Methods:
(1) thermal mass balance
(2) carbon dioxide measurements
percent outdoor air using thermal mass balance method =
= 100% * ((temperature of return air) – (temperature of mixed air)) / ((temperature of return air) – (temperature of outdoor air))
where:
• Temperatures may be in Fahrenheit or Centigrade.
• For this method to work, there must be a sufficient difference in temperature between the inside of the building and the outside. The thermal mass balance method is most suitable in the Winter and Summer.
• Multiple temperature measurements should be taken and the average value used.
percent outdoor air using carbon dioxide measurements =
= 100% * ((ppm CO2 in air supply) – (ppm CO2 in return air)) / ((ppm CO2 in outdoor air) – (ppm CO2 in return air))
where:
• ppm = parts part million
• If measured in a room, the CO2 of the air supply is measured at the air vent into the room.
• If measured at an air handler, the CO2 of the air supply is taken as the reading of mixed air.
outdoor air flow in cubic feet per minute =
= (percent outdoor air) / 100 * (total airflow in cubic feet per minute)
where:
• The number used for total airflow may be that supplied to the room or zone, the capacity on the air handler, or the total airflow of the HVAC system.
• If the air flow at the air handler is used, it should be measured and not based on the original design documents.
