Cashins & Associates Blog

Indoor Air Quality - Cold Air Dumping

Posted by Mike Cashins on Tue, Feb, 14, 2012 @ 11:02 AM

Controlling drafts can significantly control incidence of thermal discomfort complaints.

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One of the most frequent causes of drafts in clinics and offices is a condition known as “cold air dumping.” Conditioned room air is commonly delivered to occupied spaces via ceiling diffusers. The usual delivery temperature is in the 55 F. to 60 F. range. A ceiling diffuser is a supply air outlet located in the ceiling. 

Ceiling diffusers have pattern deflectors arranged to promote the mixing of supply air with the room air to produce a horizontal air pattern.  This horizontal air flow, called “surface effect,” causes the inducement of room air into the air stream.  When the outlet discharges air directly parallel with and against the ceiling, the air then tends to flow along the ceiling, gradually mixing with the room air.  The supply air temperature warms via inducement or entrainment and the velocity (draft) slows as it is delivered to the occupied space.  A high degree of surface effect is required, especially for VAV (variable air volume) systems, because it helps to reduce the “dumping” of cold air.  This “dumping” of cold air will result in employee thermal complaints.

One feature of “cold air dumping” is a localized draft. Occupants in quite proximate areas could well be at odds on perceptions of personal comfort with co-workers several feet away. Typical thermometer or temperature-relative humidity measurements will not reveal the source of problem; that is cold drafts. In such circumstances, we can see how a ‘perception’ of discomfort (sometimes viewed as whining) is, in reality, a valid thermal discomfort issue.

A common intuitive action when such cold air dumping is noted is to further reduce air flow or exit velocity. However, as indicated above, the most appropriate action would be to increase air velocity from the diffuser. Optimally, most VAV box minimum flows should be proximate to 20 – 40% of maximum. This minimizes “cold air dumping” and prevents deficiencies in total air delivery, thus helping to maintain good indoor air quality..

A fairly common phenomenon similar to “cold air dumping” is “cold air bounce.” This can occur when walls or obstructions are placed too close to ventilation supply air outlets. Here, the cold air from the diffuser strikes the obstruction and dumps down onto a nearby work station. Again, this draft causes localized discomfort. Such conditions often occur following office re-stacking when partition walls are moved.

Other cause of dumping can include:

  • Pattern deflectors are partially obstructed, missing or improperly aligned.
  • A supply air terminal has an exit velocity set too low.
  • A supply duct, diffuser, or terminal is obstructed.
  • A supply fan malfunctions or the fan’s belt slips
  • An improperly positioned terminal with pattern deflectors sends air to strike against nearby walls or other obstructions -- AKA “Cold Air Bounce.”

 What can be done to correct such problems?

  • Check to determine if the box (VAV) is reducing too far
  • Evaluate VAV box minimum setting
  • Check to see if the diffuser is too large and check installation
  • Check if the diffuser pattern or throw is incorrect causing drafts,
  • Check proper alignment of vane deflectors
  • Assure temperature sensor is located incorrectly or needs calibration
  • Check for obstructions
  • Check fan belts, if any

We have found the following actions to be quite successful in quickly controlling office thermal discomfort:

  1. Assure there are no drafts exceeding or 50 fpm for cooling or 25 fpm for heating in the areas of complaint.
  2. Place a temperature - relative humidity data logger alongside a thermal comfort chart in a visible position in area of complaint; this after assuring draft is within above criterion.
  3. If the chart indicates that the environment is within criterion, show the results to those concerned. The occupants will most often accept the results and adjust appropriately.
  4. If the chart results are outside the criterion, the system can then be objectively adjusted.

We have received feedback from several customers who have implemented the above action steps. All have reported a dramatic, upwards of 90%, reduction in thermal complaint response calls.

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Topics: industrial hygiene, indoor air quality, ventilation assessment

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