[Bldg-sim] calculating the mean monthly outdoor temp

Gaurav Mehta gmehta75 at gmail.com
Tue Dec 2 00:33:38 PST 2008


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 

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 
Sent: Monday, December 01, 2008 10:02
To: Nathan Miller ; 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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