EE4/DOE 2.1e - VAV using 100% outdoor air

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Hello all,

Since EE4 is really a front end for DOE 2.1e, I thought I should pose my question here:

What's the best way to model a 100% OA VAV system in DOE 2.1e?

We frequently need to model laboratories with fume hoods for LEED, which use 100% OA built-up VAV systems with exhaust heat recovery. EE4 does not handle VAV systems with 100% OA. A workaround we have used in the past is to establish a weighted average of the occupied/unoccupied air change rates, and input this value as the required OA. Then we use this same value as the minimum and maximum VAV terminal airflows. Of course, this turns the VAV into a CV system, which does not allow us to take credit for fan VFDs and limits the range of free cooling.

Any suggestions would be appreciated regarding functions or keywords I could implement in DOE to tell the VAV to use 100% OA while still allowing the VAV boxes to modulate. Using an example of a system requiring 4 OACH unoccupied, 10 OACH occupied, I've been experimenting with using MIN-AIR-SCH to schedule the OA (SCH = 0.4 unoccupied, 1.0 occupied), and would like to know if I'm on the right track.

Thanks in advance for your help.

Robin Thornton

---------------------------------------------------------
Robin Thornton, CMVP, CEM
Energy Analyst
CEL Gruen, sustainability group within Crossey Engineering Ltd.
www.celgruen.ca

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Were you ever successful in figuring this out?  I am in the same boat at the moment.

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Yes, there is a way to accomplish a good approximation directly in DOE-2.1 using CFM scheduling. You need to manually edit the DOE BDL file. I'm going to assume you are confident running DOE from the command line.

Build your model in EE4 as a built-up VAV System. In each Zone, enter the maximum exhaust flow rate in both the Maximum and Minimum Airflow boxes so that EE4 will size the Reference case appropriately.

You'll need to set up a spreadsheet which lists each zone's airflow at each condition. For example: 1) max flow (total CFM exhaust when fume hoods are open, e.g. 10,000 CFM); 2) total exhaust when FHs are closed and room occupied (e.g. 7,000 CFM); 3) min flow (total exhaust when FHs are closed and room unoccupied e.g. 5000 CFM). Use these values to establish fractions of full flow at each condition. In our example, this would be 1.0, 0.7, and 0.5. Estimate which hours of the day each zone will spend at each flow condition.

In both the Proposed and Reference BDL files:

For each zone within the 100% OA VAV system, comment out the OA-CFM/PER and MIN-CFM-RATIO keywords. Add a MIN-CFM-SCH = SCHED-XXX keyword (substitute your own schedule U-name e.g. SCHED-Zone8, etc). Create a separate annual and weekly schedule for each different set of fractional airflows. If all your zones have the same fractional airflows, they can all share the same schedule. Lucky you! The weekly schedule should list the appropriate airflow fraction (from your estimates above) for each hour of the day e.g. hours 0-8: 0.5, hours 9-10: 1.0, hours 11-12: 0.7, hours 13-14: 1.0, hours 15-17: 0.7, hours 18-24: 0.5.

If your labs incorporate some sort of DCV, this would be implemented by reducing the airflow fraction in the above schedules in the Proposed case.

In the VAV system itself, change the MIN-OUTSIDE-AIR keyword value to 1.00.

Run both the Proposed and Reference files from the command line. 

If you are not comfortable editing the BDL file, this may all sound like Greek to you. I spent a lot of time with the DOE-2.1E manuals... If you're stuck, contact me and I can try to assist.

Best of luck,

Robin

---------------------------------------------------------
Robin Thornton, CMVP, CEM
Energy Analyst
CEL Gruen, sustainability group within Crossey Engineering Ltd.
www.celgruen.ca

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