Where to measure

To ensure proper measurement of the fresh air ratio, readings need to be taken in the proper location. Outdoor air measurement points should always be located away from building exhaust vents. If the fresh air ratio you are interested in comes from using CO2 concentration or temperature, the measurement locations for supply and return air will be different.

For CO2, return air readings should be measured in the return duct or before the return air mixes with the outdoor air. These locations ensure a stable reading. Supply air readings should be taken inside the supply duct.

Illustration:Schematic diagram of air handling unit

If temperature measurements are used, the return air value should be read as close as possible to the air handling unit before the return air is mixed with the outside air. In the illustration given, the return air temperature (XR) is best measured in the return duct upstream of the return dampers. The supply air should be measured before air conditioning. Temperature measurements should also be taken before any fans, which can raise the temperature a few degrees. In the illustration given, the supply air temperature (XS) is best taken before the filters and chilled water coils.

　　Formula Error

This is a formula error if the formula result is a negative percentage or if the percentage is higher than 100%. An error occurs if the supply air value is not between the return air value and the outdoor air value. Usually, if all three measurements are recaptured, the problem will resolve itself. If not, then consider which one might apply to the situation:

If temperature is used to calculate %OA:

-The air has been conditioned prior to measurement -Measurement is made close to the air handling unit.

-Supplied air has been mixed with room air -Measure near the diffuser or inside the duct.

-Return air is not adequately mixed -Measure the return air inside the duct to ensure proper mixing.

-Outdoor and indoor temperatures are too close -use CO2 values as the basis for calculating %OA.

If CO2 is used to calculate %OA:

-Supplied air is mixed with room air -measurements are made close to the diffuser or inside the supply duct.

-Return air is not sufficiently mixed -Measure inside the return duct.

-Outside air measurements are contaminated -ensure that the air entering the outside air intake is fresh and clean.

-Not enough occupants producing CO2 -Use temperature values as the basis for calculating %OA.

Note:The article cited excerpts from the official public number of TSI, the layout of the content and images have been slightly modified, if there is any infringement of rights or objections can be deleted by contacting the publisher.