Indoor Air Quality in Office Buildings: A Technical Guide

5. Detailed Assessment

5.2 Individual Source Evaluation

5.2.3 Air Motion

Air motion in a building is a readily identifiable comfort parameter representing the displacement of air by convection or ventilation. If air motion in an occupied space is inadequate, it may lead to complaints of stuffiness. The air pressure in the ducts may be too low for adequate airflow, or the ventilation system may be unbalanced.

Four air changes per hour provide gentle air movement as well as continuous dispersal of contaminants. Excessive air movement causes draft or unwanted local cooling of the human body. ASHRAE recommends that the average air movement in the occupied zone for the winter period not exceed 0.15 m/s; summer air movement should not exceed 0.25 m/s.

When building occupancy or use patterns change or when office equipment such as photocopiers, computers, and printers is added, it is likely that appropriate adjustments in air supply have been overlooked. To ensure that each diffuser provides good air movement throughout the area it serves, the system may have to be rebalanced.

Airflow is influenced by the combined action of controlled mechanical systems and natural, uncontrolled forces. Pressure differentials move airborne contaminants through windows, doors, cracks, holes, utility chases, stairwells, elevator shafts, and other openings.

5.2.3.1 Checklist

• Enquire about any recent change in the physical set-up and use of the space.
• Check supply air diffusers for excess airflow or blockage.
• If diffusers have deflectors, check that they are adjusted properly.
• Check return grilles for blockage.
• Check ceiling mixing boxes for proper control and damper positions.
• Take note of any exhaust and diffuser ducts that are in close proximity.
• Check condition of filters in perimeter units and ceiling systems.
• Check that ductwork is in good condition and properly connected.

5.2.3.2 Measurement Methods and Equipment

Air movement is usually measured both in ventilation ducts, where it is relatively rapid,
and in the office, where a low speed must often be maintained.

a. Smoke tubes

One of the most useful devices for qualitative measurements of airflow and direction is the smoke tube, which can help track contaminant movement and identify pressure differentials. Smoke tubes are low-cost and are often employed during the walkthrough. Using a smoke tube in mid-room will help identify air circulation within the space. Dispersal within a few seconds suggests good circulation, whereas smoke that stays essentially still indicates poor circulation.

Smoke released near diffusers and grilles gives a general indication of air movement. The procedure assists in evaluating the supply and return system and in determining whether supply air actually reaches the work area. Since the "smoke" is usually an acid vapour take care not to inhale it.

b. Thermal anemometers

Thermal anemometers provide direct readout of air velocity. The airflow cools a sensor, usually a hot wire, in proportion to the velocity of the air. As the probe is non-directional, care must be taken to position the sensor properly.

c. Thermal comfort meters

Thermal comfort meters are capable of measuring all comfort-related parameters, such as mean radiant temperature, air temperature, humidity, and air motion. The parameters are integrated to produce a "level of comfort." The units are expensive and the results inconclusive.

5.2.3.3 Strategy for Remediation

• Unblock diffusers and return grilles where necessary. Drafts caused by supply air diffusers can be deflected with baffles.
• Sometimes uneven thermal surfaces such as cold windows, ledges or floors can create uncomfortable convection currents. These can often be alleviated by simply insulating the cold surface.
• When occupied spaces are redesigned for changed space requirements, adequate airflow to the redesigned space needs to be verified. Flow system rebalancing may be necessary.