[Bldg-sim] calculating the mean monthly outdoor temp
Chris Yates
Chris at zed-uk.com
Tue Dec 2 08:28:28 PST 2008
Thank you Gaurav,
I used the average of all (300 or so) hours for each weather month of an
EPW file. Rationale being:
- The method is comparitive so shouldn't depend too heavily on absolute
weather data (like we do in the UK)
- Unofficial advice from a technical adviser at ASHRAE confirmed it
- ASHRAE person pointed me to same results on Weather.com with close
agreement
- I calculated using the "means from means" method and it was within 0.5
of a degree
From discussion, a drawback appears to be that certain weather sets and
regions may unfairly load the result in either direction.
Joe Huang suggested that the original "means from means" method was a
legacy from the days when weather stations reported daily min and max
only. Simulation methods have moved on since. When this was put to the
ASHRAE adviser he recommended that this was put formally to ASHRAE as a
change request.
I'm intrigued as to where the naming of 80% and 90% limits come from.
Like you say Gaurav, neither of them refer to 80% or 90% of hours. The
nomenclature appears to be entirely arbitrary. In light of your
correction to your first email, it also seems evident that they can be a
little misleading.
Best regards
Chris Yates
Gaurav Mehta wrote:
> sorry, I should have read my response before hitting the send button.
> The following text would require some change-
> /
>
> /......use the internal operative temperatures during the occupied
> hours to determine if 80% of those hours are _all the hours_ are
> within the upper and the lower limit of _80% of acceptability limit_
> allowable indoor operative temperature./
>
> /
> //
> Best regards,
> Gaurav
> //
>
> *From:* Gaurav Mehta <mailto:gmehta75 at gmail.com>
> *Sent:* Tuesday, December 02, 2008 00:33
> *To:* bldg-sim at lists.onebuilding.org
> <mailto:bldg-sim at lists.onebuilding.org>
> *Subject:* Re: [Bldg-sim] calculating the mean monthly outdoor temp
>
> Mean monthly outdoor temperature was incorporated in the ASHRAE
> Standard 55-2004, because this parameter can be found easily from
> readily available climatic data, such as published by
> www.ncdc.noaa.gov <http://www.ncdc.noaa.gov>
> In the ASHRAE RP-884 project, which formed the basis of the adaptive
> thermal comfort in ASHRAE Standard 55, the outdoor temperature index
> used was the mean effective temperature (ET*). ET* combines
> temperature and humidity into a single index and for RP-884 was
> calculated based on measured data. It should be noted that RP-884
> involved nearly 21,000 sets of raw data compiled from 160 different
> office buildings located over a broad spectrum of climate zones spread
> across 4 continents. For more information regarding the relationship
> between thermal comfort and the environmental index- ET*, please refer
> to the Thermal Comfort chapter in ASHRAE Handbook of fundamentals.
> As mentioned earlier, the mean monthly outdoor temperature was
> introduced in the ASHRAE Standard 55, because it is easy to use and
> can be estimated quickly. The regressions (between the indoor
> operative temperature and the outdoor temperature index) that were
> finally incorporated in Figure 5.3 in the ASHRAE Standard 55 were
> recalculated based on mean monthly outdoor temperature (original index
> being ET*).
> It is not necessary, that everyone agrees with the adaptive thermal
> comfort envelope yielded by using the mean monthly outdoor temperature
> and figure 5.3 of ASHRAE Standard 55-2004. According to one of the
> voting members of ASHRAE Standard 55 committee, the method was
> incorporated in the Standard because the majority of the members voted
> in favor of this method. And as you are all familiar with the public
> comment procedure of ASHRAE Standards, please do voice your opinions.
> To answer the original question by Chris-
>
> One may purchase the last years weather data for a small fee at NOAA
> website which provides the mean monthly outdoor temperature or may use
> a TMY3 weather file to extract the hourly data for the month in
> question. Calculate the daily max and daily min and then calculate the
> average of daily max and average of daily min and then calculate the
> average of these two averages. One may use only the summer months
> and/or shoulder seasons (depending on the climate) for estimating the
> adaptive thermal comfort envelope, because that’s when one would use
> natural ventilation for cooling. Once you have the adaptive comfort
> envelope for the month in question, use the internal operative
> temperatures during the occupied hours to determine if 80% of those
> hours are within the upper and the lower limit of allowable indoor
> operative temperature.
> Furthermore, some researchers believe that adaptive thermal comfort is
> a much complex phenomenon and hence the adaptive comfort envelope be
> estimated using the running average of the past 30 days to calculate
> the thermal comfort envelope for a particular day in a month and some
> simplify that to only past one day. Please refer to the work by
> McCartney & Nicol (2002), Nicol & Raja (1997) and Humphreys and Nicol
> (1998).
> Thanks.
> Gaurav Mehta
>
> *From:* JRR <mailto:energy.wwind at cox.net>
> *Sent:* Monday, December 01, 2008 10:02
> *To:* Nathan Miller <mailto:nathanm at rushingco.com> ;
> bldg-sim at lists.onebuilding.org <mailto:bldg-sim at lists.onebuilding.org>
> *Subject:* Re: [Bldg-sim] calculating the mean monthly outdoor temp
>
> John Ross wrote;
>
> Using a mean monthly temperature ties the hands of innovative
> engineers in opportune climates.
> In Northern Virginia I use an opening roof skylight to dump excessive
> solar gain in the late afternoon.
> This operational variation is only practiced the last week of
> September to the second week of October
> depending depending on the particular year's weather
>
> It would be appropriate to use mean monthly temperature for
> calculating long term Geothermal field
> effects on the other hand.......
>
>
> Nathan Miller wrote:
>> I'm no statistician, but I've always been dubious about that calculation.
>> You are taking the mean of two means, which seems to be getting farther
>> and farther from actual data.
>>
>> It also seems to punish some of the climates that are best suited for
>> natural ventilation, since having cool nights significantly drops the
>> acceptability limits. I assume they are pushing you to take advantage of
>> night pre-cooling and thermal mass, but I find it curious that there are
>> no allowable hours outside of the range. All of the pre-cooling in the
>> world isn't going to help you keep the temperatures down during that one
>> string of 90 degree days in the tmy2 file...
>>
>> Nathan Miller
>>
>> Senior Energy Engineer/Mechanical Engineer
>>
>> direct: 206.788.4577
>>
>> fax: 206.285.7111
>>
>> -----Original Message-----
>> From: bldg-sim-bounces at lists.onebuilding.org
>> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Chris Yates
>> Sent: Friday, November 28, 2008 2:30 AM
>> To: Building Simulation
>> Subject: Re: [Bldg-sim] calculating the mean monthly outdoor temp
>>
>> There is a small difference between the arithmetic mean of the monthly
>> mean max and min and the overall arithmetic mean of 720 hours of weather
>> data. Like you say, it's probably a legacy from the old pen and paper
>> days.
>> I've heard that some would use occupied period only to calculate the
>> mean. However, I can not find any reference in the text of A55 to
>> justify this. Being able to average the warmer occupied hours would make
>> things a whole lot easier - even the 80% acceptability temperature can
>> regularly be lower than the summertime peaks. It's a tough call for
>> natural ventilation.
>>
>> Many thanks
>>
>> Chris
>>
>> Joe Huang wrote:
>>
>>> The cited method sounds like a carry-over from when stations reported
>>> only max/min temperatures.
>>> If hourly data is available, why wouldn't you just calculate the mean
>>> of all the temperatures ?
>>>
>>> Joe Huang
>>> White Box Technologies
>>>
>>> ----- Original Message ----- From: "Chris Yates" <Chris at zed-uk.com>
>>> To: "Building Simulation" <bldg-sim at lists.onebuilding.org>
>>> Sent: Thursday, November 27, 2008 1:58 PM
>>> Subject: [Bldg-sim] calculating the mean monthly outdoor temp
>>>
>>>
>>>
>>>> For the purpose of acceptability limits, can anybody cite any
>>>> guidance on calculating the mean monthly outdoor temperature? I've
>>>> read ASHRAE 55 and it states: "mean monthly outdoor temperature is
>>>> the arithmetic average of the mean daily minimum and the mean daily
>>>> maximum outdoor (dry bulb) temperature for the month in question."
>>>> Thanks
>>>> _______________________________________________
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>>>
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>>
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