[Equest-users] [Bldg-sim] Roof Conduction Load

[Joe Huang]

Jeremiah,

Your  question is not quite on the mark because what I've described is downstream from the 
calculation of room loads where techniques such as the Weighting Factor or the Heat 
Balance methods come into play.   Before I get to that issue, though, I want to point out 
that the "Adjust Loads" procedure I described is done automatically by DOE-2 and totally 
invisible to the User;  I did so only to explain whyBhartendu (the original poster) was 
not seeing any conduction loads through the ceiling.

Now, back to this "Adjust Loads" procedure, there is no simulation program I know of that 
solves simultaneously for the heat flows and the zone temperature.  DOE-2 does it in the 
two-step way that others and I have just described. When you said "compared to the Heat 
Balance Method", I assume you're thinking of EnergyPlus.  So, how does EnergyPlus meet 
this problem?  Whereas DOE-2 does one year of LOADS and then one year of SYSTEMS, 
EnergyPlus does them in the same time step, but also sequentially. Therefore,  EnergyPlus 
calculates the heat flows using the zone temperature from the previous time step which are 
then used in its HVAC simulation without adjustment. This should be okay when zone 
temperatures don't change, but what about at morning start-up when temperatures could 
change by 5-10 F within one hour?  I've always wondered whether that's why EnergyPlus 
gives much lower heating loads in mild California climates than does DOE-2?  As far as 
which procedure is "more realistic", I can't say.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
(o) (925)388-0265
(c) (510)928-2683
"building energy simulations at your fingertips"


On 8/19/2013 9:57 PM, Jeremiah Crossett wrote:
> Joe, Just wondering,  given your expertise how would you compare this to the heat 
> balance method.., and what do you think is more realistic?
>
> /
> /
> **
> **
> **Jeremiah D. Crossett*| Senior Analyst  |  Phase Change Energy Solutions*
> *
> *120 E. Pritchard St.  | Asheboro, NC 27203  | Mobile 503-688-8951 <tel:503-688-8951>*
> **www.phasechange.com <http://www.phasechange.com> **
> *
> **
> **
> **
> **
>
>
>
> On Mon, Aug 19, 2013 at 6:14 PM, Joe Huang <yjhuang at whiteboxtechnologies.com 
> <mailto:yjhuang at whiteboxtechnologies.com>> wrote:
>
>     Just to repeat what David and Rodrigo have already sad, DOE-2 calculates conduction
>     loads in a two step process. In LOADS, a constant reference temperature is
>     specified, which DOE-2 uses to calculate the amount of heat flow through the
>     surfaces. This is the LOADS loads summarized in the LS-E and LS-F reports.  In
>     SYSTEMS, DOE-2 calculates the true temperature of the zone based on the loads passed
>     from LOADS, as well as the actions of the HVAC system.  In the course of this
>     calculation, the heat flows ("loads") from LOADS are adjusted, but this is not done
>     surface by surface, but in total using the so-called "Zone Conductance" times the
>     difference between the actual zone temperature and the reference temperature used in
>     LOADS.  At this point, heat flows through internal walls are also considered, using
>     the  difference between this hour's zone temperature and the previous hour's zone
>     temperature of the adjacent space, e.g., the Attic or Plenum.  If you are interested
>     in this number, I believe it can be extracted in an hourly report.
>
>     In reference to Rodrigo's question, I don't see any benefit in forcing a heat flow
>     between the room and the attic by using different reference temperatures.   You will
>     simply get a constant heat flow at that temperature difference throughout the
>     simulation. In reality, attics are generally colder than the room in the winter and
>     warmer than the room in the summer, so the derived heat flow with this temperature
>     difference doesn't get you any closer to reality, in my opinion.
>
>     Joe
>
>     Joe Huang
>     White Box Technologies, Inc.
>     346 Rheem Blvd., Suite 108D
>     Moraga CA 94556
>     yjhuang at whiteboxtechnologies.com  <mailto:yjhuang at whiteboxtechnologies.com>
>     http://weather.whiteboxtechnologies.com  for simulation-ready weather data
>     (o)(925)388-0265  <tel:%28925%29388-0265>
>     (c)(510)928-2683  <tel:%28510%29928-2683>
>     "building energy simulations at your fingertips"
>
>
>     On 8/19/2013 12:05 PM, David Wood wrote:
>>     Hi All,
>>
>>     The LS-C report comes from the LOADS simulation portion of DOE 2.2. The plenum
>>     reference temperature used in LOADS is adjusted in the SYSTEMS portion of the
>>     program and the loads in the spaces adjacent to the plenum are corrected
>>     correspondingly.
>>
>>     Dave Wood
>>
>>
>>     On Thu, Aug 8, 2013 at 8:58 AM, Rodrigo Cerqueira <rodrigo.cerqueira at mra.qc.ca
>>     <mailto:rodrigo.cerqueira at mra.qc.ca>> wrote:
>>
>>         Hi,
>>
>>         This happens because the load calculation is made with a fixed temperature (see
>>         picture). All the spaces, include plenum zones, have the same temperature by
>>         default (70°F). So, all roof load appears in the plenum spaces. To considerate
>>         the roof load, you have to change this temperature in plenum spaces and a part
>>         of roof load will appear like "internal surface conduction". But I'm not sure
>>         if this is a good practice. Maybe someone else has an opinion about that.
>>
>>         *Rodrigo Cerqueira, *ing. jr, M.Ing.**
>>
>>         *Simulation énergétique *
>>
>>         signatureCouriel
>>
>>         *De :*equest-users-bounces at lists.onebuilding.org
>>         <mailto:equest-users-bounces at lists.onebuilding.org>
>>         [mailto:equest-users-bounces at lists.onebuilding.org
>>         <mailto:equest-users-bounces at lists.onebuilding.org>] *De la part de* Bhartendu
>>         Awasthi
>>         *Envoyé :* 8 août 2013 06:35
>>         *À :* equest-users at lists.onebuilding.org
>>         <mailto:equest-users at lists.onebuilding.org>; bldg-sim at lists.onebuilding.org
>>         <mailto:bldg-sim at lists.onebuilding.org>
>>         *Objet :* [Equest-users] Roof Conduction Load
>>
>>         Hi All,
>>
>>         I am doing an energy simulation model in eQuest. I noticed that the load through roof conduction in LS-C is zero where as in LS-B (for plenum space) it is showing some load.     
>>
>>                
>>
>>         As per my understanding this load should appear in LS-C also.     
>>
>>         Also, if I reduce the U value of the roof, the load through roof reduces and can be accounted in LS-B only (plenum space).     
>>
>>                
>>
>>         Refer attached LS-C and LS-B reports.     
>>
>>         Any help will be appreciated.     
>>
>>                
>>
>>                
>>
>>         Thanks&  Regards     
>>
>>                
>>
>>         Bhartendu Awasthi     
>>
>>
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>>
>>
>>
>>     -- 
>>     Regards,
>>
>>     David S. Wood
>>     Principal
>>     Wood, Byk & Associates
>>     office: 610.347.0710 <tel:610.347.0710>
>>     mobile: 610.247.1208 <tel:610.247.1208>
>>
>>
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