Neeraj, it is in fact the contrary.

ASHRAE 90.1-2007 Appendix G requires you to model Baseline Airflow Rate using a 20-degree F temperature difference between the supply air and room air temperatures. Therefore, if your room set point is 75F for cooling, set your cooling leaving air temperature to 55F and vice versa for heating; 72F heating set point requires a 92F leaving air temperature. This establishes your CFM airflow within the model. Do not vary these to adjust unmet hours.

If you have unmet heating or cooling hours (over 300 or greater than 50 between baseline and proposed), you may incrementally adjust the baseline cooling and heating capacities (originally oversized 15% and 25% respectively) accordingly to reduce the overall unmet hours (below 300) or the difference between baseline and proposed (within 50).

Before taking this step, be sure to look at your thermostat drift points. If the cooling set point is 75, but the cooling drift point is 84, when the space is unoccupied this would allow the space to move toward 84 degrees. When the space becomes occupied and the simulation tries to cool to 75, the system will not be able to achieve this within the allotted time (1 hour) and you will have an unmet cooling hour.

There are some changes to ASHRAE 90.1-2010 that will effect unmet hours, but for the time being, if you are modeling a project using 2007 or 2004, this method should help you eliminate the unmet hours.

Regards,

Michael M. Collarin, EIT, LEED AP BD+C

Hi All,

I think Nick is spot on in his discussion re unmet hours. While I'm not
saying it's a wrong thing to do, I personally have never used oversizing the
system as a way to get rid of unmet hours.

As an example, I have been helping someone off the list with his project, a
seemingly very simple building with only 5 thermal zones and one PVAVS air
handler with dx cooling and hot water heating - system 5. After running my 4
exposures, and having none of them with *any* hours of unmet loads, I
calculated my kW/cfm, heating load, cooling load, boiler kW and all of that,
put the calculated results in the baseline, reran the baseline, and for some
darn reason I had over 1000 hours of unmet cooling loads.

I checked all my inputs, made a few changes, reran the 4 exposures, and not
much changed. So, I put on my thinking cap and read Nick's email - not
necessarily in that order. One of the things that occurred to me is that
since the building is turned off all weekend my Monday morning pull down
loads during the summer might be the cause of the problem, so I put in an
optimum start schedule (this might have been required, I haven't double
checked) and my unmet hours were reduced by about 300 hours. I had an
adequate cfm/sf, something I'm not shy about raising at all since eQUEST
defaults it to 0.5 cfm/sf which is usually too small, so I didn't change it.
Then I looked at the throttling-range.

For VAV systems the eQUEST/DOE2 manuals say that the throttling range should
be *at least* 4 degrees to insure stable operation and that is what I had it
at originally. After thinking about it and doing some simple math in my head
I raised it to 6 degrees. My cooling temperature is set at 75 degrees so
this means when my room temperature reads 79, set point temp + 1/2 TR + 1
degree, my cooling coils will be activated. Before I did this I had also
looked at my SS-F reports and could see that my room temps weren't that far
off from the set point temp when I had under cooled hours reported. Using
the new throttling range brought me down to less than 500 hours of unmet
loads, which may work.

So this is an example of the thinking around what's going on and developing
a strategy that makes sense for a specific building. I also looked at my
heating/cooling temp schedules, etc., as Nick mentioned. What baffles me is
why, after none of my 4 exposures showed any unmet load hours, my baseline
did. I, like Nick, have not had that happen before. Perhaps it has something
to do with the way the building was zoned, 4 exposures and a core but I
don't know for sure.

Any ideas?

Carol

I deal with unmet hours by supplying more air rather than upsizing capacity,
and it has worked.

Jorge E. Torres Coto

Nick,

Thank you for the response. My first energy modeling experience was with eQuest, but have been using Trane?s TRACE 700 more extensively. In addition to the bldg-sim lists, the TRACE support is an excellent resource for those using the software, especially when it comes to unmet hours.

As you mention, and as I was alluding to in my third paragraph (starting with ?Before taking this step,?), there are certainly other areas that should be addressed prior to adjusting the cooling and heating capacities. I was not trying to cover all of them and did not want to get too in depth, as there are numerous areas that could be contributing to the unmet load hours and a review of the model would be necessary to determine what changes must be made to remedy the model. I?d like to note that I said the model and not the unmet hours. As you and Carol have illustrated, I fully agree that we aren?t supposed to be trying to manipulate the model to give us our desired outcome, but rather provide an accurate model. I do not think anyone should be taking liberty with the capacities to such extremes just to meet the requirements for unmet load hours. I even scratched my head the first time I read through that section and thought, ?well what?s the point??

My response wasn?t meant to suggest that modelers wildly vary cooling and heating capacities to obtain their desired results, but to clarify that Neeraj?s comment (copied below) was in fact contrary to what ASHRAE requires in Appendix G. Perhaps a few disclaimers were in order J

Correct me if I am wrong but on the contrary I think that ASHARE baseline PRM fixes the cooling and heating capacities to 1.15 and 1.25 respectively and the user is allowed to bump up the supply CFM in increments if necessary.

In conclusion, I offer these bullet points:

? Design airflow rates are set using a 20-degree delta T between supply air and room space

o These shall not be adjusted to ?fix? unmet hours

? Baseline cooling and heating capacities are initially set to 115% and 125% respectively

o These may be adjusted incrementally, but should not be relied upon to ?fix? a poorly produced model. In fact, their adjustment should be considered last, after all other approaches have failed.

? It is important to understand what an unmet hour is and why they occur

o It is of even greater importance to remedy the cause of the unmet hour rather than ?fudging? the numbers

o Refer to your software?s Building Temperature Profile report, if such a report exists, to help find the source (space, system, etc) of the unmet hours

o Thermostat Drift Points and Utilization Schedules have been primary culprits

? Other areas affecting unmet hours include but are not limited to the following: Thermostat placement, fan cycle operation, optimum starts, building envelope entries, Ventilation (outside air) schedules

Happy modeling!

Michael M. Collarin, EIT, LEED AP BD+C

I can't give an exact answer, but the first thought is that DXF is for
geometrical information, while the gbXML schema is for thermal properties
and zoning, hence the DXF isn't generally directly converted or even
convertible into gbXML.

More common though is to import DXF into Sketchup and then use the geometry
as a basis to trace the thermal zoning. I have done this using a DXF in
OpenStudio (http://apps1.eere.energy.gov/buildings/energyplus/openstudio.cfm)
to create (manually) an IDF file, which is the input file for EnergyPlus.

You could check whether DesignBuilder can import IDF geometry (it is based
on EnergyPlus after all!), and there could be your solution, let me know how
it turns out!

Cheers and good luck,

Marcus Jones, LEED AP