HVAC / Mechanical Rough Install

TLDR

• Pull mechanical & gas permits BEFORE start of rough
• Collect BTU, CFM, amp specs for every appliance
  • Including locations where utilities enter appliance
  • Range, oven, water heater, furnace, mini split, boiler, air conditioner, dryer, grill, pool heater, garage heater, grill, hood, bath fan, ERV, dehumidifier, fireplace, generator
• Don't forget thermostat locations
• Schedule electrician to pre-rough circuits in some cases
• Mark all terminations 3 ft from windows, 10 ft from intakes (2015 IRC M1506.3)
  • Install siding blocks or terminations if needed
• No return air from baths / closets (M1602.2)
• Exhaust hood > 400 cfm requires interlocked make-up air (M1503.6)
• Gas pressure test ≥10 psi × 15 min before

Overview

This page guides the builder through rough-in coordination for forced-air, hydronic (boiler + radiator & radiant-floor), mini-split, and high-velocity retrofit (Unico) systems as well as exhaust fans and appliance preparation. Scope covers layout, penetrations, clearances, pressure tests, and cross-trade hand-offs, not fabrication details.

Prerequisites

• Pull mechanical and gas permits
• Create appliance schedule - collect BTU, CFM, amp draw:
  • Gas: furnace, boiler, water heater, fireplace, range/oven, dryer, exterior grill, generator, pool heater, garage heater
  • Power: furnace blower, boiler pumps, mini-split indoor/outdoor units, ERV/HRV, dehumidifier, radiant actuators or pumps, range, hood, dryer, stove, oven, air conditioner, garage heater, bath fan
  • Water: boiler make-up, condensate, radiant supply/return
• Framing, fire-blocking, soffits complete; equipment pads framed/poured.
  • Walls thickened or furred out for duct runs where needed.
• All termination exit points marked with 3 ft / 10 ft clearances.
• Electrician coordinated for power to equipment and to install bath fan.
• Temporary heat plan (electric heaters or early-start furnace).
• Subfloor removed for remodel situations

Process

1. Layout - Walk job with trades; confirm supply/return grilles, split heads, bath-fans, ERV ports, boiler, outdoor unit, radiant manifolds, termination locations. Plan for supplies to wash windows and doors.
2. Core & Frame Penetrations - Drill/rotary-core all ducts, vents, line-sets
3. Install Ducts, Tubing, or Splits - No returns from bathrooms (M1602.2)
4. Combustion / Exhaust Terminations - Follow 3 ft window & 10 ft intake rule (M1506.3)
5. Fresh-Air vs. ERV Ducts - Provide dedicated 6 in insulated duct for make-up/fresh-air where ERV not used; interlock hoods > 400 cfm (M1503.6)
6. Install Bath Fan and Hood Ducts
7. Install Dryer Exhaust
8. Install Roof/Wall Caps & Flashings
9. Install Appliance Gas Lines
10. Condensate & Dehumidifier Drains. Trap & slope ¼ in / ft to pump or daylight.
11. Seal and Mastic, Check for Leaks

Inspections

• Gas Pressure Test - 10 psi for ≥ 15 min witnessed or timestamped photo.
• Mechanical Rough - Duct sizes, strap spacing, vent/intake clearances, fresh air in.

Client Interaction / Communication Notes

• Confirm thermostat locations, grille style / color, mini-split head positions early.
• Explain make-up-air damper and why large range hoods trigger extra duct work.
• Advise that exercising old radiant valves during purge may reveal leaks; repairs not in base scope.
• Warn of temporary heat and lack of cooling during rough phase.

Common Issues & How to Avoid Them

• Forgotten make-up air - Flag any hood > 400 cfm during design review.
• Unsealed roof caps - Assign roofer vs. HVAC responsibility before drilling.
• Old valves leak after cycling - Note pre-existing condition in contract; treat repair as change order.
• Termination clearances — Exhaust, dryer, bath-fan, and dehumidifier outlets must end ≥ 3 ft from windows/doors and ≥ 10 ft from any mechanical intake unless 3 ft above that intake (M1506.3).

Gas Pipe Sizing Reference

Resources