Bypass air
Conditioned supply air that returns to the cooling unit without passing through the equipment it was meant to cool.
Also called: Short-circuited supply air
In practice
Every cubic metre of bypass air represents fan energy and cooling work spent on nothing. The plant produced cold air, the fans moved it, and it came straight back without removing any heat. The energy is gone regardless.
It is the quieter of the two air-management failures, and that is precisely what makes it persistent. Recirculation announces itself eventually, as rising inlet temperatures and eventually as equipment complaints. Bypass produces no symptom a temperature sensor in the room will show — if anything the room looks comfortable, because plenty of cold air is circulating. The cost appears only on the energy bill and in a suspiciously small delta T across the cooling units.
The routes are mundane: perforated floor tiles placed in hot aisles or under cabinets where nothing draws from them, cable cut-outs left unsealed, gaps between cabinets and at the ends of rows, and openings around the room perimeter. Each is individually trivial and collectively significant, which is why air-management surveys tend to find a long list of small things rather than one large problem.
Fixing it is among the cheapest work available in an existing hall. Brush grommets in cut-outs, blanking panels in empty rack units, moving tiles to where air is actually drawn, and sealing row ends all reduce it without touching the plant. Containment then removes most of what remains by making the separation physical rather than aerodynamic.
Scope of this definition
This is an educational summary of how the term is used in data center practice. It is not a standard, a specification or engineering advice, and where a real decision depends on it, the current adopted standards, verified site information and qualified professional review are the correct sources.