trying to quantify VAV airflow needed in economizer mode

[BronYrAur]

I realize that this is a completely subjective question that is highly dependent on the geographic location and internal loads, but...

Does anyone have any historical data or a rough guess on how much airflow is typically required when you start running in economizer mode? In other words, if my system is designed for 50,000 CFM on a design day of 95 deg, what should I expect on a 55 deg day when I switch to economizer mode? Obviously, some boxed will be throttled back on a 55 deg day.

Does anyone have a "rule of thumb" guess on what the airflow would be?

 

[joejoedancer]

BronyrAur,

I'm thinking you could come up with this by changing the outside air design conditions in a load program such as TRACE or HAP. Assuming there exists a load calc run with one of these programs or other equal, keep all your envelope criteria and internal loads the same, and simply override the outside design conditons to 55F and see what the program kicks back for a supply air flow to satisfy the load (lock in your supply air temp at 55F in the program for it to calculate the required air flow)

 

[marcoh]

For internal zones the airflow will not change relative to ambient conditions.

For external zones at low external temperatures need need to consider day/night/clouds/solar etc. On a cold sunny days the solar load could offset any fabric losses. If no solar gain and the perimeter zones require no cooling the VAV boxes will go to their minimum setting.

Minimum air flow for the system would be with all VAV boxes turned down to minimum (say 30%?). In practice with lights on and building at least partly occupied (ie there are some internal loads) might expect 40% air flow. Programs such as HAP will provide the information for any hour of the year.

 

[imok2]

Yes my rule of thumb is to go full OSA and let the system find it's balance sensing point at the required static pressure set point