[Bldg-sim] ASHRAE 90.1 Appendix G3.1.2.2

Varkie C Thomas thomasv at iit.edu
Tue Apr 5 14:24:24 PDT 2011

Design Loads Programs use generated weather for each day of a typical day of month from the ASHRAE percent design conditions for summer & winter.  It is a guide and not a standard that has to be followed.  It is used to size equipment.  ASHRAE 99% winter design -16 degF but it can go down to -40 degF.  Actual weather data can result in overestimating and underestimating.  
See http://bepan.info/engg-calcs Item 3a - Design-Weather-Generator
and http://bepan.info/hvac-prog/x1-apec-hccv-loads
You can down the APEC HCCV Loads program and try it (does not work on 64-bit)
HCC-Disk-1 HCC-Disk2
If you did notice, HAP uses two different weather files. one for design and other for simulation. And if you dig into it you'll find out that they both are different. The simulation weather is the real time (actual) temperature and other data recorded at the airports for an extended period, which cannot be overridden, while design weather data is based on extreme or peak weather as defined by ASHRAE which you can change in the program.

Now if you are looking at LEED unmet load hours they may not be necessarily due to air sytem size, but may also be due to your plant insufficient capacity. Download HAP e help 014 from here :http://www.commercial.carrier.com/commercial/hvac/general/0,,CLI1_DIV12_ETI3906_MID1738,00.html.
In here you will find a pdf saying understanding the LEED unmet load hours in HAP.
Simulation results is just for us to know how the building would run in real time when sized on the design results. In simulation , HAP takes credit of internal heat gains, which reduces your heating load, which is OK in real time. While in design , internal heat gain is ignored in heating load calcualtions. This is just one of the difference in simulation and design in HAP.
I hope this helps Milda.

On Tue, Apr 5, 2011 at 12:01 PM, milda margarin <empire999 at hotmail.com> wrote:
hi Varun 
thanks for your reply! I have another question though based on your statement that the system should be based on the 'design results'; I thought the whole point of using simulations is to predict the peak capacity required and then base your design on this? (possibly by applying safety factors according to real life experience).
And how can you have unmet loads when you design based on the peak load? In that case the 'peak load' is not the maximum load?
Further comments anybody?
best regards
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