[Bldg-sim] System sized by "block" or "peak"?

Julia Beabout juliabeabout at yahoo.com
Thu May 1 07:08:02 PDT 2008


Hi Ying
Sizing by peak and by block is a function of the system type you are selecting.
Typically, you should not change this field and let it default based on the system type you have selected on the first tab in Create Systems in Trace.
"Peak" means the system is sized based on the sum of the peak cfms in each space.  This is typically used/the default for any constant volume system.
"Block" means that the system is sized based on the peak time in which the building as a whole peaks.    (For example: if you building's peak load occurs at 3pm, then the block load is equal to the sum of the cfms to each space at 3pm).  This is typicaly used/the default for any variable volume system.
It should be legitimate for these to vary between the base case model and proposed model as long as the sizing method is consistent with the system types that are being used for each model.  For example: if you are using a VAV system for you proposed model, then the system sizing should be "block".  But, if ASHRAE 90.1 dictates that you use say a PTAC system or a constant volume system in the base case model based on you your bldg size etc, then the base case system sizing should be "peak".
As a side note....a rule of thumb for real life use....depending on your building, there is usually/should be anywhere from a 10% to 25% difference between a system's size when sized on block versus peak  (with the block method giving you the smaller size).  If the difference is outside of this range, then you may need to check some inputs and make sure things are correct.
Julia


----- Original Message ----
From: "yizhao1 at vt.edu" <yizhao1 at vt.edu>
To: bldg-sim at lists.onebuilding..org
Sent: Thursday, May 1, 2008 3:24:56 AM
Subject: [Bldg-sim] System sized by "block" or "peak"?

Hi -
How great the difference could be if a system is sized by "block" compaired to
being sized by "peak" in Trace? If "sized by peak" means sizing the system
based on peak load, what about "sized by block"? For LEED credit application, I
thought that it is not right to size an AHU by "block" in the design case while
size it by "peak" in the baseline.

Thanks!

Ying

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