If your building is mechanically ventilated using a fan to supply outdoor air through a duct system, instruments can be used to measure the rate of mechanical outdoor air supply, although sometimes the measurement uncertainty will be high. In general, only suitable contractors have the necessary instruments and skill in instrument use. The measured outdoor air supply rate can be divided by the indoor volume, floor area, or number of occupants to determine different types of ventilation rates. Your building’s total ventilation rate may be higher than the mechanical ventilation rate due to air entry via leakage through the building envelope. Measurements of indoor air carbon dioxide concentrations are frequently used to provide a rough indication of ventilation rate per person, although there are several common sources of error. A maximum indoor carbon dioxide concentration of 1000 parts per million is often used as an indicator of adequate ventilation but this employs a set of assumptions that are not true in every situation. Suitable carbon dioxide instruments cost a few hundred dollars. For homes, contractors sometimes employ a fan system, called a blower door, installed in a doorway together with pressure sensors to measure the effective total size of leaks in the home’s envelope. A mathematical model and weather data are then used to estimate the ventilation rate. Researchers sometimes inject a tracer gas into the indoor air and measure its concentration decay over time to determine the ventilation rate; however, this measurement method is rarely used except for research. Various technical papers provide more information on measurement of ventilation rates, for example this article. A standard test method for use of blower doors is available for purchase.
How can I determine the ventilation rate in my building?
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