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# How to measure heating degree day (HDD) and cooling degree day (CDD) for a building

I am not very much familiar with EnergyPlus simulation. Recently, I have developed a PCM based gypsum board for building envelope applications, i.e., wall, ceilings and floor. I also modeled a 4X4X3.5 m building room for measuring the indoor thermal environments, number of comfortable hours monthly/yearly with range 18 to 26 °C, HDD and CDD with and without use of my developed gypsum board in Wall and Ceiling. I can measure indoor temperature and energy consumption, but I cann't measure the comfortable hours, HDD and CDD.What's the output variable for these?

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Normally HDD and CDD are associated with a specific climate rather than to a particular building. ASHRAE 90.1 defines them as:

degree-day: the difference in temperature between the outdoor mean temperature over a twenty-four-hour period and a given base temperature.

In general, HDD and CDD are used as proxies to indicate much heating and cooling a typical building might have in different climates. It is a rough proxy since all buildings are different.

HDD is often at base 65F and CDD is at base 50F. The ASHRAE Handbook of Fundamentals (2017) contains Chapter 14 which describes this in more detail and has some example. For EnergyPlus weather files, the accompanying "stat" file contains information about HDD and CDD per month and annually.

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Nice summary, but I have never seen a CDD base as low as 50F used. The CDD base is usually assumed to be either the same as the HDD base or typically higher. Having a CDD base lower than the HDD base implies a great deal of uncontrolled solar gains.

( 2019-06-10 10:36:20 -0500 )edit

@lirainer : I believe one major contextual reason for the CDD50 and HDD65 temperature conventions is that those are the average daily temperature bases used in definition of the climate zones. You'll find multiple sources tabulate CDD50/HDD65 side-by-side.

( 2019-06-11 19:34:33 -0500 )edit

@lirainer : For your 2nd assertion: For degree-day analyses, I have commonly encountered heating/cooling systems "overlap" in base temps, and 50/65 in isolation aren't totally weird... For example, bldgs in my climate have distinct heating/cooling seasons where boilers/chillers aren't needed at all for weeks at a time, but simultaneous operation is req'd in swing seasons. That equipment may be scheduled in a fashion correlative to avg OADB conditions. Knowing this, you'd want to select distinct base temperatures considering the OADB thresholds which bound heating/cooling loop operation.

( 2019-06-11 19:40:53 -0500 )edit

If you are simply looking for degree day stats to fill in a box for something prescriptive like a LEED analysis, then Jason's response is almost certainly correct and you are then just simply looking to characterize the weather/climate for someone otherwise unfamiliar.

If however your specific phrasing "... for a building" infers that you are instead looking to practically apply a degree-day analysis to a specific building/climate/historical period... Awesome! I'll provide a response aimed at that sort of scenario.

Missing from the prompt is the point/purpose of such an analysis, which is important detail... So I'll provide some such context for sake of example. Let's assume you are performing a utility analysis for an existing facility, and after asserting a base:seasonal load split on your gas/electricity consumption history you are looking to evaluate the strength of a potential correlation between your seasonal energy consumption profiles and the corresponding degree days. The degree to which you can assert the "seasonal" energies align with CDD and HDD will lend confidence to your efforts and allow you to draw up a simple model correlating input weather conditions (in say, a TMY year) to estimate future projected baseline seasonal energy consumption, upon which you might be able to reasonably bound the potential savings associated with a boiler replacement.

To get there, we could just pull some CDD65 and/or HDD50 stats from a table, but those balance points might not be a good assumption for your specific situation. Let's describe our example further:
You know from an energy audit that seasonal gas usage is defined by only boiler consumption, which in turn service only a 2-pipe heating/cooling loop. The 2-pipe loop, as operated by the plant manager, is always in one of 3 states: heating, cooling, or inoperative. We can characterize the "inoperative" state as occurring generally in a temperature band around the building's balance point, where no substantial heating/cooling is necessary to maintain comfort. Never heating/cooling simultaneously. The BAS automated switchover is shot, but the plant operator does a good job of tracking occupant comfort and weather trends, and shared with us a historical log of dates the plant has been shifted between heating, cooling, and swing season states. Combining this data with historical AMY, we can ascertain that the boilers are characteristically in operation only when the daily average OADB is 57F or less. Further, the chillers are active only when daily average OADB is 66F or higher.

NOW we've established the most appropriate, building/climate/system-specific base temperatures upon which to base the stated degree day correllation study. One could alternatively avoid much of this field work and understanding in the systems' operations, having collected the historical weather and utility data, by simply interatively "backing into" a balance point which in turn produces a good fit, but you run the risk of missing any of these important details (like the deliberate non-operation of the plant ...

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Thanks Caton for your comments. I am not very much familiar with EnergyPlus simulation. Recently, I have developed a PCM based gypsum board for building envelope applications, i.e., wall, ceilings and floor. I also modeled a 4X4X3.5 m building room for measuring the indoor thermal environments, number of comfortable hours monthly/yearly with range 18 to 26 °C, HDD and CDD with and without use of my developed gypsum board in Wall and Ceiling. I can measure indoor temperature and energy consumption, but I cann't measure the comfortable hours, HDD and CDD.What's the output variable for these?

( 2019-06-12 09:18:43 -0500 )edit

I might clarify that heating/cooling degree days (HDD/CDD) to my experience are a metric used to characterize climate/weather conditions over time. Not occupant comfort. I too am inexperienced with e+, but am hopeful someone else more practiced might constructively step in to advise whatever output metrics exist natively for quantifying thermal comfort. If for example you were using a doe2/eQuest simulation, I would point you towards the standard SIM reports summarizing zonal temperatures through the simulation, or else the option to export interval measurements for further analysis.

( 2019-06-12 19:33:39 -0500 )edit

Both HDD and CDD can be obtained via the "weather" utility in the EP Launch Utilities tab. You ask for a statistic report of your EPW filke, and he utility creates such a report. As for CDD and HDD, close to the end of the file you find these lines (taken from one of my local climate files, please note that formatting in this post is not exactly the one you get in the file):

• Heating/Cooling Degree Days/Hours calculated from this weather file.
• Heating/Cooling Degree Days/Hours from design conditions shown earlier in this report.
• Monthly Weather File Heating/Cooling Degree Days/Hours

                         Jan   Feb  Mar Apr May Jun Jul Aug Sep Oct Nov Dec


HDD base 10C 0 0 0 0 0 0 0 0 0 0 0 0
HDD base 18C 87 39 10 0 0 0 0 0 0 9 51 99

CDD base 10C 162 207 293 367 425 381 356 341 309 292 201 149
CDD base 18C 1 21 55 127 177 141 108 93 70 53 12 0

CDH base 20C 660 1062 1837 2885 3505 2610 1995 1745 1388 1427 849 574
CDH base 23C 215 510 940 1759 2133 1392 879 715 514 666 318 199
CDH base 27C 9 127 244 706 871 431 150 71 73 130 22 9

• 3484 annual (wthr file) cooling degree-days (10°C baseline)
• 0 annual (wthr file) heating degree-days (10°C baseline)

• 858 annual (wthr file) cooling degree-days (18°C baseline)

• 294 annual (wthr file) heating degree-days (18°C baseline)
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