[Equest-users] why is CDD50 used in ashrae 90

Tue Apr 12 03:15:31 PDT 2022

In my personal experience, different base temperatures are used with HDD and CDD metrics for different situations/context.  We aren’t 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’s 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.

I cannot recall, however, working with something as high as CDD78.8 (26°C), so I suspect immediately there may be some critical differences in how Chinese standards define and utilize base temperatures for degree day analysis.

To better explain, it may be helpful to observe a couple points:

  1.  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.
  2.  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.
  3.  Conversely, HDD65 infers some heating ins necessary/utilized when the daily average temperature is below 65F.
  4.  Taken as a set/pair: the fact that these bases are different infer that both heating and cooling are needed for days when the average temperature falls in between.
  5.  For example, where I live right now the daily high over the last 24 hrs was 81F and the low was 56F.  Let’s 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 zero 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’ voiced discomfort… including high internal gains and poorly selected/installed equipment (long story)… but I bring these variables up to illustrate that the base temperature behind CDD50 does not define or drive whether we need cooling.  It’s rather just a tool for bounding the threshold around which cooling is and is no longer necessary to maintain comfort.

When I teach degree days, I like to tie and relate the concept of base temperatures to building “balance points” and “thermal flywheel.”  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.

This is all just my personal perspective, of course, but I hope this may be helpful!  I’d also highlight certain individuals who occasionally frequent this mailing list like Mr. @Joe Huang<mailto:yjhuang at whiteboxtechnologies.com> whose name pops up in the recommended Fundamentals chapter could offer some deeper, possibly more accurate insights =).

Kind regards,

~Nick

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Nick Caton, P.E. (US), BEMP
ニック ケートン, P.E. (US), BEMP
Senior Energy Engineer
Energy Manager, Yokota Airbase
ESS - Energy & Sustainability Services
M JP
M US
Email
+81 . 070 . 3366 . 3317
+1   . 785 .  410  . 3317
nicholas.caton at se.com<mailto:nicholas.caton at se.com>
シニアエネルギーエンジニア
横田基地エネルギーマネージャー
ESS - エナジー持続可能性サービス

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From: Equest-users <equest-users-bounces at lists.onebuilding.org> On Behalf Of Yongqing via Equest-users
Sent: Sunday, April 10, 2022 8:49 PM
To: equest-users <equest-users at lists.onebuilding.org>
Subject: [Equest-users] why is CDD50 used in ashrae 90

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Hi, everyone

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:

cooling degree-day base 50°F (CDD50): for any one day,when the mean temperature is more than 50°F, there are as many degree-days as degrees Fahrenheit temperature difference between the mean temperature for the day and 50°F. Annual cooling degree-days (CDDs) are the sum of the degree-days over a calendar year.
However, I am very confused why 50°F 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℃ is as the base of CDD.

Yongqing

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