Tolerance for unmet load hours

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Does anyone know what the ASHRAE limitation on the tolerance (in F) for unmet load hours is. Design Builder/EnergyPlus appears to have this tolerance for the heating and cooling setpoint. It is defined as the maximum difference from the heating setpoint when the reporting the time setpoints are not met (for both heating and cooling). If the zone temperature is below the heating/cooling setpoint by more than this value an unmet load hour will be reported.
There is a similar input in eQUEST called the 'Cool control range' and 'Heat control range'.
If you increase the value of this tolerance you will get smaller number of unmet load hours. I was told once that ASHRAE has a limit on what this tolerance is however I am unable to locate it.
Any help would be greatly appreciated.
Thank you!

Best regards
for KEMA Services Inc., USA

Celia King-Scott PE, BEMP, LEED(r) AP BD+C
Senior Engineer, Sustainable Buildings & Communities

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King-Scott, Celia3's picture
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Hi Celia,

ASHRAE 90.1 Appendix G would simply require that the tolerance be the same
in both the baseline and proposed design energy models.

Cheers,

Mike

Michael Tillou, PE, LEED AP BD+C, ASHRAE BEMP

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I had the very same question and couldn't find the answer, but I was told that Trane Trace uses 1.1?F / 0.83?C. So we use that as a default when modelling for LEED. But the only thing I could find was as Michael Tillou says, it must be the same for baseline and design case.

I am interested to know what other people have experienced.

Thanks!

Annie

ANNIE MARSTON, Ph.D., LEED AP, BEMP
Senior Consultant, Head of Building Performance
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Beware - don't get seduced. This setting is a "gaming" temptation for some software. Bigger tolerance means less unmet load hours and can become a simple way to get within the ASHRAE limits. Better to figure out why you have the unmet hours in the first place!
That said, I had a project a while back with too many unmet load hours and figured out why they were occurring. Without going into details, the model was correct and the unmet load hours were also correct. As it happened, I had left the tolerance at the default value of .2C. That was unrealistically low, so I changed it to .5C (.9F) and unmet load hours were within ASHRAE tolerances.

James V Dirkes II, PE, BEMP, LEED AP
www.buildingperformanceteam.com
Energy Analysis, Commissioning & Training Services
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Oh I definitely agree about gaming the system, it is very easy with this. I also agree about finding the root cause of your unmet load hours that is very important. It would just be nice to have some guidance as it can take a long time to get your unmet load hours down if you are using the tolerance of 0.2?C and make very little difference to the energy savings between the two buildings. I just felt the fairest way to do it was to follow a software that had the tolerance set that was also used commonly to submit to LEED.

ANNIE MARSTON, Ph.D., LEED AP, BEMP
Senior Consultant, Head of Building Performance
[cid:image009.jpg at 01CF2284.8C44F2B0]
1424 K Street NW | Suite 500 | Washington, D.C. 20005
main +1 202 608 1334 | direct +1 202 407 9395
email a.marston at baumann-us.com
www.baumann-us.com

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