[Bldg-sim] three questions about how to size HVAC equpment and systems in building design

[Jin Minming]

Dear All,
 
I have three problems about how to size HVAC equipment. Any advice
would be appreciated.
 
1: The peak cooling load is usually calculated by using 1% or 0.4% dry
bulb temperature and daily temperature range from the historical data. In building
energy simulation, we usually use typical year weather data. If there are still
many (for example, above 300 hours) unmet hours for cooling in a large office
building after you make sure that all the inputs for models are reasonable, you
will increase system coil or plant capacity. Or you just think the unmet hours for
cooling may be highly dependent on software or other factors. Then we can usually ignore
it. For unmet hours for heating in building performance simulation, this seems not to be a problem as the internal
heat gains are usually assumed to be zero in sizing heating equipment. 
 
2: In sizing HVAC systems and equipment, is it common to use operative
temperature? I found that the results (capacity of boiler and chiller) will be
very different by using air and operative temperature for a building with a
high window-wall-ratio. 
 
3: Urban heat island is well documented in the literature. But it seems
it is still not clear how to consider this urbanization effect in choosing heating
and cooling design temperature for HVAC equipment. Do you think this is because
this temperature differences are too small to consider in building design? This
may be related to another problem: how the final cooling coil, fan, pump, and
chiller capacity is sensitivity to the external dry bulb temperature for
cooling design. For example, the 0.4% and 1% dry bulb temperatures are 93.8 F and
91.5 F in Atlanta. This temperature difference (2.3 F) will lead to obvious
differences in HVAC equipment capacity for a large office building. Or other
factors (like safety factors) have more significant influences on final choice
of HVAC equipment? How does the uncertainty in cooling design temperature
affect the HVAC system and plant capacity in building design?
 
Thank you in advance,
 
Jim Terry


      
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