Tolerance for unmet load hours for LEED

I was wondering what people use as their tolerance for unmet load hours for LEED, this has been touched on before in some of the previous questions but I think it is a really interesting subject as you can't submit LEED EAp2 credit without getting your unmet hours down below 300 and with only 50 hours difference between the baseline and design case, the rules are very strict.

In EnergyPlus we can specify our tolerances and the default is a delta T of 0.2°C which is a very tight tolerance for unmet load hours. I have spoken with others and some software packages set their unmet load hours tolerance to around 1.8°F (1°C). The difference between these two values is enormous when it comes to deciding whether a zone has its tolerance met. Specifically, does anyone have any guidance from LEED as to what value to use. Clearly we could all use a delta T of 10°C and have no unmet hours but the systems are likely not doing what they are supposed to and the points would vary wildly.

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Annie Marston

asked 2014-10-16 15:48:47 -0500

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JasonGlazer

updated 2015-07-10 06:41:29 -0500

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You might have already seen it but check out @Matt Larson's answer in the thread below...

https://unmethours.com/question/359/w...

MatthewSteen (2014-10-16 17:56:11 -0500) edit

Where can we change the tolerance? i tried under simulation settings -> simulation control but it is fixed at 0.72R

TheEnergyTutor (2020-05-22 08:36:09 -0500) edit

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To quote AHSRAE 90.1-2007 Addendum cr:

temperature control throttling range: 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.

unmet load hour: an hour in which one or more zones is outside of the thermostat setpoint plus or minus one half of the temperature control throttling range. Any hour with one or more zones with an unmet cooling load or unmet heating load is defined as an unmet load hour.

For reference, the Commercial Reference Building Models use a reporting tolerance of 0.556 deltaC, or 1.0 deltaF.

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Matt Larson

answered 2014-10-17 12:34:13 -0500, updated 2014-10-17 12:51:44 -0500

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Great! thanks for this, I have been looking for guidance for ages this is perfect! and close to what we have been using but we were really just going off judgement. I think you wrote this in your other answer but this is fantastically clear.

Annie Marston (2014-10-17 13:17:31 -0500) edit

eQuest defaults to 2F, which I think is accepted industry wide.

Benjamin (2014-10-17 15:48:47 -0500) edit

That 1F delta from the Commercial Reference Building Models is not a very hard value, I am sure we'd have gone larger if the controls were challenged. I think 2F is quite okay as the eQuest/DOE2 software is probably what 90.1 committee had in mind. 90.1 really needs to add a specific value for use with modeling software that doesn't actually throttle.

Archmage (2014-10-20 18:18:58 -0500) edit

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I'm most familiar with eQUEST for this issue.

Sometimes you have to hard enter coil capacities as the autosize routine doesn't work correctly sometimes. This happens frequently/randomly on preheat coils often. To meet the heating load eQUEST has the preheat coil, reheat coil, and possibly a baseboard in many of my models. I think it uses all of these together rather than individually as intended. In other words, the preheat coil may be set for 55F, but may not achieve that instead relying on the zone devices to meet the load. This doesn't always work so hard entering capacities can be a solution. I've had to do this with DX cooling coils too.

Changing reset schedules to gradually heat up or cool down after a nighttime setback can help too if the temperature difference is large.

Hope that helps.