Economizer exhaust / return airflows

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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

Lyle Keck's picture
Joined: 2011-09-30
Reputation: 1


The return air flow rate is what is returning to the airhandler,
regardless of whether it is ultimately exhausted, sent through a heat
recovery device, or is recirculated to be mixed with the outside air. I
wouldn't expect it to change relative to the supply airflow, unless you
have variable zone exhaust. The difference between supply and return
flows is the sum of zonal exhaust and infiltration/exfiltration. You
have probably specified exhaust fans in one or more of your zones
totaling to 870 cfm. You should only be entering values for zone-level
exhaust if you want to model air that doesn't return to the airhandler,
such as bathroom, kitchen, and laundry exhaust fans. System-level
exhaust is automatically determined by the outside air and
infiltration/exfiltration flows.

I don't know if there is an hourly report option for system level
exhaust. I see one for "Total ZONE exhaust air flow rate (cfm)", and
another for "Plenum exhaust flow rate (cfm)". To verify economizer
operation, I would do runs with and without the economizer, and look at
the hourly OA-to-supply-air ratio (which you are already tracking) and
the energy savings.



Bishop, Bill2's picture
Joined: 2011-09-30
Reputation: 0