Unmet Hours

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For LEED compliance under ASHRAE 90.1, are the 300 unmet hours the sum of all the unmet hours in each zone, or the peak per zone?  In other words if I have 250 unmet hours in one zone and 250 unmet hours in another zone, does this comply, or is it considered 500 unmet hours total?  By zones, I mean thermal zones served by the same air handling unit.  Thanks.

Brad Robinson

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that would be 500 unmet hours.

the bigger leed review problem i've had is when i have more unmet hours
in one category in the proposed (like 200 unmet in cooling) and fewer
unmet hours in the other (like 0 in heating) but the baseline is
opposite. certain leed reviewers think this is a problem. the 90.1
requirement addresses the unmet hours per the entire building design
(both heating and cooling) not heating and cooling individually.

Patrick J. O'Leary, Jr.'s picture
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Hmmm..

Does that mean you need to sum the unmet hours of ALL separate systems as well?

In regards to the unmet cooling, LEED NC-2009 (Canada) notes that it is acceptable to have unmet cooling hours.  I interpreted this to mean that the unmet hours of concern are only on the heating side, not the cooling.

Brad Robinson's picture
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Brad:

Please see attachment. It is a excerpt from 90.1-2010 that clarifies the
definition of an unmet load hour.

If the 250 unmet hours in each zone occur at the same time, you would have only
250 unmet hours.

Paul Diglio

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Brad,

I believe the unmet hours refers to the entire building. If the unmet hours for the different systems occur at the same time they are counted as one unmet hours regardless of the number of systems that indicate that the unmet hours have occurred at that time.
Robert O'Connell P.E. CEM CEA

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