Can I use “indirect” methods to determine my outside air intake flow rates?
No. Methods that determine outside airflow rates by taking the difference between supply and return airflow rates, or by using temperature and mass balance methods produce significant errors that result in unreliable control of outside air. These errors can be easily demonstrated with simple mathematical calculations. Other indirect methods that rely on maintaining the pressure drop across a fixed orifice (i.e. baffle, damper or louver) are subject to the difficulties of accurately measuring low pressures or the unreliability of static set-up requirements. In the case of fixed dampers, the inability of the damper blades and operators to consistently return to the same position, especially as the damper ages, renders this method inappropriate at best.
