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Diagnosing and Solving IAQ Problems: Basic Measurement Techniques

Most IAQ problems can be effectively diagnosed with educated observations, an awareness of odors, a sense of temperature and relative humidity, and a smoke pencil to observe the existence of and direction of air flows. It is often useful to also measure thermal comfort parameters using various instruments to measure temperature and relative humidity.


Occasionally, the diagnostician will want to measure airflow into, through, or out of ducts/vents, or calculate the percent of outdoor air in the supply air stream. Some diagnosticians may also find it useful to track carbon dioxide levels in occupied spaces over the course of the occupancy period, or compare carbon dioxide levels in the complaint area with levels in non-complaint areas.


The measurement techniques described here are simple and comparatively easy to perform. More complex measurement methods and those involving measurement of specific contaminants are usually not needed, and would normally require outside expertise. Diligently follow manufacturer's instructions on all measurement instructions.


Observing or Measuring Airflow

Using Smoke Tubes

Smoke tubes or smoke guns can provide a quick visualization of the path of the airstream and thus help identify pressure differentials. By dispensing a series of small "puffs" or "dribbles" the smoke can provide more information than a single large "cloud" In occupied areas, if the smoke is dispersed in several seconds, this suggests good air circulation. Smoke tubes have a variety of uses and should be a staple of the building diagnostician. Use smoke tubes:


  • Near supply air outlets - the dispersal pattern provides information about the velocity and direction of the supply air.
  • Near exhaust vents to make sure the exhaust is drawing air out of the room.
  • Near combustion chamber of combustion appliances to insure that there is no backdraft of flue gases.
    • Turn the appliance on.
    • Close all doors or other openings that might possibly be closed in normal operating conditions, and turn on all exhaust fans or other equipment (e.g., clothes dryer) that may exhaust air from the room.
    • Release puffs of smoke next to the combustion chamber and around the flue fittings (where flue gases might leak into the room) to detect any air movement from the flue or combustion chamber into the room.
    • If any flue gas leakage is occurring, turn off the appliance, open windows and doors, and exit the room. This is a potentially life threatening hazard
  • At drain trap openings make sure no air is flowing up from the trap.
  • At duct seams to check for leakage.
  • At the entrance to an exhausted room to insure the room is under negative pressure relative to the occupied spaces.
  • At the entrance to a clean room to insure room is under positive pressure.


More generally, identify spaces that are positive or negative relative to other spaces to determine if this may be causing pollutants or moisture to travel in undesirable paths.


Measuring Outdoor Air Flow

    The quantity of outdoor air supplied to a building can be ascertained using a flow hood or air velocity measurements. To check that outside air requirements are being met under all operating conditions, take measurements at minimum airflow.

  • If an economizer is operating, turn the economizer off, or set the economizer shutoff temperature to the lowest setting.
  • Since airflow varies during the day and over the seasons in VAV systems, VAV system controls should be made to operate at minimum flow conditions.


Measuring Air Flow with a Flow Hood (Preferred)

Flow hoods are designed to measure airflow through an opening (e.g., at supply air diffusers or return air grills, outdoor air intake) by placing the face of the flow hood over the opening and reading the computed airflow volume. The flow hood must completely cover the opening. If the opening is larger than the flow hood, the user can obtain a reasonable estimate by following manufacturer's recommendations. Flow hoods will show the airflow in an easier, quicker, and more consistent manner than using a series of pitot tube or rotating-vane anemometer readings.


Measuring Air Flow with a Velocity Meter (Less Preferred)

Velocity meters (anemometer or pitot tube) are less expensive than flow hoods, but their accuracy is more problematic and they require greater attention to detail. An anemometer or Pitot tube can be used to measure the velocity of air passing over a point. To accurately represent the velocity through a duct or opening, multiple measurements must be taken. It is extremely important to follow measurement instructions supplied with the equipment to obtain an accurate reading. Airflow through the duct or opening is then measured by multiplying the area of the opening (e.g., square feet) by the average velocity of the air (e.g., feet per minute) to obtain airflow (cubic feet per minute).


Airflow = Free Open Area x Average Velocity


(ft3/m or m3/m) (ft2 or m2) (ft/min or m/min)


Calculating the Percent of Outdoor Air Using Carbon Dioxide (CO2) Measurements (Not Preferred)

As a last resort, if outdoor air cannot be measured directly, one can measure the supply airflow and calculate the percentage of outdoor air using CO2 measurements.

  • Measure the total supply airflow using a flow hood or velocity meter.
  • In quick succession, measure carbon dioxide in the supply air, the return air, and outdoor air (do not use calorimetric tubes for these measurements). An average of more than one measurement in each air stream is advisable to obtain accurate estimates.
  • Calculate the percent outdoor air as follows:

{CO2 (return) - CO2 (supply)} x 100

{CO2 (return)-CO2 (outside)} =% outdoor air


Outdoor air supply=(Supply airflow) x (% outdoor air)


Measurement of supply and return are best accomplished in an indoor space serviced by an AHU of interest. Outdoor air should be measured at the outdoor air intake of the AHU. Percent outdoor air is the same throughout the supply air stream of the AHU.


Note: It may be tempting to use temperature rather than CO2 to measure percent of outdoor air. This is not advisable because as the three airstreams approach each other, varying temperatures affect accuracy and the calculated result can deteriorate very significantly. For the same reason, the difference in CO2 measurements in each air stream should be at least 200 PPM to insure an acceptable level of accuracy for this method.



Measurement of Thermal Comfort

Generally, independent measurements of temperature and relative humidity will be sufficient. However, some instruments will integrate these and other measurements and provide a read out of thermal comfort consistent with ASHRAE Standard 55-1992.


Temperature and Humidity

For temperature and humidity measurements, instruments can be a simple thermometer and humidity gauge, a sling psychrometer, or an electronic thermo hygrometer. Accuracy to within + or -1 degree F and + or - 5% RH is the objective of thermal comfort measurements. Readings should be made 3-6 feet off the floor, and at floor level. Persons suffering from cold feet may report that the room is "too cold."


Be sure that the meter is located away from direct sunlight or near a supply air outlet, or other heating/cooling sources. Refer to the manufacturer recommendations for the time needed to stabilize the reading, and maintain the frequency of calibration.


Acceptable Values

Direct reading instruments are readily available. Readings within acceptable ranges for thermal comfort.


Alternatively, a thermal comfort meter can be used. Such meters integrate several thermal comfort parameters and will provide a direct indication as to whether thermal comfort is in the acceptable range according to ASHRAE Standard 55-1992.



Measurement of Light

Measurements should be to ascertain that lighting conditions are adequate for the space. Use of a light meter to measure foot-candles (FC) should be conducted at the working surface in a horizontal plane 30 inches above the floor. Since the objective is to measure the task illuminant, daylight should be excluded. Thus, in an occupied area with windows, readings should be taken with the full use of interior shading (blinds or draperies) to reduce direct solar gain. When measurements are taken, the reflections from other strong light sources should be minimized. Light meters are generally sensitive to ambient temperatures, and should be only operated according the temperature range recommended by the manufacturer. Calibration is as important for light meters as it is for temperature and humidity measuring instruments.


Acceptable Values

Minimum foot-candle (FC) levels measured at the working surface:

  • Ambient lighting for PC use: 20 FC
  • Visual tasks of high contrast or large size office: 20 FC
  • Visual tasks of medium contrast or small size: 50 FC
  • Visual tasks of low contrast or very small size: 100 FC
  • Hallways and stairwells measured at floor level: 5 FC
  • Public spaces with dark surroundings (sidewalks, garages), measured at the floor: 2 FC


Measuring and Interpreting Carbon Dioxide (CO2)

COMeasuring Instruments

Sorbent tubes are readily available for measuring CO2 and they are inexpensive. However, with accuracy of only + 25%, sorbent tubes are not of much value for indoor air quality diagnostics. Though more expensive, instruments using infrared spectrometry with digital read-outs are more accurate and appropriate.


COValue Indicators

The exhaled breath of occupants is the main source of carbon dioxide (CO2) in buildings. Because the concentration of CO2 is highly correlated with levels of human bioeffluents (body odor), COmeasurements are often used to indicate whether the outdoor air ventilation rate in the building is sufficient to handle the bioeffluents load.


ASHRAE ventilation standards of 15-20 cfm of outdoor air per occupant are designed to dilute human bioeffluents odor to an acceptable level. In general, 15 cfm per occupant will keep indoor minus outdoor CO2 levels below 700 PPM, while 20 cfm will keep indoor minus outdoor CO2 levels below 500 PPM. (This corresponds to indoor values of 1000 PPM and 800 PPM when outdoor values are 300 PPM, which is assumed by ASHRAE.)


Interpreting CO2 Measurements Above Threshold Values

Indoor CO2 should be measured at peak values. Peaks usually occur around 11am and 3pm in a typical office environment. However, if measurements in the occupied space are ever above 1000 PPM:

  • Check for improperly vented combustion appliances, which could also be producing carbon monoxide.
  • Check the COlevels outside; and calculate the indoor-outdoor values and compare with the above mentioned thresholds for 15 and 20 cfm per occupant.

If neither of these conditions can explain why the CO2 levels are above 1000 PPM, it is a valid presumption that the outdoor air ventilation rate is too low.


Interpreting CO2 Below Threshold Values

If CO2 levels are below the identified guidelines, this does not mean that the ventilation rate or that indoor air quality is satisfactory. CO2 measurements below the designated threshold levels are not an indicator that either IAQ or outdoor air ventilation rates are satisfactory.


As a general measure of indoor air quality, CO2 measurements do not account for non-occupant related contaminants, which can dominate the indoor environment. And as an indicator of the outdoor air ventilation rate, the use of CO2 measurements will almost always tend to overestimate the true outdoor air ventilation rate, often by as much as 100% to 200%.


This is because the threshold values are based on the assumption that COhas risen to its theoretical steady state condition. As people occupy the building, when occupancy stabilizes, and when ventilation rates remain constant, CO2 levels will rise and eventually reach steady state condition, and go no higher. The steady state value will be greater with higher occupant densities and lower outdoor air ventilation rates.


Unfortunately, it is extremely unlikely that steady state will have been reached.

  • Under a typical outdoor air exchange rate of 0.5 air changes per hour, it would take 6 hours to achieve 95% of steady state conditions.
  • But constant full occupancy in a building is seldom longer than 3 hours in the morning or 3 hours in the afternoon.
  • Thus, measuring CO2 prior to steady state will always underestimate the true ventilation rate.

The extent of overestimation increases as occupant density decreases, and as the outdoor air ventilation rate decreases. In the majority of circumstances, CO2 levels in occupied spaces will not achieve 1,000 PPM even when outdoor air ventilation rates are unacceptably low. The matter is further complicated by the fact that outdoor air ventilation rates are often not steady in VAV systems.


Observing Changes to CO2 Values Over Time

Real-time measurements of CO2 with data-logging equipment can be also be used to see how CO2 values rise and fall in an occupied space during the day, reflecting the pattern of changing occupancy, or changing outdoor air ventilation rates. This can provide clues as to what is happening in the building and this information can help in the diagnostic process.


Comparing CO2 Values of Different Spaces in the Same Building

The investigator may wish to compare CO2 values in the complaint area with values in other parts of the building. CO2 values in the complaint area higher than values in non-complaint areas suggest that outdoor air ventilation rates in the complaint area may be causing the problem.



Measuring Contaminants

Most often IAQ problems can be solved without measuring specific contaminants. Measurements are sometimes helpful:

  • to test for clearly identified sources and clearly identified target contaminants
  • to measure specific contaminants, such as radon, that have no acute affects but which could cause serious long term illness
  • to test for mitigation effectiveness in controlling a source
  • to compare with levels found in non-complaint buildings
  • to insure the containment of a work area/construction area in an occupied building
  • to document for liability or other legal or administrative reasons. When measurements are taken, qualified, experienced persons should take them and adhere to protocols and quality assurance procedures.

Diagnosing and Solving IAQ Problems: Basic Measurement Techniques

Created on November 22nd, 2011.  Last Modified on January 8th, 2017

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The Healthy Facilities Institute provides the information on as a free service to the public.


While an effort is made to ensure the quality of the content and credibility of sources listed on this site, HFI provides no warranty - expressed or implied - and assumes no legal liability for the accuracy, completeness, or usefulness of any information, product or process disclosed on or in conjunction with the site. The views and opinions of the authors or originators expressed herein do not necessarily state or reflect those of HFI: its principals, executives, board members, advisors or affiliates.