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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">In my personal experience, different base temperatures are used with HDD and CDD metrics for different situations/context.  We aren$B!G(Bt locked to CDD50/HDD65,
 per se, and it can sometimes be insightful to plot things like chiller/boiler operations and utility billings against mean OADB temperatures to discover a useful and unique set of base temperatures specific to a given system/building/campus.  I recommend reading
 up on the degree day analysis chapter/section in ASHRAE Fundamentals for more info/ideas!  It$B!G(Bs my hazy recollection (it has been some years) that prescriptive standards like 90.1 primarily utilize/define degree days with the isolated intention to quantify
 and bound the prescriptive applications/requirements of specific energy measures like economizers based on climate characteristics.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">I cannot recall, however, working with something as high as CDD78.8 (26$B!k(BC), so I suspect immediately there may be some critical differences in how Chinese
 standards define and utilize base temperatures for degree day analysis.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">To better explain, it may be helpful to observe a couple points:<o:p></o:p></span></p>
<ol style="margin-top:0in" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo3"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">The base temperature itself reflects an average (mean) temperature over a 24 hour period.  If
 a given day has an average temperature above or below that threshold, that infers some degree of heating or cooling is necessary to maintain comfort. 
<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo3"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">CDD50 more specifically infers that some mechanical cooling is necessary/utilized for at least
 part of the day when the daily average temperature is higher than 50F.  <o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo3"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">Conversely, HDD65 infers some heating ins necessary/utilized when the daily average temperature
 is below 65F.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo3"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">Taken as a set/pair: the fact that these bases are different infer that both heating
<u>and</u> cooling are needed for days when the average temperature falls in between. 
<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0in;mso-list:l0 level1 lfo3"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">For example, where I live right now the daily high over the last 24 hrs was 81F and the low
 was 56F.  Let$B!G(Bs assume a really balanced sine curve so the average was exactly 68.5, right in the middle.  That would result in accruing 65.5-50 = 18.5 CDD50 degree days, and
<u>zero</u> HDD65 degree days, as 68.5 > 65.  Anecdotally and subjectively, while it was a little chilly during my early morning commute, I felt no particular need for heating while in the office today.  On the other hand, a few of my colleagues were complaining
 about the space getting too hot this afternoon.  Now, there are a lot of variables leading to my colleagues$B!G(B voiced discomfort$B!D(B including high internal gains and poorly selected/installed equipment (long story)$B!D(B but I bring these variables up to illustrate
 that the base temperature behind CDD50 does not define or drive whether we need cooling.  It$B!G(Bs rather just a tool for bounding the threshold around which cooling is and is no longer necessary to maintain comfort. 
<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">When I teach degree days, I like to tie and relate the concept of base temperatures to building $B!H(Bbalance points$B!I(B and $B!H(Bthermal flywheel.$B!I(B  These are
 not all the same thing but it can help to illustrate the function/purpose of defining your own base temperatures for reasons outside of prescriptive mandates/standards.  Base temperatures for example can be used to bound things like loop operations boiler/chiller
 plant cutoff/availability on the basis of exterior temperature measurements.  <o:p>
</o:p></span></p>
<p class="MsoListParagraph"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">This is all just my personal perspective, of course, but
<b><u>I hope this may be helpful!</u></b>  I$B!G(Bd also highlight certain individuals who occasionally frequent this mailing list like Mr.
<a id="OWAAM5EC0405EA235482F84FE7C0C85498B2B" href="mailto:yjhuang@whiteboxtechnologies.com">
<span style="font-family:"Calibri",sans-serif;text-decoration:none">@Joe Huang</span></a> whose name pops up in the recommended Fundamentals chapter could offer some deeper, possibly more accurate insights =).<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">Kind regards,<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA">~Nick<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;mso-fareast-language:JA"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Arial",sans-serif;color:#3A7A46;mso-fareast-language:JA">Nick Caton, P.E. (US), BEMP<o:p></o:p></span></b></p>
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</span><span style="font-size:9.0pt;line-height:150%;font-family:"Arial",sans-serif;color:#626469;mso-fareast-language:JA">Energy Manager, Yokota Airbase
<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Equest-users <equest-users-bounces@lists.onebuilding.org>
<b>On Behalf Of </b>Yongqing via Equest-users<br>
<b>Sent:</b> Sunday, April 10, 2022 8:49 PM<br>
<b>To:</b> equest-users <equest-users@lists.onebuilding.org><br>
<b>Subject:</b> [Equest-users] why is CDD50 used in ashrae 90<o:p></o:p></span></p>
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<p><span style="color:red">[External email: Use caution with links and attachments]</span><o:p></o:p></p>
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<span style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#4B4B4B">Hi, everyone<o:p></o:p></span></p>
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<span style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#4B4B4B">A question was confused me for a long time, so have to the email to ask you, which is why CDD50 is used in Ashrae 90, CDD50 is defined as following:<o:p></o:p></span></p>
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<p style="margin:0in;background:whitesmoke"><span style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#4B4B4B">cooling degree-day base 50$B!k(BF (CDD50): for any one day,when the mean temperature is more than 50$B!k(BF, there are as many degree-days as degrees
 Fahrenheit temperature difference between the mean temperature for the day and 50$B!k(BF. Annual cooling degree-days (CDDs) are the sum of the degree-days over a calendar year.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#4B4B4B">However, I am very confused why 50$B!k(BF is used as the threshlod between cooling and heating? That means when the outdoor air temperature is more then 50F, the cooling
 starts to work?In chinese standard, 26</span><span style="font-size:10.5pt;font-family:"Cambria Math",serif;color:#4B4B4B">$B!n(B</span><span style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#4B4B4B"> is as the base of CDD.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:10.5pt;font-family:"Arial",sans-serif;color:#4B4B4B">Yongqing<o:p></o:p></span></p>
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