Unmet Load Hours Calc

7 posts / 0 new
Last post

I have a question regarding the way unmet load hours are calculated in
eQuest. I have received the following comments from the USGBC reviewer
after submitting my energy model.

1. The response narrative to preliminary comment 1 states that the SS-R
reports have been used to determine the number of unmet load hours for
each case however this method does not take into account coincident
hours. Additionally, the number of hours any system outside of
throttling range is not the number of hours the HVAC system is in
operation but rather it is the number of hours each zone exists during
the building simulation, or 8,760 hours. Therefore the number of unmet
load hours is 333 in the Baseline case and 508 in the Proposed case and
the simulation does not meet the requirements of Section G3.1.2.2. If
appealing this credit please revise both cases using the prescriptions
of Section G3.1.2.2 until the number of unmet load hours is less than
300 for both cases and the number in the Proposed case does not exceed
the number in the Baseline case by more than 50.

I thought the unmet load hours were calculated only during times when
the fans are actually running in the simulation. This is why I always
pull my unmet hours from the SS-R report, adding up the unmet hours for
both cooling and heating for each individual unit. I can also go into
the SS-E report and calculate the number of hours my fans are actually
running and then calculate the total number of hours by using the
percentage of hours outside the throttling range given in the BEPS
report.

If you calculate the hours in the manner the reviewer has outlined then
there is no way to show which hours are heating and which are cooling
and you have to provide that breakdown in the EAc1 template.

Any input on this topic would be greatly appreciated.

Thanks in advance.

BRETT M. WEST

Brett M. West's picture
Offline
Joined: 2011-09-30
Reputation: 0

Brett,

Let me state a few things, and then I?ll offer my interpretation of the comment:

? The numbers in the SS-R report are the same numbers shown in the Air-Side Summary report. These reports will yield inflated system totals, and therefore simulation totals, for multi-zone systems due to simultaneous counting of separate zones which belong to the same system. Use them to determine the ratio of unmet heating and cooling hours relative to the two combined, but the sum of hours in these reports is not necessarily the same as multiplying the BEPS percentage by ?hours fans on?.

? Multiplying the BEPS percentage by the ?hours fans on? in the SS-E report yields the correct sum of unmet heating and cooling hours, regardless of whether you are modeling multi- or single-zone systems. Use the ratio obtained above to allocate this total between heating and cooling hours (I have posted previously that there may be a more accurate method of allocating heating/cooling hours, but no one has corrected me).

? The ?hours fans on? includes hours the fans are scheduled on and the hours they are scheduled off but cycle on. Refer to DOE2.2 Volume 4 (build 47), page 138 (SS-C report description), for the ?hours fans on? definition. This definition is echoed on page 142 (SS-E report description). Therefore, for any system that cycles during off hours, all 8,760 hours have the potential to be counted as an unmet hour.

Now for the review comment. The ?hours each zone exists? phrase doesn?t make sense on the first pass. However, due to the last bullet point above and baseline requirement G3.1.2.4, ?hours each zone exists? is practically the same thing as ?hours the systems are on?. All 8,760 hours have the potential to be counted as an unmet hour if the hourly T-stat setting & throttling range are not satisfied. What if the proposed model doesn?t cycle over night? I say that?s a bad idea; I would never design a system that way unless an owner wants mold, frozen pipes, etc. Define a reasonable set back temperature, allow fans to cycle, and they will stay off if cycling is unnecessary.

Where the comment is confusing/wrong is: ?SS-R reports have been used?does not take into account coincident hours?. Yes, it does. It double-counts coincident hours on a multi-zone system, and because it is a system-level report, each system?s zones are counted separately. It is as ?coincident? as it gets. I?m not sure how the reviewer rejects the SS-R report but then arrives at totals which are apparently greater than what was reported. I have no .SIM file to review, but the smell of nonsense is strong.

DAKOTA KELLEY

Dakota Kelley's picture
Offline
Joined: 2011-09-30
Reputation: 1

You have a good nose, Dakota, so if you smell something fishy or otherwise,
I will probably agree with you.

Until you "kids" started talking about SS-R and SS-O reports I had never
even really looked at them. I always used the SS-F report to calculate my
hours. It reports hours underheated, hours undercooled, what the temperature
is (so you can see how far it goes out of range), and the time it occurs.
All of this information is very useful. For instance, you can quickly see if
you maybe just need to adjust your throttling range, from say 4 to 6, to
solve your problem, a frequently a reasonable and justifiable thing to do.
You can also see if your hours outside of throttling range happen to be
occurring on a Monday during the warm-up period during the winter or during
the late afternoon in a west facing zone during the summer, which might also
suggest an action to take. And, lastly, since you sometimes have hours in
both the heating and cooling columns, you have your coincident hours, too.
Then all you have to do is add them up. Granted that could be a chore if you
have 300 hours of unmet loads, but if I had 300 hours of unmet loads I'd
figure I hadn't modeled the building as well as I should have and I'd go
back and try to figure out why. I wouldn't be satisfied that the delta
between baseline and proposed didn't exceed the accepted 50 hours difference
until I had given it another shot to reduce the overall totals. And, like
many people have recently pointed out, and correctly I think, you've got to
know when to say "Enough".

That's MHO.

Carol

cmg750's picture
Offline
Joined: 2010-10-05
Reputation: 0

I recently received a similar response from a reviewer (same person?). Unfortunately my initial response was not enough so now the team must appeal the denial. See below. Per the attached screenshot from the eQuest detailed reports help file, my understanding of the hours outside of the throttling range is the same as Dakota?s: BEPS % Hours Outside * SS-E hours fans on = Total # of Hours Outside Throttling Range. Correct? If it is, then I guess I just need a better way to explain it to the reviewer?

Initial comment:
1. Section 1.3 of the Template provides the total unmet load hours for the Proposed (74 hours) and Baseline models (65 hours); however, the BEPS report for the Baseline and Proposed model indicates that the total number of unmet load hours is 70 (0.8% * 8,760 hours) and 140 (1.6% * 8,760 hours), respectively. The total unmet load hours for each model is determined by multiplying the number of hours in a year (8,760) by the ?Percent of hours any system zone outside of the throttling range? in the BEPS report or the BEPU report. With the revision, the total unmet load hours for the Proposed model exceeds the total unmet load hours for the Baseline model by more than 50; however, Section G3.1.2.2 requires that the Proposed model does not exceed the total unmet load hours for the Baseline model by more than 50. Please revise the baseline model per the prescriptions of Section G3.1.2.2 so the total unmet load hours for the proposed model does not exceed the total unmet load hours for the baseline model by more than 50. Please note that the total unmet load hours for the Proposed and Baseline models cannot exceed 300 as required by Section G3.1.2.2.

My initial response:
Per eQuest help documentation, the ?percent hours any system zone outside of the throttling range? should be multiplied by the hours the fans are operating in order to calculate total underheated/undercooled hours. Per the SS-L [same as SS-E since this building only has one system] report, the fans are operating on average
7388 hours per year in the Baseline and 4706 hours per year in the Proposed. That means the Baseline and Proposed models indicate that the total number of unmet
load hours is 59.1 (0.8% * 7388 hours) and 70.6 (1.5% * 4706 hours), respectively. The different fan runtimes are due to the time the fans must run during unoccupied hours to maintain setback temperatures.

Denied!
1. The response narrative to preliminary comment 1 states that the hours outside throttling range apply only to hours in which the interior fans operate however the eQUEST Help states that the ?percent of hours any system zone outside of throttling range is the percentage of hours when the temperature in any conditioned zone is outside of the thermostat?s throttling range?. If appealing this credit please revise the Baseline case per the prescriptions of Section G3.1.2.2 so that the difference in the number of unmet load hours is less than 50.

Matt Dubrovich, PE, CEM, BEMP

Matthew Dubrovich's picture
Joined: 2011-09-30
Reputation: 0

Either the reviewer is wrong (and they need to be educated so that LEED project Owners don?t have to keep writing $500 appeal checks) or that help file is wrong. Or they are both right J

There is an ?Important Note? in that help file that says ?Any hours outside throttling range that occur during night cycle control hours are included in the total hours outside throttling range implied in the percentage above.?

That?s an interesting statement. If you set your single system to have a night cycle control strategy of ?cycle on first? (where the system only kicks on if the control zone is outside of the throttling range), the help file is saying that ANY after-hours temperatures outside of the throttling ranges will be included in the BEPU % calculation if a night cycle control strategy is in place. With ?cycle on first?, there could be MANY hours where certain zones are outside of the throttling ranges, yet the system fan is OFF. If those hours are being added to the numerator, then it is implied that they are also being added to the denominator (even though the system fan is OFF).

So to me, I read this to say that if you have ANY night cycle control strategy, the denominator automatically goes to 8760.

I could be wrong, but that?s why I?ve always multiplied the BEPU % by 8760.

James Hansen, PE, LEED AP

James Hansen's picture
Offline
Joined: 2011-09-30
Reputation: 200

Good point but then I would expect (hope?) that the reviewer would ask what the night cycle control is set to. In this case, the single system is a VAV with the NCC set to ?Any? which means that the fans should be cycling anytime any zone is outside of the throttling range. That should allow me to follow the procedure outlined in the help file. Using 8760 does give the most conservative answer though. Brett, maybe you should wait to submit your response until after I send in my appeal, you might save your team $500.

Matt Dubrovich, PE, CEM, BEMP

Matthew Dubrovich's picture
Joined: 2011-09-30
Reputation: 0

I'm sure most of us have stories about mis-understandings between the LEED
reviewer and energy modeler. I must resist the urge to speak negatively
about them , but I have received many comments that show lack of
understanding of very basic modeling procedures. Anyone reading this forum
will also note that many users have similar issues. I would expect this
from beginners , but at the same time I would expect someone providing third
party review to be more knowledgeable about energy simulation.

These type of questions are probably at the root of USGBC's reasoning for
bringing the review process back in house.

Does anyone know of a list where we could post review comments and our
solutions to the more common questions?

I found an older post from Scott at DOE2.com that explains everything.
Since this is a developers explanation you could use it in your response. I
wish there was a way to distribute this to every LEED review team so we can
stop getting this question.

"

Thomas Serra's picture
Offline
Joined: 2011-09-30
Reputation: 0