Most accept blower testing as a method of determining the theoritical building air infiltration. This is great but the test only provides the theoritical amount of fresh air infiltration during the average winter temperature/wind. As temperature rise and fall along with wind, we more or less fresh air in the home. All so the mechanicals on the home are not operating while the blower test is conducted.
With some care, measuring CO2 concentrations during steady state occupancy and internal conditions, we have real data regarding the actual amount of fresh air entering the home. The number of occupants generate a specific amount of CO2. The CO2 is mixed with the fresh air infiltration/ventilation (typically 450 ppm), generating a part per million (ppm) concentration of CO2. The resulting ppm determines the cfm of fresh air being mixed with occupants CO2. Reliable CO2 meters are becoming inexpensive, providing confirmation of the amount of fresh entering the home.
I recently found a blocked fresh air inlet by observing the slow rise of the CO2 levels in my home.
I like the idea of checking the amount of fresh air infiltration/ventilation to match the ideal building air change rate to purge indoor pollutants and occupancy density. Building need an air change in 4-5 hours for ideal purging of pollutants.
Determining Fresh infiltration/ventilation by monitoring CO2 PPM levels. Locate the CO2 meter in the middle of the home. Operate air circulation with occupants away from the meter. Monitor for several hours until the CO2 levels stablize. With an interior volume of 32,000 cubic ft. and CO2 meter reading of 800 ppm indicates 27 cfm per occupant of fresh air infiltration.
With 2 occupants, 2 X 27 cfm/ = 54 cfm total effective fresh air blending with the CO2 from the occupants. 32,000 FT^3 / 54 cfm = 592 minutes or 10 hours to change the air in the home with perfect air mixing. I measured 60 cfm of fresh make-up air plus infiltration. My guess is 50% of the fresh air gets out of the home without mixing with the CO2 because of slow mixing. I am under-stating the amount of fresh air by 505. I could use a little more fresh air, but its ok for me. As the outside temps drop and wind rises (winter), the CO2 levels decline, indicating an increase in fresh air infiltration. I can discontinue fresh make-up air during the coldest wind months of winter while maintaining low CO2 levels.
Attached is a table of various CO2 levels and the amount of fresh air infiltration/ventilation per occupant and the square footage of the home with a 9 ft. ceiling. These numbers are based on perfect mixing of the fresh air and CO2 prior to being metered. Because of poor mixing of the fresh air and the CO2 from the occupants, expect an underestimate of the amount of fresh by 25-50%. Operating the air handle and allowing the CO2 levels to stablize a couple hour while the occupants are located away from the meter will provide a more accurate reading.
This an interesting concept to provide real time data of air change depending on the different conditions home are exposed to.
This the best tool we have to monitor levels of fresh air in the home.
I am attaching a chart illistrating the concept with typical fresh air/CO2 levels.