[Bldg-sim] Residential System Sizing

[Joe Huang]

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

  8=! 8=) :=) 8=) ;=) 8=) 8=( 8=) :=') 8=) 8=) 8=?
  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 
  Texas A&M University..............................77843-3581
  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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