Oversized cooling

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Hi,

I have a problem with the cooling capacity. The Max cooling load is 927 kbtu/hr, but the cooling capacity is 7501 kbtu/h. I auto-sized the equipment (system and cooling coil) for 1.00, but I don't understand why the factor seems to be 8x.

[cid:image005.jpg at 01D007F1.5E8DABC0]

[cid:image003.png at 01D007E1.48C9B8C0]

The system fan Works in part-load 0-20% all the time:

[cid:image001.png at 01D007E1.84CAE5B0]

Does anybody knows what could be causing this?

Thanks.

SUMAC
Gonzalo Lema | Mechanical Engineer | LEED GA
Tel: +1.773.857.7906 Ext 7009 (Chicago, USA)
Tel: +1.305.814.8805 (Miami, USA)
Tel: +511.253.5355 (Lima, Per?)
Tel: +593.4.600.0613 (Guayaquil, Ecuador)
Tel: + 571.3151985 (Bogot?, Colombia)
glema at sumacinc.com
www.sumacinc.com

lemagonzalo's picture
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Curious.

I do think the higher load seems to be more like what you should expect for a building this size (7501 kBTU/h = 625 tons. For a 102,000 ft2 building that's 163 ft2/ton). That might be slightly oversized for Ft Carson, although Colorado springs gets hot in the summer. Not sure what your building loads are, but 163 ft2/ton is not unheard of for a heavy use building. I would expect something more in the range of 250-300. So the question is why is the maximum cooling load reporting as only 927 kBTU/hr (77.25 tons = 1320 ft2/ton)

Just my two cents from looking at this for five minutes - I might be wrong.

Vikram Sami
BEMP, LEED BD+C
Associate Partner

ZGF ARCHITECTS LLP
T 206.521.3509 E vikram.sami at zgf.com
925 Fourth Avenue, Suite 2400
Seattle, WA 98104

vsami's picture
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Hi Vikram,

Actually, the proposed design has a 90 ton chiller, so the 77.25 tons maximum cooling load should be fine. My concern is that the auto-size PVAVS for baseline design is too big compared to the load and to the proposed design.

Is this something a LEED reviewer might object?

Thanks.

SUMAC
Gonzalo Lema | Mechanical Engineer | LEED GA
Tel: +1.773.857.7906 Ext 7009 (Chicago, USA)
Tel: +1.305.814.8805 (Miami, USA)
Tel: +511.253.5355 (Lima, Per?)
Tel: +593.4.600.0613 (Guayaquil, Ecuador)
Tel: + 571.3151985 (Bogot?, Colombia)
glema at sumacinc.com
www.sumacinc.com

lemagonzalo's picture
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Joined: 2014-08-11
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Without looking at the DOE-2.2 input and output files, it's difficult to
give you a definitive answer, but from my experience explanations for
such sizing questions can always be found with a little digging and
understanding of how DOE-2 does autosizing.

When DOE-2 does autosizing, it takes the maximum loads reported from
LOADS and calculates the equipment capacity (heating, cooling, and fan)
needed to meet those loads. The fact that the building is in a cool
climate (Colorado Springs) has several implications on the sizing
calculation - (1) fan size is probably determined by the maximum
heating, not the cooling, load, (2) fan size will also be increased
significantly (1.24) due to the altitude, (3) substantial difference
between the peak cooling load from LOADS and SYSTEMS, since the former
is derived at a constant reference temperature with no outside air,
while the latter is derived at the actual zone temperature and with
outside air (e.g., economizer running earlier in the day, etc.). To
trace these differences, compare the peak loads in the LOADS reports to
those in the SYSTEM reports.

Joe

Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
yjhuang at whiteboxtechnologies.com
http://weather.whiteboxtechnologies.com for simulation-ready weather data
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