Ways of estimating the amount of fresh air Infiltration/Ventilation in a Home
ASHRAE, American Medical Ass., American Lung Ass, and local codes are concerned with the amount of fresh air that occupied space recieve. Some suggest the "Blower Door Test" and then deciding on the need for mechanical fresh air and the amount. Testing for pollutants is expensive and difficult because of variable conditions and the lack understanding all of the pollutants that sensitive occupants are affected by.
Unfortunately, the blower door only measures the air leaks with specified negative or positive pressure. The air leakage amount is calculated to represent an average winter day at a average wind velocity. This an accurate measure of total air leakage holes in the home but not a measure of natural air leakage during any condition. What is missing is the location of the air leaks in the home. For the estimate to be representative, the air leaks must be evenly distributed throughout the exterior of the home. Stack effect and wind have a minimal effect on single large leaks. Stack effect needs high/low leaks. Wind need upwind/down wind air leaks.
There are more problems. The fresh air infiltration is dependent on the stack effect, wind, operation of air pressure generating devices, exterior surfaces, and open doors/windows. Pressure from the air handler, bath fans, clothes drier, and kitchen home are common.
Monitoring the real fresh air movement through our homes may be the most critical issue regarding indoor air quality and long term health for sensitive occupants. Purging indoor pollutants and renew oxygen should be high on out lists. Instead, we have lengthly discussions about sizing and mechanical features without a word about fresh air ventilation and controlling high moisture in out equipment which inturn leaves us with potiential high levels of pollutants and less than optimum oxygen levels.
Consider that the air in a home is initially outside air and the outside air is slowly moving through our occupied spaces. In homes with minimal amounts of indoor pollutants, a fresh air change in 4-5 hours is suggested as minimal. Some suggest that this could be reduced when the home is not occupied. When the wind is +7 mph or the outside temperature is at it's winter low, most homes get more than their needed air change in 4 hours. An indication of the air change rate is the inside air dew point verses the outdoor dew point.
In the winter, outdoor dew points are from 0^F-50^F. Each occupants adding .5lbs. of moisture. Low indoor dew points indicate low occupancy and high air change rates. High indoor dew points indicate low fresh air infiltration rates or high occupancy.
In the summer, the opposite is true. Humidifiers or dehumidifiers are needed to supplement the moisture levels in a home.
CO2 levels in the home verses outside CO2 levels also provide solid information about the fresh air change rate when compared to the number of occupants in the home.
CO2.com recently introduced the $100 CO2 meter. It operates with usb connection from a computer. The meter monitors temp and CO2 PPm without data logging.
If the outdoor CO2 level is 450, the inside level will indicate the cfm of fresh air infiltrating the space.
An example is:
550 ppm is 106 cfm of fresh air per person.
650 ppm = 53 cfm
750 ppm = 35 cfm
850 ppm = 26 cfm
This gets you an approximate estimate of real fresh air change rate in a space.
With the unknown effects of indoor pollutants, fresh air change is a basic for indoor air quality.
Fresh air a critical part of any space occupied by people and pets.
REgards TB
Bear Rules: Keep our home <50% RH summer, controls mites/mold and very comfortable.
Provide 60-100 cfm of fresh air when occupied to purge indoor pollutants and keep window dry during cold weather. T-stat setup/setback +8 hrs. saves energy
Use +Merv 10 air filter. -Don't forget the "Golden Rule"