) and each supply register is a boot dropping through the CEILING plane like a recessed light. trunkAboveWallTopM = joist depth + working clearance above the tallest plate; topPlateBandM = the doubled 2x plate band [wall.height - 0.09, wall.height] no duct member may enter."
    },
    "ducted": {
      "cfmPerTon": 400,
      "cfmPerTonNote": "Industry baseline; ~350 in hot-humid (latent priority) to ~450 in hot-dry climates; manufacturers publish an acceptable 350-450 band (hvacprocalculator, MEP Academy, ACCA discussion).",
      "frictionRateInWgPer100ft": 0.1,
      "frictionRateNote": "Legacy default duct-sizing friction rate. Real Manual D computes friction rate from available static pressure — HVAC School's worked example lands at 0.05 in wg/100 ft on a low-static ducted unit.",
      "trunkSizeInByTons": {
        "2": [8, 14],
        "2.5": [8, 16],
        "3": [8, 18],
        "3.5": [8, 20],
        "4": [10, 20],
        "5": [10, 24]
      },
      "trunkSizeNote": "APPROXIMATE contractor convention: rectangular sheet-metal supply trunk [depth, width] in inches, derived from 400 cfm/ton at ~700-900 fpm trunk velocity. Manual D (and the actual blower's static) governs; taper trunk after each takeoff group.",
      "branchRoundIn": 6,
      "branchRoundNote": "6-in rigid round ~100-125 CFM is the standard bedroom branch. Flex duct needs upsizing at low static: HVAC School's Manual D example used 8-in FLEX for 100 CFM and 14-in flex for a 400 CFM return at 0.05 in wg/100 ft.",
      "registerCfmTypical": 100,
      "registerCfmNote": "Approximate: 75-150 CFM per supply register; big rooms get 2+ registers.",
      "supplyRegisterPerRoom": true,
      "supplyRegisterPlacement": "perimeter — floor or low wall under windows (heating-dominant) or ceiling/high wall (cooling-dominant)",
      "returnCentralOrPerFloor": true,
      "returnPathFromClosedRooms": "door undercut or transfer grille required so bedrooms can return air to a central hallway return",
      "returnNote": "One large central return per floor is standard practice; Manual D allows either central or per-room returns."
    },
    "ventilation": {
      "bathFanCfm": 50,
      "bathFanContinuousCfm": 20,
      "kitchenExhaustCfm": 100,
      "kitchenExhaustContinuousCfm": 25,
      "localExhaustNote": "IRC Table M1505.4.4: bathrooms 50 CFM intermittent / 20 continuous; kitchens 100 CFM intermittent / 25 continuous. Range hood = the usual kitchen intermittent device.",
      "rangeHoodCfm": 100,
      "rangeHoodMakeupAirThresholdCfm": 400,
      "rangeHoodMakeupAirNote": "IRC M1503.6: exhaust > 400 CFM requires makeup air where fuel-burning appliances are not direct-vent/mechanical-draft.",
      "bathFanDuctIn": 4,
      "bathFanDuctNote": "Approximate practice: 4-in round for 50-80 CFM fans, 6-in for 110-150 CFM; manufacturer static tables govern.",
      "dryerVentMaxFt": 35,
      "dryerVentDeductPer90ElbowFt": 5,
      "dryerVentIn": 4,
      "dryerVentNote": "IRC M1502.4.6.1: 35 ft max developed length from transition duct to terminal; Table M1502.4.6.1 deducts 5 ft per 4-in 90-degree elbow (2.5 ft per 45). 4-in smooth metal duct is standard.",
      "exhaustTerminationMinFtFromOpenings": 3,
      "exhaustTerminationMinFtFromMechanicalIntake": 10,
      "exhaustTerminationNote": "IRC M1504.3: >= 3 ft from property lines, operable windows/doors and gravity intakes; >= 10 ft from mechanical intakes unless 3 ft above them.",
      "wholeHouse": {
        "basis": "IRC Table M1505.4.3(1) — continuous whole-house mechanical ventilation, CFM",
        "floorAreaBandsMaxSqft": [1500, 3000, 4500, 6000, 7500, 999999],
        "bedroomBands": ["0-1", "2-3", "4-5", "6-7", ">7"],
        "continuousCfm": [
          [30, 45, 60, 75, 90],
          [45, 60, 75, 90, 105],
          [60, 75, 90, 105, 120],
          [75, 90, 105, 120, 135],
          [90, 105, 120, 135, 150],
          [105, 120, 135, 150, 165]
        ]
      }
    },
    "equipment": {
      "sizingCodeRequirement": "IRC M1401.3: equipment sized per ACCA Manual S from loads per ACCA Manual J (or other approved methods)",
      "serviceSpaceDeepIn": 30,
      "serviceSpaceWideIn": 30,
      "garageIgnitionElevationIn": 18,
      "atticAppliancePassagewayMaxFt": 20,
      "note": "IRC M1305.1 working space and access rules apply to furnaces/air handlers exactly as to water heaters."
    },
    "condenser": {
      "sqftPerTonByZoneBand": { "hot": 450, "mid": 550, "cold": 650 },
      "zoneBandNote": "ASSUMPTION (Manual J/S govern, M1401.3): rule-of-thumb cooling load divisor by IECC climate zone band — zones 1-2 (FL/TX/AZ style) ~1 ton/450 sqft, zones 3-4 ~1 ton/550 sqft, zones 5+ ~1 ton/650 sqft. Zone read from wall-assemblies.json exterior.stateClimateZone; unknown/INTL codes assume the mid band.",
      "maxTonsPerUnit": 5,
      "maxTonsPerUnitNote": "Residential split-system condensers top out around 5 tons — bigger loads take multiple outdoor units (unit count = ceil(totalTons/5)).",
      "minTons": 1.5,
      "padSideM": 0.95,
      "padThicknessM": 0.1016,
      "padNote": "4-in concrete equipment pad; unit anchored per IRC M1403 (heat pumps on a level base ~3 in above grade) + manufacturer clearance.",
      "unitDimsM": [0.9, 0.8, 0.35],
      "unitClearM": 0.6,
      "wallClearM": 0.3,
      "clearanceNote": "≥0.6 m clear between units, ≥0.3 m off the wall face — per mfr clearance + IRC M1403.",
      "linesetSuctionDiaM": 0.019,
      "linesetLiquidDiaM": 0.0095,
      "linesetHeightM": 0.4,
      "linesetMaxLenAdvisoryM": 15,
      "linesetLateralM": 0.035,
      "linesetLateralNote": "Cross-trade lateral: the pair rides 3.5 cm off the wall centerline (across-wall) so it never shares the plumbing plane (supply risers + DWV stack stand on the centerline; night-5 D2 trade-skin convention). Thin walls clamp the offset so the OUTBOARD suction riser (the liquid riser rolls back toward the plumbing plane) keeps its full section inside the wall body — clamped runs carry a coordination flag.",
      "linesetNote": "Refrigerant line-set per unit: suction 3/4 in (19 mm, insulated — the COLD line) + liquid 3/8 in (9.5 mm, warm) as a parallel pair, through ONE exterior-wall penetration at ~0.4 m (clear of ROs), then wall-following to the indoor air handler coil (M1411). Runs over ~15 m carry a 'verify manufacturer max line-set length / oil return' advisory — typical residential mfr charts top out 15-30 m; the manufacturer's line-set table governs.",
      "disconnectAboveUnitM": 0.3,
      "disconnectNote": "NEC 440.14: disconnecting means within sight of the unit — wall-mounted beside each condenser; the dedicated branch circuit is routed separately."
    }
  },
  "placementHeuristics": {
    "wetWall": "Share one 2x6 plumbing wall between back-to-back wet rooms (bath/bath, bath/kitchen, bath/laundry); stack wet rooms vertically across floors so one DWV stack serves all.",
    "ventStack": "Main 3-in stack rises in the wet wall of the primary bath straight through the roof; terminate >= 6 in above roof (IRC P3103.1) and >= 3 in diameter in cold climates.",
    "buildingDrain": "Lowest run under slab / in crawlspace, falling at 1/4 in/ft (1/8 in/ft ok for 3in+) toward the street/septic side of the foundation.",
    "waterHeater": "Garage, basement, or utility closet near the wet core; 30x30 in service space in front, 18 in ignition elevation in garages, drain pan when above living space.",
    "airHandlerFurnace": "Central closet, basement, garage or attic near the geometric center of the duct network to minimize trunk length.",
    "supplyTrunk": "Rectangular trunk down the hallway spine — dropped hallway ceiling chase, open-web floor trusses, or basement/crawl ceiling — tapering after takeoffs.",
    "supplyRegisters": "One per habitable room at the perimeter, ideally under/near windows (counteracts envelope loads).",
    "returnGrilles": "One central return per floor in the hallway; transfer path (undercut/grille) from every closable room.",
    "condenser": "Outdoor pad on a side/rear wall, short lineset to air handler; respect manufacturer clearances and local setback rules.",
    "dryerVent": "Laundry on an exterior wall if possible; shortest straight 4-in run, budget 5 ft of the 35-ft allowance per 90-degree elbow.",
    "rangeHood": "Duct up through cabinet+roof or straight out the exterior wall behind the range; makeup air needed above 400 CFM.",
    "miniSplitHeads": "High on wall ~84 in AFF, >= 3 in from ceiling, exterior wall preferred for short linesets; one head per major zone using the roomBtuBySqftEnergyStar table."
  }
}