[Bldg-sim] calculating the mean monthly outdoor temp

Nathan Miller nathanm at rushingco.com
Mon Dec 1 09:12:21 PST 2008


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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