[Bldg-sim] Residential System Sizing

[Bereket Nigusse]

Hi All,

I am modeling a residential building with RESYS as a system, and using auto sizing for system sizing.

The space temperature (space design temperature) for load calculation is set at 75 oF while the Design-Cool-T is set at 78 oF.  I believe that this has the effect of decreasing the system cooling capacity. There is no set-back or set-up applied.    No ventilation outdoor air is specified.  Infiltration, which is already captured in the building load calculation, is part of the building load.  

How does the system cooling capacity increases by more than 50% (in the case of Minneapolis and Duluth, Minnesota) compared to the building peak load in the absence of any out door air load for ventilation?  

Thanks,
 
Bereket





________________________________
From: Joe Huang <joe at drawbdl.com>
To: Ellen Franconi <EFranconi at archenergy.com>; bldg-sim at lists.onebuilding.org; Jeff Haberl <jeffhaberl at tees.tamus.edu>; Bereket Nigusse <nigusse_ba at yahoo.com>
Sent: Tuesday, February 3, 2009 2:16:32 AM
Subject: Re: [Bldg-sim] Residential System Sizing

 
Ellen, 
 
Not exactly. What you wrote is definitely true during  the SYSTEMS simulation and peak loads reported in SS-F (?), but Bereket was  referring to the SV-A, which is the systems sizing report. Assuming that he's  doing "auto-sizing",  the DOE-2 sizing routine will take the peak loads  from LOADS and compute the design cooling capacity at the  COOLING-DESIGN-TEMPERATURE. (I may have the keyword wrong, since I don't have a  DOE-2 manual handy).  Thus, the termperature discrepancy between the single  LOADS temperature and the COOLING-DESIGN-TEMPERATURE does exist, as you've  mentioned, but transient loads due to tempeature set-up or set-back  or interzone heat flows are not reflected. That's why an autosized  SYSTEM will often still result in a few undercooled or underheated  hours.
 
My suggestion to Bereket in making sense of the numbers is  to compare the TEMPERATURE under SPACE-CONDITIONS for the LOADS reference  temperature, to the COOLING-DESIGN-TEMPERATURE (I may have this keyword  wrong) is ZONE-CONDITIONS in SYSTEMS. What kind of HVAC are you modeling?   It sounds like a residential system (RESYS), but that system doesn't have any  OUTSIDE-AIR, does it ? 
 
Joe Huang
White Box Technologies, Inc.
 
----- Original Message ----- 
From: Ellen    Franconi 
To: bldg-sim at lists.onebuilding.org ; Jeff Haberl ; Bereket    Nigusse 
Sent: Monday, February 02, 2009 7:50    PM
Subject: Re: [Bldg-sim] Residential    System Sizing

Another reason that the results are different is that the space    temperature may be different in the "loads" part of the DOE-2 program than in    the "systems" part of the program. In "loads", the space    temperatures are set equal to the space design temperate. In "systems",    the space temperature is based on the zone setpoint schedule and whatever    temperature is actually achieved in the space. Thus, if you have a set up or a    setback from the design space temperature, it will be different. 
Ellen
 
Ellen Franconi, Ph.D., LEED AP
Energy Analysis Group Manager
Architectural Energy Corporation
2540 Frontier Avenue
Boulder, CO 80301
tel. 303-444-4149
fax 303-444-4303
efranconi at archenergy.com
http://www.archenergy.com/

>>>    Jeff Haberl <jeffhaberl at tees.tamus.edu> 2/2/2009 8:40 PM    >>>
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
Professor......................................................Office    Ph: 979-845-6507
Department of Architecture.......................Lab Ph:    979-845-6065 
Energy Systems Laboratory.......................FAX:    979-862-2457 
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College Station, Texas,    USA.......................URL: www-esl.tamu.edu
8=/ 8=) :=) 8=) ;=) 8=)    8=() 8=) 8=? 8=) 8=) 8=) 

________________________________

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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