Maximum Z ratio.

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Does anybody know what value works best for the max z ratio? I'm pretty sure it is file specific, but I don't have a very clear understanding, as one small change is made and my results are all over the place.

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Hahahahaha...OK so I went to this roll-out seminar on 90.1-2010 updates coming out for release this fall. You'll never guess that they actually did explain that the 2010 will allow a penalty on the baseline building for ventilation. The ventilation rates between the Proposed Design and Baseline doesn't need to match. The 'rates' are defined as the rates within the 62.1 charts for ventilation that need to be the same. So 5cfm/Person and 0.06 cfm/sf is the same between the two. Which is the way I did it on a project I had earlier the Spring...So at this point in time, you can still do it this way described above and no one would question you since you are modeling them ventilation energy neutral, but when 2010 rolls out officially, I believe you can use this new definition to your advantage in the current LEED v3.0 (though compared to a 2007 Baseline model, you'd be able to say it was vague and will comply with the current code).

I had a 5 story office space that was baseline modeled as a Sys#5 VAVDX, hydronic gas fire boiler in which it took me a better part of 2 weeks to get the vent rates to equal htg and cooling and equal the Proposed case. Then some genius came along and said it was ok to take the vent rate from Proposed and divide it by the area and apply to all spaces the cfm/sf...one and done, but in the near future is not allowed.

This was very exciting to hear all the updates, oh crap my geak was showing

I don't understand the humor or how this is directly related to Max Vent Z ratio?

We did this to get rid of the fact that projects that take the 30% increased ventilation credit in LEED were not having to pay the energy penalty for increased ventilation in their proposed over the minimum required by 62.1. For 2010 and beyond the baseline would have to be the minimum required by the rating authority (i.e. LEED) and anything greater in the proposed for credit by a rating authority (again LEED) would not be modeled in the baseline.

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Hahahahaha...OK so I went to this roll-out seminar on 90.1-2010 updates coming out for release this fall. You'll never guess that they actually did explain that the 2010 will allow a penalty on the baseline building for ventilation. The ventilation rates between the Proposed Design and Baseline doesn't need to match. The 'rates' are defined as the rates within the 62.1 charts for ventilation that need to be the same. So 5cfm/Person and 0.06 cfm/sf is the same between the two. Which is the way I did it on a project I had earlier the Spring...So at this point in time, you can still do it this way described above and no one would question you since you are modeling them ventilation energy neutral, but when 2010 rolls out officially, I believe you can use this new definition to your advantage in the current LEED v3.0 (though compared to a 2007 Baseline model, you'd be able to say it was vague and will comply with the current code). I had a 5 story office space that was baseline modeled as a Sys#5 VAVDX, hydronic gas fire boiler in which it took me a better part of 2 weeks to get the vent rates to equal htg and cooling and equal the Proposed case. Then some genius came along and said it was ok to take the vent rate from Proposed and divide it by the area and apply to all spaces the cfm/sf...one and done, but in the near future is not allowed. This was very exciting to hear all the updates, oh crap my geak was showing

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Bvogt is spot on. Standard 62.1 allows the design engineer the freedom to decide what the max vent ratio should be. Run the design calculations first with this field blank then review the ASHRAE 62.1 report to see which spaces are "hijacking" the system. More times than not high density rooms with infrequent peak occupancy are the cause and its relatively straightforward to decide that this room or rooms will not be starved for ventilation due to people diversity and a max vent z ratio that best aligns with more consistently populated spaces can be used. The Zd value for each space is what you'll be looking at in deciding what the max vent z should be.
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Bob, that is what I have heard too. It saves you from messing with your VAV mins one by one.
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I believe this fixes the OA Fraction for the system and will overide the calulated ratios from each room. You would normally review the Trace A62 report first and determine what spaces are dictatingb the Max Z, then use this to save the iterative process of increasing VAV minimums. Some else chime in if I am wrong. Thanks Bob
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To get a better understanding, checkout: http://energy-models.com/content/understanding-max-z-ratio

It runs through an example calculation, step by step.

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