Modeling Actual Design cooling and heating airflow in Trace for Proposed Case model

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A project I’m working on recently submitted a Design Preliminary Review to USGBC and received comments back.  One of the comments on the model is that my Proposed Case design airflows reported (from Table 1.4.2 in the LEED Output summary report) are not consistent with the Mechanical Schedules.  I admittedly missed this requirement.  My model is set up so that Main airflow for cooling and heating are "to be calculated." Actual airflow on the design documents and Mechanical Schedules is different due to rounding, over-sizing for future, etc.  My question is this - what is the best way to model actual cooling and heating airflow in Trace? I have a very large building (almost 200,000 SF) and very large model due to large number of rooms.  I've now learned i can group like rooms into thermal blocks, will do that next time.  So, inputting actual room airflow into the hundreds of rooms will be incredibly time consuming, albeit the most accurate and full proof approach.  My templates are set up so that "airflow" is set by AHU, meaning each room is assigned to an AHU System by using the templates.  I'm thinking of changing the main airflow template to the % of design option, then calculating the ratio of actual airflow to calculated, and putting that in the template.  If Trace calculated a system needs 1,000 cfm, but my AHU is scheduled for 1,500 cfm, then I can put in 150% into the template so that all rooms are increase proportionally.  Anyone agree with this approach or have other ideas?  My systems are VAV with 30% minimum, hydronic reheat.

barbaram's picture
Joined: 2012-03-20
Reputation: 0

A few things. It sounds like you are using your design model for LEED. However, the design model was used to make equipment selections (that don't fully coincide with your load calcs).

Remember that the LEED model is purely an energy model and not a load design model.

Your method of setting the templates to 150% works, and you can use this to get close.

Then, once you system checksums supply airflow is close to what your AHU selection is,  you can simply set the VAV block cooling airflow. (under the selection tab--> advanced button in create systems). Then, make sure your Fan HP/KW is based off of that cfm.

This works, because the reality is that you have oversized the AHU anyway, and this allows it to properly unload the fan. Which helps fan unloading. The downside of oversizing on this type of system is that it raises your VAV minimum. Hopefully you keep this below .4 cfm/sq ft - or you are hurting yourself vs LEED baseline.

For the record, the block VAV cooling only works for VAV systems and it is not something I'd recommend just changing for the heck of it. It is actually a reasonably advanced topic and I'd suggest running some test to make sure you understand it fully before submitting to LEED (and subsequently making them think this very useful method is invalid!)

Bob's picture
Joined: 2010-06-30
Reputation: 475