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How VAV systems handle Canadian climate extremes
A variable air volume system serves multiple zones from one air handling unit. The AHU maintains a constant cold supply air temperature, typically 55°F, and each VAV terminal box throttles airflow up or down to match the zone's actual load at any moment. On a hot July afternoon in Toronto, a south-facing conference room might need its box wide open while a north-facing interior office needs only 30% of its maximum flow.
Canadian office buildings face a specific challenge: exterior zones need heating in winter while interior zones with high occupancy and equipment loads may need cooling year-round. A reheat VAV box solves this. When the exterior zone drops below setpoint in January, the box holds at minimum CFM and the hot water or electric coil heats that small volume of cold supply air to whatever discharge temperature the zone needs. The result is precise zone control without overcooling or reheating waste beyond what that single zone requires.
The relationship between max CFM, min CFM, and diversity
The max CFM for each box gets sized from peak zone load. But not every zone peaks at the same time. A VAV system with 20 zones rarely needs all 20 at their maximum simultaneously. This diversity factor, typically 0.7 to 0.85 for commercial office buildings, means the AHU can be sized smaller than the sum of all box maximums. Use the VAV system diversity calculator after sizing individual boxes to determine the correct AHU capacity.
Minimum CFM and the ventilation trap
Setting a minimum CFM floor that satisfies ASHRAE 62.1 is not optional. If the minimum CFM drops below the ventilation requirement during low-load periods, occupants receive inadequate fresh air. This is a code violation in all Canadian provinces that reference ASHRAE 62.1 or the National Building Code ventilation provisions. The minimum CFM must be at least equal to the ASHRAE 62.1 zone minimum air flow, calculated as the sum of the people component (0.15 L/s per person for offices) and the area component (0.3 L/s per square metre). Enter occupant count and floor area above to have the calculator check this automatically.
Frequently Asked Questions
Size the VAV box maximum CFM from the zone peak sensible cooling load divided by 1.08 times the supply-to-room temperature difference. In Canadian offices with a 55°F supply air temperature and a 72°F room setpoint, the delta-T is 17°F. The minimum CFM must satisfy ASHRAE 62.1 ventilation requirements, typically 5 CFM per person plus 0.06 CFM per square foot. Select a box whose maximum flow rating matches your calculated maximum and whose minimum controllable flow can reach the ventilation minimum. Cross-reference results with the available static pressure calculator to confirm the box operates within the system's pressure budget.
A cooling-only VAV box throttles airflow to its minimum setting during low-load or heating periods. The room gets cold supply air at minimum CFM, which may not provide enough heat in Canadian winters. A reheat VAV box adds a hot water or electric heating coil at the outlet. During heating mode, the box stays at minimum cooling CFM and the coil heats that air to the temperature needed to meet the zone heating load. Reheat boxes are standard in Canadian office perimeter zones where winter heating loads are significant. Use the heating load inputs above to size the reheat coil capacity for your zone. See the heat load calculator to determine zone heating loads first.