Every rule in 9.32.3 exists because modern Ontario houses are built tight — too tight to ventilate themselves. The draftiness that kept a 1950s bungalow dry in January is gone from a 2026 build, and with it the passive air swap that used to happen whether you wanted it or not. 9.32.3 fills that hole with a mechanical system plus room-by-room exhaust fans that cover the moisture and contaminant peaks the principal fan cannot chase fast enough.
Three problems pile up in an airtight house with no mechanical ventilation. The first is moisture: a family of four generates roughly 14 litres of water vapour per day through breathing, cooking, and showering. Without a fan to push it outside, that vapour condenses on every surface cooler than the dew point — window frames in January, the cold corner behind a closet, the underside of roof sheathing in a cathedral ceiling. Mould follows wherever the liquid lingers. The second is indoor air quality — carbon dioxide from occupants climbs past 1 200 ppm in a few hours in a sealed bedroom, cooking produces nitrogen dioxide and fine particulate, and new finishes off-gas formaldehyde and VOCs for months. None of that self-ventilates out of a tight house. The third is radon — the inert radioactive gas that seeps up from Canadian Shield bedrock and accumulates in basements, which the mechanical ventilation system dilutes as a side effect of meeting the 9.32.3.3 rate.
Natural ventilation — open a window — is how houses ventilated for two hundred years and it is legally useless under the current OBC for a new dwelling. The airflow is stochastic, it stops entirely when doors and windows are closed, and it throws away every joule of heat in the exhaust stream. 9.32.3.3 requires a mechanical fan, full stop. Exhaust-only ventilation — a continuous bathroom fan running 24/7 that sucks stale air out, with outdoor air bleeding in through envelope leaks — is what most 1990s homes have and what 9.32.3.6 permits under narrow conditions. It works in a leaky house because the leaks provide the makeup air; it fails in a modern tight house because there is no leak path for the replacement air to arrive, so the fan just depressurizes the house until flow backs down to almost zero. Balanced ventilation — an HRV or ERV — uses two fans moving equal volumes of air in opposite directions through a heat-exchange core, recovering 60 to 80 percent of the heat in the outgoing stream. Indoor pressure stays neutral, the house gets full design flow in any weather, and the energy penalty drops by a factor of four compared to exhaust-only. Under OBC 9.36 and SB-12 (2024), the prescriptive energy packages assume an HRV; exhaust-only rarely meets the energy code on a new Part 9 build.
Table 9.32.3.3 sets the minimum continuous capacity by bedroom count — 16 L/s for a 1-bed up to 30 L/s for a 5-bed. Most installations land at the minimum because the HRV model selected hits the required flow on its lowest continuous speed. Intakes must draw from kitchens, bathrooms, and water-closet rooms — the three places moisture and contaminants originate — located in the ceiling or on the wall at least 2 metres above the floor per 9.32.3.3.(10). Ducting runs short and straight. Every 90-degree elbow costs roughly 6 equivalent metres of static pressure, and every metre of flex duct reduces delivered flow 20 to 30 percent compared to sealed rigid sheet metal. The principal fan is controlled by a manual switch in the living area clearly marked VENTILATION FAN per 9.32.3.3.(5). Sound matters — a fan less than 1 metre from finished interior surfaces must meet the sone rating in Table 9.32.3.10.-B, typically 1.0 sone or quieter. A noisy fan gets switched off by the occupant, and a switched-off ventilation system is worse than no ventilation system at all.
9.32.3.7.(4) gives you two legal options. The intermittent option — a 50 CFM (25 L/s) fan on a wall switch, run by the occupant during and after showers — is simple, cheap, well-understood. The downside is humans forget to turn it on or off; moisture gets ahead of the fan before the shower ends, and the fan runs for hours after because someone walked away without flicking the switch. The continuous option — a 21 CFM (10 L/s) fan running 24/7, either as its own fixture or as a branch of the HRV — skips the human-reliability problem entirely. Most modern Ontario builds run the HRV in continuous mode with a boost switch in the bathroom: 10 L/s baseline, 20-minute 25 L/s burst during a shower, back to baseline. That combination meets the code both ways and produces the driest bathroom.
9.32.3.7.(1) sets the kitchen minimum at 50 L/s (about 106 CFM). Every residential range hood sold in Canada exceeds this — a basic 200 CFM builder-grade microwave hood is 2x the minimum, and enthusiast kitchens routinely spec 600 to 1200 CFM hoods. 9.32.3.8 adds a second layer: once mechanical exhaust exceeds the Table 9.32.3.3 maximum AND the dwelling contains a non-direct-vented fuel-fired appliance, a dedicated makeup-air fan is required, rated within plus-or-minus 10 percent of the exhaust rate. The practical rule every AHJ applies — regardless of appliance type — is that once the hood exceeds roughly 75 L/s (159 CFM) of continuous exhaust, makeup air becomes necessary to prevent door-slam, window whistle, and exfiltration through every envelope detail. Recirculating hoods do not satisfy 9.32.3.7 — they exhaust nothing to outdoors. Specify a ducted hood with a short run to a wall cap, and size makeup air at purchase time, not at final inspection.
Three failures drive most AHJ rejections. First, undersized principal fan — someone picks the minimum rate as the HRV capacity, not realizing the nameplate value is at 100 Pa static pressure with a clean filter and zero duct losses. Once ductwork, filter, and defrost cycle are factored in, delivered flow drops 20 to 40 percent. Specify the HRV from a manufacturer's table at actual system static pressure, not the marketing sheet. Second, shared laundry and bathroom exhaust — builders sometimes duct a laundry room into the bathroom fan on the theory that both generate moisture. 9.32.3.7.(4) does not recognize that; the bathroom gets its own fan, and a dedicated laundry room most AHJs treat as needing its own 25 L/s intake. Third, HRV without balancing — an HRV commissioned without measuring supply and exhaust flows drifts 20 to 50 percent out of balance within a year. Unbalanced HRV depressurizes or pressurizes the house and drives electricity cost to where most owners switch it off. Hire an HRAI-certified installer for commissioning — the small extra cost pays back in a single heating season.