Part 9 Residential HVAC Design: Loads, Equipment, and Controls
Part 9 residential HVAC design starts with a real load calculation. For houses, the OBC references CSA F280 for determining required heating and cooling capacity. The calculation must reflect the actual envelope, windows, orientation, airtightness, ventilation load, and outdoor design conditions. Room-by-room loads are needed for proper air distribution; a single whole-house number is not enough to size ducts and terminals. Equipment output should match the design load within normal sizing tolerances-oversizing causes short cycling, poor humidity control, and comfort complaints.
Equipment and controls must be certified, installed per the manufacturer's instructions, and arranged for safe service. Controls include thermostats, fan controls, high-limit and safety devices, and zone dampers where used. For cooling, condensate must be drained safely; for heating, clearances and access must be maintained. Duct static pressure and available external static pressure must be checked so the selected blower can deliver the required airflow.
Traps: sizing by floor area ('one ton per 600 square feet'), ignoring duct losses, using heating airflow for cooling, and forgetting ventilation and infiltration loads. Another trap is choosing equipment from a catalogue without verifying the actual output at the design conditions.
Worked example: a 2,200-square-foot house is not sized by a rule of thumb. Calculate the room-by-room load with CSA F280, add the ventilation load, select equipment whose rated capacity covers the load, then design ducts for the required airflow at a reasonable friction rate. If the calculated heating load is 48,000 Btu/h, a 60,000 Btu/h output furnace may be reasonable; a 100,000 Btu/h furnace is likely oversized and should be questioned.