The definition of Unmet Load Hours has been vague for quite some time. ASHRAE 90.1 2010 was supposed to address this issue. Do we have some resolution about this yet? For example: 1) Do we count the sum of all the unmetloadhours for all the zones, or just the zone with the most unmetloadhours (this was a problem for 200 zone models)? 2) What is the tolerance of the zone temperature to the zone setpoint before the timestep is counted as unmetloadhour time, and what is it based on. 3) Is there an allowance for "pull down time", i.e. the time that it takes for the zone to reach setpoint after recoving from the setpoint change after night setback. During this time the loads will be unmet.

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Marcus Sheffer

LEED Fellow

7group / Energy Opportunities

LEEDuser Expert

4454 thumbs up

Fri, 11/09/2012 - 20:17

Looks like 2010 dropped the 50 differential but kept the 300 overall limit. The 2010 language indicates projects can exceed the 300 limit by providing sufficient justification that the accuracy of the simulation is not compromised. I do not see that 2010 specifically addressed the issues you raise. Here is my understanding - 1) coincident unmet load hours in multiple zones only count as one unmet load hour; 2) probably depends on the software; 3) whether it counts or not depends on how you model the transition period - for example, if you can model steps or optimal start it can reduce the unmet load hours.

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