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 that 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 of 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 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.

Acceptable Values

Minimum foot-candle 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

CO₂Measuring 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.

CO₂Value 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), CO₂measurements 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 CO₂levels 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 CO2levels 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 CO₂has 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 CO2prior 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 CO₂with 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.

	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.