How to Prevent Cold-Air Dumping from Ceiling Diffusers
Updated: 3 days ago
Cold-air dumping is a common air-distribution problem in air-conditioned buildings. It occurs when cool supply air discharged from a ceiling diffuser loses its horizontal momentum too quickly and falls directly into the occupied zone. Occupants may experience cold drafts, uneven temperatures and discomfort even when the room thermostat indicates that the desired temperature has been achieved.
The problem is often blamed on low supply-air temperature. However, cold-air dumping is usually caused by a combination of diffuser selection, insufficient airflow, poor diffuser placement, excessive system turndown, incorrect neck connections or unsuitable control strategies. (How to Prevent Cold-Air Dumping from Ceiling Diffusers)
This article explains why cold-air dumping occurs and how HVAC designers, contractors and commissioning teams can prevent it.
1. What Is Cold-Air Dumping?
A ceiling diffuser is normally designed to discharge supply air horizontally along the ceiling. The air jet should remain attached to the ceiling, travel a calculated distance and gradually mix with room air before entering the occupied zone.
This ceiling attachment is commonly associated with the Coanda effect. When the diffuser is properly selected and installed close to a flat ceiling, the supply-air jet entrains nearby room air and tends to remain attached to the ceiling surface.
Cold-air dumping occurs when this attachment is lost too early. The cold supply air, which is denser than the surrounding room air, then descends rapidly into the occupied zone.
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