<div dir="ltr">To try to add to that, the sequential calculation of load then system reaction is more a result of the calculation engines preferring to use only linear solvers than the fundamentals of the procedure used. The main issue arises because the load generally depends on the action of the HVAC system (e.g. surface temperatures depend on convection coefficients) and the HVAC system action depends on the load, which is a nonlinear relationship.<div>
<div><br></div><div>The Heat Balance Method (weird name because there is no such thing as a conservation of heat) simply enforces an energy conservation equation at each discrete temperature/enthalpy node. For N nodal temperatures, this gives a set of N simultaneous equations to solve at each time step. If all of these equations are linear, the calculation engine can use more efficient linear solvers to resolve the temperatures. Nonlinear solvers would require some sub-timestep iterations, which adds calculation time, and there's also the problem that nonlinear solvers do not always converge and can be sensitive to initial guesses. The preference for linear solvers is also the reason that radiation is usually linearized.</div>
<div><br></div><div>Strictly speaking, Energy Plus is an "integrative" modeling tool and does solve the loads and system response "simultaneously". The use of previous time step temperatures are used because the energy balance equations general involve transience which requires a finite difference approximation of heat storage. As Joe pointed out, this can lead to errors especially when large time steps are used in the vicinity of large temperature swings. The integrative technique of Energy Plus is to first assume the thermostat setpoint is exactly met, then calculate the system response to meet this load, then to recalculate the zone temperature caused by this system action. I'm not sure how many of these iterations are made each timestep, but there is some information passing back from the system reaction to the zonal temperatures. Similarly, this method (they call it Predictor Corrector method) is used to model system controllers. Again, I'm not sure if the information loop stops with one iteration or continues until a convergence criterion is met.</div>
<div><br></div><div>The weight factor method used by DOE-2 relies on the convolution principle of linear systems, so there is no way to obtain a simultaneous solution (unless there is some way to first deconvolve the weight factors). On the other hand, network or quasi-network solutions like the Heat Balance Method have the ability to solve for space temperature and HVAC reactions simultaneously, but this is not a requirement of the method, and true simultaneous HBM solvers are either rare or do not exist because of the computational expense.</div>
<div><br></div><div>Aaron<br><div><br></div><div><br></div><div><br></div><div><span style="font-family:arial,sans-serif;font-size:13px">Jeremiah,</span><br style="font-family:arial,sans-serif;font-size:13px"><br style="font-family:arial,sans-serif;font-size:13px">
<span style="font-family:arial,sans-serif;font-size:13px">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 why</span><span style="font-family:arial,sans-serif;font-size:13px"> Bhartendu</span><span style="font-family:arial,sans-serif;font-size:13px"> (the original poster) was not seeing any conduction loads through the ceiling. </span><br style="font-family:arial,sans-serif;font-size:13px">
<br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">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.</span><br style="font-family:arial,sans-serif;font-size:13px">
<br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">Joe</span><br style="font-family:arial,sans-serif;font-size:13px"><pre cols="90" style="white-space:pre-wrap">
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
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</pre><br style="font-family:arial,sans-serif;font-size:13px"><span style="font-family:arial,sans-serif;font-size:13px">On 8/19/2013 9:57 PM, Jeremiah Crossett wrote:</span><blockquote type="cite" style="font-family:arial,sans-serif;font-size:13px">
<div dir="ltr"><div class="gmail_default" style="font-family:tahoma,sans-serif;font-size:small;color:rgb(51,51,51)">Joe, Just wondering, given your expertise how would you compare this to the heat balance method.., and what do you think is more realistic? </div>
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<b><font color="#000099" size="1"><b style="font-family:Verdana,sans-serif">Jeremiah D. Crossett</b><span style="font-family:Verdana,sans-serif"> </span><font face="Verdana, sans-serif">| Senior </font><font face="Verdana, sans-serif">Analyst</font><font face="Verdana, sans-serif"> | Phase Change Energy Solutions</font></font></b></div>
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</span></font></div></div><br><br><div class="gmail_quote">On Mon, Aug 19, 2013 at 6:14 PM, Joe Huang <span dir="ltr"><<a href="mailto:yjhuang@whiteboxtechnologies.com" target="_blank">yjhuang@whiteboxtechnologies.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0pt 0pt 0pt 0.8ex;border-left-width:1px;border-left-style:solid;border-left-color:rgb(204,204,204);padding-left:1ex"><div text="#000000" bgcolor="#ffffff">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. <br>
<br>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.<br>
<br>Joe<br><pre cols="90" style="white-space:pre-wrap">Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 108D
Moraga CA 94556
<a href="mailto:yjhuang@whiteboxtechnologies.com" target="_blank">yjhuang@whiteboxtechnologies.com</a>
<a href="http://weather.whiteboxtechnologies.com/" target="_blank">http://weather.whiteboxtechnologies.com</a> for simulation-ready weather data
(o) <a href="tel:%28925%29388-0265" value="+19253880265" target="_blank">(925)388-0265</a>
(c) <a href="tel:%28510%29928-2683" value="+15109282683" target="_blank">(510)928-2683</a>
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</pre><div><br>On 8/19/2013 12:05 PM, David Wood wrote:<blockquote type="cite"><div><div><div>Hi All,<br><br></div>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 <br>
correspondingly.<br><br></div>Dave Wood<br></div><div><br><br><div>On Thu, Aug 8, 2013 at 8:58 AM, Rodrigo Cerqueira <<a href="mailto:rodrigo.cerqueira@mra.qc.ca" target="_blank">rodrigo.cerqueira@mra.qc.ca</a>> wrote:<br>
<blockquote><div lang="FR-CA"><p><span lang="EN-US">Hi,</span></p><p><span lang="EN-US">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. </span>Maybe someone else has an opinion about that.</p>
<p> </p><p><img></p><p> </p><p><b>Rodrigo Cerqueira, </b>ing. jr, M.Ing.<b></b></p><p><b>Simulation énergétique</b></p><p> </p><p><img alt="signatureCouriel"></p><p> </p><p> </p><p> </p><div><p><b><span lang="FR">De :</span></b><span lang="FR"> <a href="mailto:equest-users-bounces@lists.onebuilding.org" target="_blank">equest-users-bounces@lists.onebuilding.org</a> [mailto:<a href="mailto:equest-users-bounces@lists.onebuilding.org" target="_blank">equest-users-bounces@lists.onebuilding.org</a>] <b>De la part de</b> Bhartendu Awasthi<br>
<b>Envoyé :</b> 8 août 2013 06:35<br><b>À :</b> <a href="mailto:equest-users@lists.onebuilding.org" target="_blank">equest-users@lists.onebuilding.org</a>; <a href="mailto:bldg-sim@lists.onebuilding.org" target="_blank">bldg-sim@lists.onebuilding.org</a><br>
<b>Objet :</b> [Equest-users] Roof Conduction Load</span></p></div><p> </p><div><p>Hi All,</p><div><p> </p></div><div><pre style="white-space:pre-wrap">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. </pre>
<pre style="white-space:pre-wrap"> </pre><pre style="white-space:pre-wrap">As per my understanding this load should appear in LS-C also. </pre><pre style="white-space:pre-wrap">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). </pre>
<pre style="white-space:pre-wrap"> </pre><pre style="white-space:pre-wrap">Refer attached LS-C and LS-B reports. </pre><pre style="white-space:pre-wrap">Any help will be appreciated. </pre><pre style="white-space:pre-wrap">
</pre><pre style="white-space:pre-wrap"> </pre><pre style="white-space:pre-wrap">Thanks & Regards </pre><pre style="white-space:pre-wrap"> </pre><pre style="white-space:pre-wrap">Bhartendu Awasthi </pre>
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