| Zone | Supply CFM | Return CFM | Net | Status |
|---|
Why supply/return balance determines comfort and air quality
Every cubic foot of supply air delivered to a room must have a return path. When it doesn't, the room pressurizes and air is forced through every crack, gap around doors, and penetration in the building envelope. In winter, this pressurized air moving outward carries moisture into wall cavities, causing mould and structural damage. In summer, hot humid outdoor air is drawn into depressurized rooms through the same paths. Most Canadian homes with closed bedroom doors during HVAC operation experience exactly this problem — the bedroom has supply air but no return path, creating pressure-driven air movement that adds to heating/cooling load and moisture problems.
The bedroom door problem in Canadian homes
A typical bedroom with 100 CFM supply and a closed door with no return grille will pressurize to 5-8 Pascals above the hallway. At that pressure, the bedroom door exerts 2-4 lbs of force against the frame. Air moves out through ceiling penetrations, exterior wall outlets, and around window frames. In cold Canadian winters, this moisture-laden exfiltration condenses inside wall cavities. The solutions: add a return grille from the bedroom to the hallway, install an undercut door (minimum 1.5 inches clearance), or install a jump duct above the ceiling between bedroom and hallway. Use the register sizing calculator to size the transfer grille correctly.
Frequently Asked Questions
Each room needs a return air path equal to its supply CFM. Return grilles don't need to be in every room — a central return in the hallway can serve multiple bedrooms IF the bedrooms have a clear air path to the hallway, either through door undercuts (minimum 1-1.5 inch clearance), transfer grilles through the wall, or jump ducts over the ceiling. Standard duct design practice requires that rooms with supply registers and closable doors have a return path that limits room pressurization to 3 Pascals or less. A 100 CFM bedroom needs approximately 30-40 CFM of return path area through door undercuts to stay within 3 Pa — usually requiring a 1.5 inch door undercut or a dedicated transfer grille.
Residential buildings should be slightly positive (supply 2-5% more than return) to minimize infiltration of cold outdoor air in winter. Commercial buildings often target slightly positive pressure to keep unconditioned air and pollutants from entering through doors and loading docks. Negative pressure (return greater than supply) causes cold infiltration in winter and pulls humid air through the building envelope — increasing heating load and moisture risk. However, excessive positive pressure forces warm moist air outward through the envelope in winter, which can also cause moisture damage in cold climates. Balanced to slightly positive is the target for most Canadian residential buildings. See the ASHRAE 62.1 calculator for ventilation requirements that affect the supply/return balance.