OBC HVAC - House common mistakes
OBC HVAC - House · Understand the traps and how to avoid them.
Treating Part 9 as a complete stand-alone code for every house HVAC item and never checking whether a Part 6 provision or referenced standard has been triggered for the specific house system.
Candidates memorize Part 9 sections and assume Part 6 is only for larger buildings, then miss house-specific Part 6 cross-references and referenced standards.
What to do instead
Before designing or reviewing, classify the building and each HVAC element, then trace Part 9 applicability and any Part 6 cross-reference; keep a checklist showing which Part and referenced standard governs each item.
Proceeding with an HVAC alteration or replacement without confirming permit triggers and required inspection stages, then concealing work before the inspector sees it.
Candidates assume like-for-like HVAC work is exempt or rely on verbal assurances from the contractor instead of checking the local permit process.
What to do instead
Verify permit and inspection requirements before work starts; list inspection hold points such as rough-in, venting, and final; schedule inspections before concealment and record the results.
Sizing heating or cooling equipment from a rule of thumb, the old nameplate, or the largest room rather than a room-by-room load calculation.
Time pressure and field habit encourage quick replacement sizing, especially when the existing system 'worked'.
What to do instead
Perform a documented heat-loss and heat-gain calculation using the OBC-referenced method, including envelope areas, orientation, infiltration, ventilation loads, and duct gains/losses; keep the calculation in the project file.
Providing combustion air by relying on loose construction or by taking air from a bedroom, bathroom, or contaminated area instead of a calculated permanent opening.
Candidates misunderstand natural infiltration and assume a basement window or gaps are adequate combustion air.
What to do instead
Calculate the required combustion-air free area for the appliance input, provide permanent openings or a ducted supply from an acceptable location, keep intakes clear of contaminants, and document the calculation on the drawing.
Locating a vent terminal by eye or by the old installation instead of measuring the required clearances to windows, doors, air intakes, combustibles, and grade.
Field convenience and unfamiliarity with manufacturer-certified clearances lead to 'close enough' placement.
What to do instead
Use the appliance manufacturer's certified venting instructions and the OBC minimum clearances; measure each terminal, record dimensions on the site review, and relocate before final inspection if any clearance is short.
Routing a duct through or too close to a fire separation or combustible assembly without recognizing the need for a fire damper, firestop, or listed clearance.
Candidates focus on airflow and ignore the rating of the wall or floor assembly the duct crosses.
What to do instead
Identify the fire-resistance rating of each penetration, select a listed fire damper or firestop system appropriate to the assembly, provide required access, and inspect the seal before concealment.
Installing supply ducts, plenums, or flue connectors at less than the listed clearance to combustible material because the mechanical space is tight.
Candidates prioritize fitting equipment into a cramped room and do not check the listing or code clearance table.
What to do instead
Check the appliance and duct listing for required clearances; maintain the greater of the listed and code clearance; use an approved heat shield only where permitted; photograph and record clearances at rough-in.
Treating the HVAC system as independent of the building envelope and failing to seal duct, vent, and service penetrations or to maintain insulation and air-barrier continuity.
Candidates draw a line between mechanical and envelope work and do not coordinate trade sequencing.
What to do instead
Mark every penetration on the envelope drawings; require sealed, insulated, and air-barrier-compatible details; inspect before drywall and confirm duct insulation in unconditioned space.
Confusing local intermittent exhaust with whole-house ventilation and never commissioning or balancing the HRV or ventilation system.
Candidates equate a bathroom fan with the code's ventilation requirement and skip airflow measurement.
What to do instead
Provide the required whole-house ventilation method, balance the HRV or ventilation system, measure exhaust and supply airflows, record the results, and confirm controls operate as intended.
Submitting drawings or specifications that omit equipment capacity, model, clearances, venting arrangement, controls, and inspection records, then relying on site judgment to fill the gaps.
Documentation is deferred until the end and is treated as paperwork rather than part of the code compliance evidence.
What to do instead
Prepare a complete HVAC schedule and drawings before installation; include load calculation summary, equipment data, clearances, venting, combustion air, controls, and inspection hold points; update as-built records at completion.