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MEP (Plumbing + HVAC) — research notes for the Bones inference engines

Status: researched 2026-08-13 against the 2021 IRC (read via the Pennsylvania adoption on UpCodes, which carries the model-code text for these chapters) plus industry references. Companion dataset: data/mep-rules.json.

Disclaimer: drafting aid, not engineering. Everything here is typical/approximate and meant to let a plugin draw a plausible system. Real designs need a licensed plumber/HVAC designer, ACCA Manual J/S/D calculations, and the local authority having jurisdiction (AHJ). Jurisdictions on the UPC (IAPMO — most of the US West) differ materially from the IRC/IPC numbers below; the biggest geometric difference (trap-arm length) is called out explicitly.

1. Code landscape (what governs what)

  • IRC Part VII, chapters 25–33 is the residential plumbing code in IRC states: ch. 27 fixtures, ch. 28 water heaters, ch. 29 water supply, ch. 30 sanitary drainage, ch. 31 vents, ch. 32 traps. (IRC ch.30, ch.31, ch.29)
  • IRC Part V, chapters 12–24 is the residential mechanical code: ch. 13 general (access/clearance), ch. 14 heating/cooling equipment, ch. 15 exhaust, ch. 16 ducts. (IRC ch.15, ch.14, ch.13)
  • UPC/IMC states: California, Washington, Oregon, Nevada, etc. use the IAPMO UPC for plumbing. Values differ (see §2.4). Flag jurisdiction in the plugin UI.
  • HVAC design proper is not in the code text: the code (IRC M1401.3) delegates to ACCA Manual J (room-by-room loads), Manual S (equipment selection), Manual D (duct design). Exact M1401.3 text: "Heating and cooling equipment and appliances shall be sized in accordance with ACCA Manual S or other approved sizing methodologies based on building loads calculated in accordance with ACCA Manual J..." (IRC ch.14)

2. Plumbing — DWV (drain, waste, vent)

2.1 Fixture drain and trap sizes (IRC Table P3201.7)

Fixture Min trap/drain Notes
Lavatory 1¼ in
Kitchen sink 1½ in often run as 2 in in practice for grease margin (approximate)
Bathtub 1½ in
Shower 2 in 1½ in only allowed for ≤ 5.7 gpm flow; model 2 in
Clothes washer standpipe 2 in
Water closet 3 in via Table P3005.4.1 — no WC on pipe < 3 in

Trap seal 2–4 in deep (P3201.2). Source: IRC ch.32.

2.2 Slope, branch and building-drain capacity

  • Slope (P3005.3): 2½ in and smaller → ¼ in/ft (2%); 3 in and larger → ⅛ in/ft (1%) minimum. An engine should drop drain elevation by run_ft × slope when routing.
  • DFU loads (Table P3004.1): lav 1, kitchen sink 2, shower 2, tub 2, WC (1.6 gpf) 3, full-bath group 5, half-bath group 4, washer standpipe 2.
  • Capacity (Tables P3005.4.1/.4.2): horizontal branch: 1½ in → 3 DFU, 2 in → 6, 3 in → 20, 4 in → 160. Stack: 3 in → 48. Building drain at ¼ in/ft: 3 in → 42 DFU, 4 in → 216 — i.e. a 3-in building drain covers essentially any ≤ 3-bath house; use 4 in beyond that.
  • Source: IRC ch.30.

Engine sizing algorithm: sum DFU downstream of each node → pick smallest pipe whose table capacity ≥ sum → never decrease size downstream → force ≥ 3 in once a WC is upstream.

2.3 Venting and trap arms (IRC ch. 31)

  • Every trap needs a vent within the trap-arm limit (Table P3105.1, at the slopes above): 1¼ in → 5 ft, 1½ in → 6 ft, 2 in → 8 ft, 3 in → 12 ft, 4 in → 16 ft. WCs (self-siphoning) are exempt from the distance limit in the IRC.
  • Vent size: ≥ half the drain served, min 1¼ in (P3113.1); +1 pipe size if developed length > 40 ft.
  • At least one vent pipe must extend to outdoors per building drain (P3102.1). Roof termination ≥ 6 in above the roof (or snow line, P3103.1); where the 97.5% design temp ≤ 0 °F, roof penetrations must be ≥ 3 in diameter (frost closure, P3103.2) — so a full-height 3-in stack is the correct default model everywhere.
  • Source: IRC ch.31.

2.4 UPC difference that changes geometry

UPC Table 1002.2 trap arms are far shorter: 1¼ in → 2 ft 6 in, 1½ in → 3 ft 6 in, 2 in → 5 ft, 3 in → 6 ft, 4 in → 10 ft (HQ Plumbing code library). In UPC states fixtures must sit much closer to a vented wall — the dataset carries both maps (maxTrapArmFtBySize vs maxTrapArmFtBySizeUpc).

2.5 Water supply (IRC ch. 29)

  • Service: min ¾ in (P2903.7); 1 in typical for larger homes (approximate).
  • Distribution: industry practice is a ¾ in trunk (or manifold) with ½ in branches to fixtures; formal sizing per Table P2903.6 / Appendix AP.
  • Pressure: max 80 psi static (PRV above that, P2903.3.2); minimum is "meets fixture flow pressures" per Table P2903.1 (e.g. shower 2.5 gpm @ 20 psi, WC tank 3 gpm @ 20 psi, lav 0.8 gpm @ 8 psi). 40–60 psi is the usual design band (approximate).
  • Velocity caps (Table P2903.8.1, manifolds): 12 ft/s plastic, 8 ft/s metallic.
  • Source: IRC ch.29.

2.6 Fixture clearances and rough-in dimensions

Code (IRC P2705.1, P2708.1 — ch.27):

  • WC/lav/bidet: ≥ 15 in centerline to side wall/vanity, ≥ 30 in center-to-center between fixtures, ≥ 21 in clear in front.
  • Shower: ≥ 900 in² interior, ≥ 30 in min dimension, maintained to 70 in above drain.

Industry rough-in (approximate — not code; horow.com guide):

  • Toilet flange 12 in from finished wall (10/14 in variants); supply ~6 in left of centerline, ~7 in AFF.
  • Lav drain 18–20 in AFF, supplies 2–3 in above drain at 8-in spread; rim 30–32 in (vanity-dependent).
  • Shower valve 38–48 in AFF (guide cites ~48; 44 is a common drafting default), head ~80 in; tub valve 28–36 in, spout 4–6 in above rim.

2.7 Wet walls (where plumbing lives)

A wet wall is a 2x6 (or double-2x4) wall carrying the stack, vents and supplies. 2x4 = 3.5 in actual is too shallow for a 3-in DWV stack, which needs ~3.5 in clearance at fittings; 2x6 gives 5.5 in (washingtonplumbers.net). Stud boring limits (~40% bearing/60% non-bearing) reinforce this. Steel shield plates required where pipes pass < 1.5 in from the stud face (IRC P2603.2.1).

2.8 Water heater placement

30 in × 30 in level working space at the control side (M1305.1); ignition source 18 in above garage floor (M1307.3); attic installs need a 30×22 in passageway ≤ 20 ft with flooring (M1305.1.2); drain pan when above finished space (P2801). Source: IRC ch.13.

3. HVAC

3.1 Sizing (all rules of thumb — Manual J/S is the code-required method)

  • Cooling: baseline ~20 Btu/h·ft² ⇒ ~500 ft²/ton (range 400–700 by climate/vintage). ENERGY STAR's room-AC table (e.g. 450–550 ft² → 12,000 Btu/h) confirms ~500–600 ft²/ton for average envelopes (energystar.gov); PNNL notes well-built low-load homes exceed 1,000 ft²/ton (basc.pnnl.gov).
  • Heating: 30–60 Btu/h·ft² by climate zone — hot 30–35, warm 35–40, moderate 40–45, cool 45–50, cold 50–60; ±10% for insulation (inchcalculator.com).
  • Selection band: Manual S / PNNL — capacity at design conditions should be 95–115% of design load; never oversize cooling (humidity control fails); furnaces tolerate ~150% (basc.pnnl.gov).

3.2 Ducted systems (Manual D territory)

  • Airflow: 400 CFM/ton nominal; ~350 hot-humid, ~450 hot-dry; manufacturers publish a 350–450 band — check the blower table (hvacprocalculator, MEP Academy).
  • Friction rate: legacy default 0.1 in wg/100 ft; real Manual D derives it from available static — HVAC School's worked example computes 0.05 in wg/100 ft on a 0.2 in wg low-static unit and sizes 8-in flex per 100 CFM branch, 14-in flex for a 400 CFM return (hvacrschool.com). Rigid round runs smaller: 6-in ≈ 100–125 CFM is the classic bedroom branch (approximate).
  • Trunks (approximate contractor convention, derived from 400 CFM/ton at ~700–900 fpm): 2 ton ≈ 8×14 in, 3 ton ≈ 8×18 in, 4 ton ≈ 10×20 in; taper after takeoffs. Flagged: no single primary source — Manual D governs.
  • Registers/returns: one supply register per habitable room at the perimeter (under windows for heating climates); one large central return per floor with door undercuts/transfer grilles from closable rooms (Manual D practice, basc.pnnl.gov).

3.3 Ductless mini-splits

  • Wall-cassette capacity steps: 6k / 9k / 12k / 18k / 24k Btu/h; 9–12k for bedrooms, 12–18k living areas (manufacturer lineups; ENERGY STAR table above for per-room selection).
  • Mounting: ~7 ft (84 in) AFF, ≥ 3 in below ceiling (8+ in better), ≥ 2 in side clearance; exterior wall shortens linesets; don't blow directly on beds/occupants; avoid recessed pockets that starve return air (aircondlounge.com).

3.4 Ventilation & exhaust (IRC ch. 15 — hard code numbers)

Item Value Section
Bath fan 50 CFM intermittent / 20 continuous Table M1505.4.4
Kitchen exhaust (range hood) 100 CFM intermittent / 25 continuous Table M1505.4.4
Range hood makeup air trigger > 400 CFM M1503.6
Dryer duct 35 ft max developed length, −5 ft per 4-in 90° elbow, 4-in duct M1502.4.6.1
Exhaust termination ≥ 3 ft from openings/property line, ≥ 10 ft from mechanical intakes M1504.3
Whole-house ventilation 30–165 CFM continuous by area × bedrooms Table M1505.4.3(1) — full matrix in dataset

Source: IRC ch.15.

3.5 Equipment access

Same M1305.1 rules as water heaters: 30×30 in working space, 24-in access doors, 18-in garage ignition elevation, attic passageway limits (IRC ch.13).

3.6 Ducts vs. the structure — why the trunk lives in the ATTIC (prod report 2026-08-16)

A user's production scene showed the supply trunk boring straight through wall top plates. That is not a thing framers or mechanicals do, and the code chain says why:

  • IRC R602.6 (drilling/notching of studs) + R602.6.1 (top plate): where a top plate is cut, drilled or notched more than 50 % of its width, a galvanized metal tie not less than 0.054 in thick (16 ga) and 1½ in wide must be fastened across the opening with eight 10d nails each side. Those limits exist for pipes — a 14×8 in (or even 6 in round) supply duct can never satisfy a plate-boring limit, so ducts simply do not pass through top plates. (IRC 2021 ch. 6, wall construction — same UpCodes source family as the rest of this file.)
  • IRC M1601 (duct systems): duct installation rules (M1601.4 support/joints; R-value per N1103.3 when outside conditioned space). Nothing in M1601 offers a plate-penetration path; installation guidance assumes ducts run in open framing cavities: attics, floor trusses, dropped chases.
  • Residential practice (Manual D guidance, §4 item 6 above): single-storey slab homes run the rectangular trunk in the attic above the ceiling joists, branches tee off at attic elevation, and each supply register is a ceiling boot dropping through the ceiling plane between joists — geometrically identical to a recessed-light rough-in. Two-storey supply alternatives (floor-truss webs, dropped hallway soffits) still never cross a plate: they run below it or above it.

Engine model (dataset hvac.attic): trunk + branches route at wall.height + trunkAboveWallTopM (0.30 m ≈ ceiling-joist depth 0.24 m + clearance); supply registers sit at the ceiling plane (meta ceiling:true) fed by short vertical boots; bath/dryer exhaust exits through a stud bay below the plate band. Gate: no duct-run member's OBB may enter any wall's top-plate band [wall.height − topPlateBandM, wall.height] (0.09 m ≈ doubled 2x plate).

4. Spatial placement — how an inference engine should lay this out

This is the "WHERE" logic. Items 1–4 are strong industry conventions; code cites are noted.

  1. Find the wet core. Cluster wet rooms (baths, kitchen, laundry). The shared wall between back-to-back fixtures (bath/bath, bath/kitchen, bath/laundry) becomes the 2x6 wet wall. Multi-storey: prefer stacking wet rooms vertically so one DWV stack serves all floors.
  2. Vent stack: place the 3-in stack in the main bath's wet wall, behind/near the WC (shortest trap arms), rising straight through the roof, terminating ≥ 6 in above it (P3103.1). Secondary wet areas beyond trap-arm reach (§2.3/2.4) get their own 1½–2 in vents, which may re-join the stack ≥ 6 in above the highest fixture flood rim or exit the roof separately.
  3. Drains fall downhill: route fixture drains within trap-arm limits to the wet wall, drop down the stack, then run the building drain at ¼ in/ft (⅛ in/ft ok at 3 in+) under the slab / along the basement ceiling toward the street or septic side. Every horizontal length costs elevation — check crossings with footings/beams.
  4. Supply tree: ¾-in main enters near the water heater location; ¾-in hot+cold trunks chase alongside the DWV in the same wet walls/joist bays; ½-in branches to each fixture group. Keep hot runs short: put the water heater adjacent to the wet core (garage/basement/utility closet), 30×30 in clear in front, 18 in ignition lift in garages.
  5. Air handler/furnace near the duct centroid: central closet, basement, garage, or attic. Attic installs must satisfy the M1305.1.2 passageway.
  6. Supply trunk down the hallway spine: rectangular trunk in a dropped hallway ceiling chase, open-web floor trusses, or under the basement ceiling — hallways run down the middle of houses, which minimizes branch lengths and keeps the soffit out of rooms. Taper the trunk after each group of takeoffs.
  7. Branches & registers: 6-in round (8-in flex) per room to a perimeter register under/near windows; 2+ registers for large rooms (> ~150 CFM).
  8. Returns: one big central hallway return per floor (filter grille), plus a return path (undercut/transfer grille) from every closable room.
  9. Condenser/heat-pump outdoor unit: side/rear exterior pad, minimizing lineset to the air handler; keep manufacturer clearances and local setbacks.
  10. Point exhausts are exterior-wall seekers: dryer (35-ft budget, elbows expensive) wants the laundry on/near an exterior wall; range hood ducts up through the roof or straight out behind the range; bath fans duct up to roof caps or out gable/soffit-adjacent wall caps, ≥ 3 ft from openings (M1504.3).
  11. Mini-split alternative: if no duct network, place one head per thermal zone (ENERGY STAR table per room), high on an exterior wall ~84 in AFF, linesets dropping outside to a shared outdoor unit.

5. What is code vs. what is approximate

Hard code numbers (IRC 2021): trap/drain sizes, DFU tables, slopes, trap-arm lengths, vent min sizes, roof termination, 15/21/30-in fixture clearances, 900 in² shower, ¾-in service, 80 psi max, 50/100 CFM exhaust, 400 CFM makeup-air trigger, 35-ft dryer duct, whole-house ventilation matrix, 30×30 service space, 18-in garage elevation.

Approximate / practice (flag in UI): 2x6 wet wall, ½-in branch convention, rough-in heights (valve 38–48 in etc.), all ft²/ton and Btu/ft² sizing, 400 CFM/ton, 0.1 friction rate, trunk-by-tonnage table (weakest-sourced item in the dataset), register CFM, mini-split mount height, 4-in bath-fan duct, every placement heuristic in §4.

Known jurisdiction deltas: UPC trap arms (§2.4); UPC also requires different venting details and (in CA) water-heater seismic strapping — not modeled. IPC (non-IRC states) trap arms match IRC values (IPC Table 909.1).

Sources

All consulted 2026-08-13: