The HVAC system is going to be a big part of most indoor air quality investigations. The quality of the air in a building is so closely tied to the ventilation of the building that we have decided to address these two issues together here. The HVAC system is almost always found to play a major role in IAQ problems in one of two ways. It is either related to the problem or it will be related to correcting the problem. Either way, the IAQ practitioner must understand the systems and their function before proceeding with any IAQ investigation. Taking a look at the mechanical plans and equipment schedules is actually a good place to begin an IAQ investigation. After gaining an understanding of the systems and their operation, the IAQ investigator might be able to make better sense of occupant complaints, etc.

The photo's below were taken at a fast food restaurant and an assisted living facility. In the restaurant, high energy costs were the concern, the assisted living facility had indoor air quality complaints. In both cases, the HVAC system components were found to be in poor condition and lacking maintenance. These conditions are typical of what we find during both energy and IAQ investigations. It is often the owner that makes the decision to contact us after spending thousands of dollars to make repairs, etc. but getting the same results. In the words of David Bearg, "maintenance treats symptoms, but not causes". The fact is, not all contractors are capable of performing the diagnostics necessary to correctly identify the problem and so many thousands of dollars can be spent guessing with less than satisfactory results.

How Much Ventilation Do You Need?

The algorithm is based on the ASHRAE 62.2 Mechanical Ventilation Standard (February 2003). This standard calls for the following ventilation rates:

For a four-bedroom 2,000 square foot home, this is 57.5 cubic feet per minute (cfm).

(The above table is for residential buildings)

Tracer Gas Ventilation Assessment

Tracer gas measurements are the most accurate way to determine the percentage of outdoor air in a building. This can be performed using Carbon Dioxide (CO²) or some other tracer gas such as Sulfur Hexafluoride (SF₆). There are also some ways of equating this number without tracer gases using temperature, but the methods may not be as accurate for several reasons. In most cases, CO²  is the gas of choice for ventilation assessments. Although there are those that would argue that using SF₆ is more accurate, the extremely high cost of the gas would not justify it's use on the typical investigation. The reason for the difference in accuracy between the two is that there are fewer confounding factors when using SF₆ due to the fact that it is not found in any measurable quantity in outdoor (background) air. The use of CO² provides acceptable accuracy at reasonable costs.

Other Ventilation Assessments

The above graph is from a data logger setup that we use during evaluations of indoor air quality and ventilation in buildings. In this particular system CO2 is measured and recorded as voltage (far right). This monitor indicated climbing CO2 levels from 8am until lunch time, but levels still in the acceptable range. The levels continued to climb throughout the day to approximately 2500ppm CO2 at the end of the work day. The levels the next morning were still at 2000ppm when workers returned which told us that the outdoor air was reduced too early and turned back on too late. The relative humidity levels were also going up and down which gives a general idea of the cycling of air handlers. By using data loggers we can gain valuable information on the operations and indoor air quality in a building. We use multiple data loggers so that we can easily spot major differences in the air quality within the building, from zone to zone and from suspect to non-suspect areas. In performing IAQ and ventilation assessments, one data logger may be all that is necessary in a small residential building, while many systems are likely to be used in a commercial or industrial facility.