Indoor Air Quality in Office Buildings: A Technical Guide

5. Detailed Assessment

5.2 Individual Source Evaluation

5.2.2 Carbon Dioxide

Carbon dioxide is a colourless, odourless gas. It is a normal constituent of the atmosphere at 330-350 ppm. Its concentration in indoor air in office buildings can, under certain conditions, provide a good indication of the ventilation rate. It is generated indoors primarily through human metabolism. People in the office environment exhale carbon dioxide at a rate of about 0.3 L/min when performing light office duties.

Although the primary function of an HVAC system is to provide thermal comfort, some outside air must be introduced in order to dilute workplace-generated contaminants and odours. Because modern buildings have less natural ventilation (infiltration) than older ones, and because occupants, office equipment, and furnishings produce chemical contaminants, it is important to add relatively clean outdoor air to the occupied work space. The case against using outside air during the heating and cooling seasons is that additional costs are incurred to filter, heat/cool, humidify/dehumidify, and distribute the air. It is recognized today, however, that the requirements of energy conservation and IAQ must be balanced in order to provide occupants with a healthy, comfortable, and productive workplace. Salary costs for absent or unproductive employees can far exceed building operating costs.

The concentration of carbon dioxide indoors varies according to location, occupancy, and time of day, tending to increase during the day. Typical office levels are in the range of 600-800 ppm. ASHRAE Standard 62-1989, Ventilation for Acceptable Indoor Air Quality, recommends a minimum ventilation rate of 10 L/s per person to ensure good IAQ in the office, using the ventilation rate procedure. The ASHRAE standard also provides an alternative performance method, the IAQ procedure, which uses guidelines for acceptable concentrations of certain contaminants as a means of achieving good IAQ. For normal occupancy and activities, this minimum outdoor ventilation rate of 10 L/s per person would result in a carbon dioxide concentration of 850 ppm at steady state conditions in the occupied space.

Carbon dioxide levels should be used with caution as an indicator of acceptable IAQ. The basic premise is that if the HVAC system is not removing carbon dioxide, then other indoor contaminants are probably accumulating proportionately. However, there may be a high indoor source of a contaminant irrespective of a low carbon dioxide level. Comparison of peak carbon dioxide readings between rooms and between air handler zones may help to identify and diagnose various ventilation deficiencies.

5.2.2.1 Checklist

• Study floor plans and details of renovations to assess potential problem areas, such as:
  
  
  • open office space that has been converted to closed offices (such closed offices may not contain thermostats, return or supply air diffusers)
  • places where structural alterations have resulted in space usage of a type different from that indicated in the plans (e.g., trans-formation of office space into a waiting room, conference room, or computer room).
    
    
• Note complaints about lack of ventilation, "lack of oxygen," stuffi-ness, and symptoms such as headaches and fatigue, which may signal problem areas.
• Answer the following questions:
  
  
  • What are the high average or peak occupant densities? What was the design occupancy?
  • Are the outside air controls and dampers functioning properly?
  • Is the minimum outside air damper opening set at approximately 15%?
  • Is there a source of contamination from the outside air? Are all air supply diffusers operational? Is the air stuffy or are odours present?
  • What time of day does the air seem to be worst? Is there an increased problem toward the end of the day?
  • Are diffusers and exhaust outlets close together, thereby causing short-circuiting?
  • Has any construction (e.g., walls, partitions) modified the ceiling return air pathway?

5.2.2.2 Measurement Methods and Equipment

Levels of carbon dioxide are normally highest in the late morning and late afternoon and vary with occupancy during the day. Also, outdoor air intake is usually at a minimum during the peak heating and cooling seasons.

Measurements should be taken at control locations, such as the outdoor air intake, the mixed air supply, the exhaust air plenum, places where the initial assessment has indicated high occupancy levels, and other locations where there are complaints of poor air quality. Carbon dioxide measurements taken at the air intake should be close to outdoor levels, otherwise exhaust is being entrained. The exhaust carbon dioxide level will indicate the average level for the building.

Spot samples can be taken, or continuous measurements can provide a detailed profile of concentration over time. To sample, the operator should stand away from the sampler/analyser to prevent contamination of the air sample with carbon dioxide from breath. Measurements are usually made between desk and head level.

It is good HVAC operating practice that indoor carbon dioxide levels at the start of the morning shift be close to outdoor levels. Extended system operating times and natural air infiltration should be used to achieve this goal. Although measurement of the volume of outdoor air may be beyond the capability of building personnel unless the air rate from the supply fan is known, the proportion of outside air can be estimated from temperature measurements of the outside air, return air, and mixed. The percentage of outdoor air is calculated as follows:

Outside air (%) = [(Tmixed air - Treturn air) / (T outside air -T return air)] x 100

The accuracy of the calculation is proportional to the temperature differences. Alternatively, the percentage of outside air can be calculated in the same way using carbon dioxide measurements.

a. Direct-reading tubes

In the direct-reading colorimetric method, a hand pump is used to draw air through a glass tube packed with a specific compound. The length of stain observed in the sampling tube is proportional to the carbon dioxide concentration and is read directly from the sample tube. The tube can be used only once. The accuracy of the direct-reading method is ae 25%.

Other direct-reading monitors sample by diffusion and are deployed for 1-8 hours. These devices provide an average carbon dioxide level for the measurement period and are an inexpensive way of obtaining a time-weighted average value.

b. Infrared analysers

Infrared analysers consist of sample and reference cells, a detector, and a broad band source of infrared radiation. These direct-reading continuous analysers respond quickly and can be moved from location to location to provide an immediate measurement of carbon dioxide. Care must be exercised to properly establish the zero and span settings and the device should be calibrated before and after each day of testing. The instrument must be allowed to reach thermal equilibrium before operation.

Advantages of the infrared analyser are portability, sensitivity, and instant and continuous monitoring capability. Disadvantages are its cost, its tendency to drift with time, its susceptibility to mechanical shock, and the need for frequent calibration.

5.2.2.3 Strategy for Remediation

Ventilation effectiveness can be improved by:

• adjusting and rebalancing the ventilation system (supply and return diffusers) to reflect current occupancy levels, and heat and contaminant generation locations
• increasing the outside air supply
• removing obstructions from the return plenum
• controlling pressure relationships, exhausting areas where pollutants are generatedaltering the source/distribution relationship by changing the physical arrangements of supply and return diffusers or furniture and partitions
• upgrading the air distribution system by increasing fan capacity in either the supply or the return system.