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4. Outdoor Air Measurement Technologies (OAMTs)

Several outdoor airflow measurement technologies (OAMT) are marketed to meet the need for better measurement and control of outdoor air ventilation rates in commercial buildings. These OAMTs consists of sensors, sometime together with hardware, installed in the stream of outdoor air entering the AHU upstream of the location where outdoor air mixes with recirculated indoor air. Typically, the outdoor air enters an AHU through a louver or air intake hood which are present to limit the rates at which moisture (e.g., rain drops) enter the air handler.

This figure schematically shows a common outdoor air intake with a louver (shaded gray) on the left through which outdoor enters and an outdoor air damper (shaded blue) located downstream on the right. The red arrows illustrate an example of the resulting airflow pattern. Often, recirculated indoor air mixes with the outdoor air just downstream of the outdoor air damper; thus, the OAMT must be placed upstream of damper in the region with a complex airflow pattern. 

It is difficult to accurately measure the airflow rate through an airflow passage if the airflow is not parallel to the perimeter of that passage. Accurate measurements are also complicated by highly non-uniform air speeds. Low air speeds pose further challenges to accurate measurements of outdoor air intake rates. 

Why measurements are needed

Minimum ventilation rate standards [1, 2] specify minimum rates of outdoor air supply per unit floor area, per person, or per unit volume or specify combinations of these metrics. The ventilation rates in mechanical ventilated buildings are largely determined by the rates of outdoor air flow into air handling units (AHUs). It is common for outdoor air entry rates to be estimated based on data provided by air balance companies who measure outdoor air flow rates with various settings of the AHU’s airflow control dampers. Damper settings, e.g., degree of opening of the outdoor air dampers, are then used to estimate the outdoor air entry rates. 

However, there are several potential sources of errors in outdoor air flow rates when this process is employed. Accurate measurements of outdoor air flow rates are challenging, particularly when minimum rates of outdoor air are being supplied; thus, the outdoor air flow rates provided by air balance companies can be inaccurate. Also, outdoor air entry rates can vary with the total air supply rate of the AHU and as outdoor wind speed and direction change, such that damper positions do not uniquely indicate outdoor air supply rates. Wear or adjustment of damper hardware since air balance data were collected, often several years prior, can also lead to errors. 

Data available on ventilation rates in commercial buildings indicate that ventilation rates are often very poorly controlled, with minimum ventilation rates often far from the rates specified in standards [14-17]. There is a need for improved measurement and control of minimum ventilation rates in commercial buildings.