That is a very phillosofical question.
The sizing day can be assigned. We usually use ASHRAE sizing day "weather" which is based on the statistical likelyhood in % of such extremes occuring. These were developed using advanced statisticle methods and many years worth of historical weather data.
It does not account for future weather or unknown changes to your building and its future usage. Engineers often include a safety factor for such unknowns. ASHRAE 90.1 App. G stipulates a factor of 1.25 for heating and 1.15 for cooling for the baseline building probably because this has been common past practice in the US.
However, oversizing comes with a price...possibly. For example, most chiller manufacturers know that chillers, when they run, typically spend most of their time at 40-60% of full load and design them to be quite efficient in this band (possibly even more efficient than at full load). However, because the industry uses IPLV (partload efficiency), and full load efficiency as benchmarks to sell their equipment, they can't afford to penalize the full load performance too much in favour of better partload performance.
Each chiller is different (has different performance matrices at differing load, ambient air temps and humidities, water supply and return temperatures. So picking the "right" chiller for the job requires testing several over a year simulation... Enter the world of parametric and optimization simulation studies.
Mit freundlichen Grü�en- Sent from my iPhone (excuse the brevity)
i. A.
Jean Marais
b.i.g. bechtold
Tel. +49 30 6706662-23
On 30.07.2014, at 16:20, "benjamin_khuong@xxxxxxxxx [EnergyPlus_Support]" <EnergyPlus_Support@xxxxxxxxxxxxxxx> wrote:
Ah, I found it. To size my HVAC system would knowing the peak demands be enough? I'm assuming so.
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