Virtual outdoor air flow meter for an existing HVAC system in heating mode

Measurements from the operation of Heating, Ventilation and Air conditioning (HVAC) system are collected by Building Automation System (BAS) mostly for control purpose. In many air handling units (AHUs) the airflow meter is not installed on the outdoor air stream to reduce the initial costs. To provide such a missing information needed for the ongoing commissioning of HVAC system, a virtual flow meter (VFM) should be implemented in the BAS. This paper presents VFMs for the heating season when the heat recovery coils are used. Two models are presented which predict the outdoor air ratio (factor α), in the absence of such an air flow meter, by using BAS trend data. Two energy balance equations, one for the air mixing box and another for the heat recovery coils, are coupled. The results from a case study building show that the two models predict the daily average outdoor air ratio of 0.89 and 0.87, respectively, compared with the reference daily average ratio of 0.86.