[Bldg-sim] Residential System Sizing

[Bereket Nigusse]

Jeff,

Thank you for the explanation.  

Yes I am modelig a residential building and using RESYS as a system.  Infiltration is specified and the zone load calculation reflects infiltration contribution to building load.  Since the outdoor air fraction is set to zero, I assume that  ventilation contribution to the building load or system capacity is also zero.  The space temperature for load calculation is set at 75 oF while the Design-Cool-T is 78 oF.  I beileive, this has the effect of decreasing the system cooling load.  

Jeff the peak design load day is different from that of the peak system days for some of the loacations.  However, the cooling load differences due to peak load and system peak day differences are quite small compared to the change I am looking at the peak load and system cooling capacity.

It is not obvious to me as to how the low temperature night cooling (due to 24 hour infiltration) results in in increasing the cooling capacity for the system?   

 
Bereket





________________________________
From: Jeff Haberl <jeffhaberl at tees.tamus.edu>
To: Bereket Nigusse <nigusse_ba at yahoo.com>; bldg-sim at lists.onebuilding.org
Sent: Monday, February 2, 2009 10:40:59 PM
Subject: RE: [Bldg-sim] Residential System Sizing

Bereket:

I would not expect the values in SV-A and LS-B to agree for most buildings. There are many reasons for this.

First, peak loads from LOADS do not include ventilation loads as would be the case in SYSTEMS, only infiltration. 
Second, you may be looking at different days on the weather file between LOADS and SYSTEMS, so this would 
need to be checked. 

In addition, DOE-2 sizes for the absolute peak, whereas other methods, such as Manual J, have some undersizing
built into the procedures, hence you'll almost always get a much larger system size in DOE-2. 

Both the SV-A and LS-B lists you show seem reasonable, with the exception of the MN #s, which I suspect that you
may have a fair bit of 24 hour infiltration in your input file...(i.e., low night time temps in MN?) 
However, without looking at the file I can only guess. 

There are 100s of reasons for the funny little differences you show.  You're probabloy better off running a Manual J 
calculation on the house if it is residential. DOE-2 is known for doing a very bad job of sizing for residential.

Jeff

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Jeff S. Haberl, Ph.D., P.E.............................jhaberl at esl.tamu.edu
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________________________________

From: bldg-sim-bounces at lists.onebuilding.org on behalf of Bereket Nigusse
Sent: Mon 2/2/2009 5:59 PM
To: bldg-sim at lists.onebuilding.org
Subject: [Bldg-sim] Residential System Sizing



Hello All,

I have a problem with DOE2.1E cooling system sizing.  The cooling capacity reported by DOE2.1e in SV-A and LS-B reports are quite different and shows different sensitivity trends depending locations.  

Question 1
For the case with out door air flow ratio input set to zero DOE2.1E calculated cooling capacity and the peak coolig load are shown below.   I assume that the out door air contribution to the system load is zero for zero out door air fraction and with this assumption the system cooling capacity will be different from the peak cooling load only due to difference in ARI and the peak cooling load hour weather conditions.  What else could cause the cooling capacity to be different from the peak load for zero out door air flow rate case?  I have looked into to DOE2.1E manual but wasn't able connect the steps from the peak cooling load to the system capacity?  

Question 2
Does the change from the peak cooling load to the  system cooling capacity seem a reasonable for the different loacations given below? 
The building is 2100 sqft conditioned floor area, two story, single family house.  Each floor is modeled as a single zone.


           Report AV-A     Report LS-B                
Locations     Supply Flow, CFM     Cooling Capacity, kBtu/hr     Peak Cooling Load, kBtu/hr     Outdoor DB Temp at Peak Load, oF     Outdoor WB Temp at Peak Load, oF    
Baltimore, MD      850.0     25.7     22.1     93     73    
Houston, TX      813.0     27.0     20.2     93     78    
Chicago, IL      1054.0     29.3     20.3     89     72    
Minneapolis, MN      1226.0     33.5     20.3     89     79    
Duluth, MN      1258.0     34.8     18.1     87     72    
Phoenix, AZ      1281.0     37.4     31.6     114     71    


Thank you in advance for the help,


Bereket


      
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