Monitoring OA via calculation for IEQc1?

This is more than likely possible. However, you will need to keep a few things in mind. Your AFMS's will need to be located such that significant duct leakage is negligible. In other words, if you were only taking the sums of a lot of VAV supply and return boxes, I would not consider that to be very accurate. Also, I would imagine that you would be required to show that the compound error of the multiple sensors is still within the bounds of the credit requirement. I really do not see where this is a beneficial approach.

For an economizer ( Mixing box ) AHU, can you also do the following. We have AHU's for Cleanroom applications- The cleanrooms AHUs have 2 settings and act as a constant volume system Normal mode- and night time set back mode ( reduced flow rate but still maintain room pressurization requirements), Both modes have a variable outside air depending on the outside air conditions in terms of temperature and humidity associated with the mixing box.   We are measuring the total supply airflow (mixed Air) flow rate to give a calculation on  the Air change rate of the space. We are also measuring the Return Air flowrate to see the difference with regards room pressurization requirements. The difference between the supply and the return will be the minimum air for the cleanroom area's pressurization- Make up air- Fresh Air requirements. This can be set to be above the minimum fresh air requirements for Leed.  On the fresh air  ( Outside Air) we are also measuring the Temperature and humidity. We don't have a flowrate grid on the Outside air ( Fresh Air). The return air from the rooms we are also measuring the temperature and humidity. The air being mixed (Mixing Air) we are measuring also the mixed air Temperature and Humidity   Where we are mixing the  A. Outside Air ( Fresh Air ) + B  Return Air is your total flow rate for the Supply ( mixed) where again we are measuring the mixed air temperature and humidity. We can use the BMS system, to calculate the outside air as a percentage to the fresh air based on the above calculation- mixed air temperature . On the motorized dampers - Fresh Air mixing damper, Mixing Air damper and exhaust air damper. The percentage opening of the damper is also displayed on the BMS . When the system is exhausting a section of the return, this can be calculated from the % opening of the exhaust damper and the fresh air damper to give the percentage  return air being mixed. The mixing Air temperature and humidity sensors shall provide optimized mixing of the Air to ensure unnecessary cooling during the summer time i.e. avail of free cooling (mixing) instead of availing of mechanical cooling. The mixing Air temperature and humidity sensors is to measure the total energy content of the air and the modulating dampers will have to be set up to ensure that the optimum mixing ratio of fresh air to re-circulated air is obtained on exiting the mixing section of the AHU, thereby minimizing the end energy use of the unit when humidity and temperature are critical parameters that must be maintained.   Mixing is to be done in a vigilant manner incorporating and controlling the pressure profile and keep the required minimum outside air requirement. I don't see the point of putting another flow meter on the outside air ( Fresh Air) , when we have all the necessary flow meters on the supply and return ducts. Positional sensors on the Fresh Air damper and exhaust dampers. We also have all the temperature and humidity properties of the above , to calculate the the actual Outside Air as a percentage.   Appreciate any feedback on what we are proposing above. Has anyone else tried this method. Appreciate any comments.