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Unmet hours according Ashrae 90.1/2010

asked 2023-03-16 13:19:49 -0500

Nathalia's avatar

I have a question regarding the Unmet Hours required by ASHRAE 90.1/2010. I've always checked the 300 hours parsing only during occupied cooling or heating. However LEED has questioned that the 300 hours must be achieved both during occupied and unoccupied heating and cooling, is this correct?

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answered 2023-03-16 23:58:29 -0500

Keigo's avatar

updated 2023-03-28 23:37:54 -0500

That's a good issue to raise. In my opinion, that's correct. Unmet hours must not exceed 300 hours during both occupied and unoccupied heating and cooing.

ASHRAE90.1 defines "unmet load hour" as follows. There is no mention of "during occupied time".

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And the requirment of unmet hours is as follows.

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I think the consufing thing for EnergyPlus users is that the summary reort (the outpu of the simulation result in html or csv format) shows the following Comfort and Setpoint Not Met Summary which reports unmet hours only during occupied time as shown below. But it's ok, this summary is not wrong because this summary is about Comfort.

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The bad thing is that LEED summary also reports unmet hours only during occupied time as shown below. I think this is a defect and should be fixed to unmet hours during both occupied time and unoccupied time (i.e. 8760hours).

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We can see unmet hours during both occupied time and unoccupied time in System Summary, but anyway the LEED summary should be fixed.

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I would like to hear others' opinions.


This website shows the opposite opinion i.e. LEED counts Unmet Hours only during the occupied hours. However, I cannot find any evidence for it. Does anyone know?

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ASHRAE 90.1 User's Manual seems to support the opinion that the Unmet Hour is for both occupied and unoccupied hours (i.e. throughout the year) because the denominator is 8760 hours in the manuals as shown below. I vote for this opinion so far.

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Hi Keigo! Thanks for the answer. For me, reading item G3.1.2.3, it is not clear whether what is being required are the hours occupied or not occupied. For not making it clear, I believe I should evaluate both. And what left me more confused is that in the emissions of other projects sent to LEED, I always indicated the hours occupied in the table, and we were not questioned beforehand. Hence my doubt. But for the next ones, I'll check the two o'clock. But now I have another question. Would it be 300 hours during occupancy plus 300 hours during non-occupied hours? or would it be 300 hours all?

Nathalia's avatar Nathalia  ( 2023-03-17 06:26:14 -0500 )edit

300 hours all. What is not pointed out in the LEED Certification Review Report is not necessarily correct. GBCI do not review every detail of energy modelling, and they often overlook incorrect inputs.

Keigo's avatar Keigo  ( 2023-03-17 06:52:02 -0500 )edit

You could potentially expand your tolerances for zone heating/cooling setpoint not met. If cooling setpoint is 21C and heating setpoint is 23C, the throttling range would be 23 - 21 = 2C. Therefore, your tolerances (half the throttling range) would be +/- 1 deltaC.

ericmartinpe's avatar ericmartinpe  ( 2023-03-21 12:37:14 -0500 )edit

@ericmartinpe, FYI, the definition of temperature control throttling range in ASHRAE90.1 is as follows: the number of degrees that room temperature must change in order to go from full heating to no heating or from full cooling to no cooling. It is not the delta T between cooling setpoint and heating setpoint. The throttling range for coolng and the throttling range for heating are set independently. The graphic on this page helps.

Keigo's avatar Keigo  ( 2023-03-21 21:39:31 -0500 )edit

@Keigo interesting, thanks for that info. So, how would you determine the throttling range in a model? Wouldn't that require plotting temperature and flow alongside setpoints for each zone? In practice, I've always used 2 degrees F (1.11C) tolerance for zone setpoint not met in LEED models.

ericmartinpe's avatar ericmartinpe  ( 2023-03-24 18:50:15 -0500 )edit

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Asked: 2023-03-16 13:19:49 -0500

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Last updated: Mar 28 '23