All,

I am having trouble believing the results from my economizer operation. I have an OA economizer set up with a DB high limit of 72 degF (per design).
I created hourly reports that show (1) total system supply air flowrate in cfm (2) total system return air flowrate in cfm (3) exhaust flowrate in cfm and (4) ratio of outside air to total supply air.

Ideally, as the economizer lets more outside air in, and the ratio of outside air to total supply air goes up, less air should be returned and more air should be exhausted.

Unfortunately in my model, as the ratio of outside air to total supply air fluctuates (typically 0.5 up to 1) my exhaust airflow rate and return airflow rate stay constant.
Total supply is around 59,000 cfm, total return stays constant around 58,000 cfm, and exhaust airflow is a meager 870 cfm, also constant.
How can outside air to total supply air yield a value of 0.9 and yet the return airflow rate is 98% of the supply airflow rate?

Any insight would be well received! Thanks!

Lyle Keck