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Re: [EnergyPlus_Support] how do I verify zone heating loads?
I will offer some quick answers to help you understand this better. But
ultimately, you have to simplify your simulation greatly in order to create a
case that can be verified by simple hand caclulations such as UADeltaT. I
suggest that you start with a very simple box - no windows, no infilration, no
internal gains and satify yourself that the caculations are reliable, then add
features one by one.
1. No valid range found for Loop Demand=726610.25 means that the last upper
load range limit was not large enough. This load may have occurred during
warmup and may not relate to any real loads in the building once the simulation
has intialized. Always put a very large number as the final upper limit so
that equipment will always operate no matter how large the load gets.
2. Report variables (nearly all) are defined in the Input Output Reference.
Search for the report variable name.
3. If you do not have any window frame and divider objects, then center of
glass u-value applies over the entire area of the window subsurface.
4. UADeltaT applies if the following conditions are met: constant indoor
temperature, constant outdoor temperature, no sun, exterior radiant (night-sky
and ground) turned off by setting exterior thermal absorptance to near zero.
5. U values in the eio output file are at nominal conditions. Interio and
exterior convection coefficients change continually by default (and can be
reported) unless you specify constants for these using the CONVECTION
COEFFICIENTS object.
6. You infiltration caclulation is probably off because EnergyPlus by default
varies windspeed and outdoor temperatures with height off the ground. The
windspeed on a groundfloor zone is generally about 50% of the measured wind
speed on the weather file. Report the local values or turn off the height-
dependency.
Mike
On 22 Feb 2007 at 21:01, brianstreby wrote:
> In trying to resolve heating plant issues, I am first looking at what
> appears to be a load error:
>
> ** Warning ** No valid range found for Loop Demand=726610.25 in Load
> Range Based Operation Scheme=BLR STAGING.
> ** ~~~ ** All equipment will be turned off. Other cases of
> this condition will not be reported.
>
> I assume that units for Loop Demand are watts, in which case this load
> is far higher than expected.
>
> I chose a zone at random and selected report variable Zone/Sys
> Sensible Load to Heating Setpoint, which I also found was about 200%
> of design when run on a random winter day with a higher-than-design
> outdoor drybulb. Question (1) does the value for this report variable
> include ventilation loads?
>
> With the intent to verify each component heat loss for that zone, I
> first reported construction and surface details and checked u-values
> in the .eio file. I was able to verify all u-values except the windows
> which appeared to be center-of-glass u-values rather than overall
> u-values. I was not able to find any frame u-values in this output
> file, which leads to my second question: (2) is there any way to
> verify the overall window u-value, including glass, edge effects, and
> frame?
>
> Next, to verify heat loss through the envelope, I selected the
> following report variables:
> -outdoor drybulb,
> -zone mean air temperature,
> -window heat loss (for each window in the zone),
> -window frame heat loss (for each window in the zone),
> -surface internal convection heat rate (for each surface in the
> zone), and
> -zone infiltration sensible heat loss.
>
> Using a simple Q = UA delta T for each window/surface, I could not
> match my calculated loads to the ones in the report (reported window
> heat loss more than double what I expected, wall heat loss less than
> 1/4 expected). This leads to the next two questions: (3) is UA delta T
> valid in this case, or does night sky radiation significantly factor
> into the heat losses? and, (4) Is my interpretation that the report
> variable "surface internal convection heat rate" represents the
> surface heat loss from the zone correct?
>
> I also tried verifying the infiltration load with no success. My input
> file uses 52 cfm with 1,0,0,0 factors allowing for a constant
> infiltration rate. Using 52 cfm at 35 deg F OA and 43 deg F indoor
> temp (low because the HW plant is not working), I should get a heat
> loss of 1853 Btu/h or 534 Watts or 325,918 Joules over a ten minute
> time step. The reported infiltration heat loss is 83,456 J. Question
> (5) why the difference?
>
> One final question: (6) there does not appear to be any report
> variable that shows the actual zone heat loss rate (at the actual zone
> node air temperature), whereby i could sum the losses through the heat
> transfer surfaces, sub-surfaces, and by infiltration for comparison
> purposes (assuming internal loads = 0). am i missing this somewhere
> in the reports?
>
> Two departing comments: (a) I find it difficult in some cases to
> interpret what was meant by a particular report-variable. is there a
> dictionary of these somewhere? if not, this would be very helpful. and
> (b) units for similar report variable headings are not consistent, e.g
> "window heat loss" and "window frame heat loss" carry the unit Watts
> while "zone infiltration sensible heat loss" carries the unit Joules.
>
> Many thanks in advance for your help!!!
>
> Brian
>
>
>
> The primary EnergyPlus web site is found at:
> http://www.energyplus.gov
>
> The group web site is:
> http://groups.yahoo.com/group/EnergyPlus_Support/
>
> Attachments are not allowed -- please post any files to the appropriate folder
> in the Files area of the Support Web Site.
>
> EnergyPlus Documentation is searchable. Open EPlusMainMenu.idf and press the
> "search" button.
>
> Yahoo! Groups - Join or create groups, clubs, forums & communities. Links
>
>
>
>
>
>
========================================================
Michael J. Witte, GARD Analytics, Inc.
EnergyPlus Testing and Support
EnergyPlus-Support@xxxxxxxx
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