[Bldg-sim] Conservative design cooling load calculations vs baseline sizing

Chip Barnaby cbarnaby at wrightsoft.com
Thu Dec 5 07:34:45 PST 2013


All,

An under-appreciated aspect of residential loads calculations is 
"swing."  The building and its contents will absorb a lot of energy 
if the air temperature is allowed to rise a few degrees during peak 
events.  Modeling work that I have done in ASHRAE 1199-RP and 
elsewhere shows that tolerating an occasional 3 F temp rise above the 
set point can reduce the required equipment size by up to 35% 
(depending on building mass etc. etc.)

It is excessively conservative for cooling calcs to assume peak 
internal gains coincident with peak outdoor conditions.  On hot 
afternoons, people very rarely cook full turkey dinners with all the 
TVs on.  If they do, let them warm up a little.  Better than paying 
for an oversized system and running it at lower part load all the time.

A major (and perhaps the only significant) distinction between 
residential and non-residential design philosophy is that in non-res 
situations, people are constrained about what they can do to control 
their environment.  In non-res, the worker must stay at his/her desk 
and slave on ... in res, the occupants can close the shades, move out 
of the sun, and decide to barbecue when it is hot.  Much more 
forgiving situation, so it is nuts to double-oversize res cooling 
systems for the worst worst conditions.  In my humble opinion.

All of which relates to actually designing a good system, as opposed 
to doing 90.1 analysis.  The two activities probably have minimal 
overlap at best.

Chip Barnaby


At 09:48 AM 12/5/2013, Robby Oylear wrote:
>To be honest, the only potential "overly conservative" sizing I can 
>see going on here is if the building is served by a central cooling 
>plant and that plant has been sized based on the sum of the peak 
>loads and not the peak coincident block load for the building.  If 
>this is in fact how they sized the system then there is potentially 
>a large cost savings that could be had by appropriately sizing the 
>central plant.
>
>The "worst case" scenario that you describe is a real scenario that 
>can happen fairly easily.  All it takes is someone to be at home 
>with their lights on and appliances running on a peak summer 
>day.  That doesn't seem like a very unlikely scenario at the zone level.
>
>As for how the Baseline equipment should be sized, it should be no 
>different in terms of lighting, plug loads, and solar gains.  The 
>only stipulation on the baseline simulation is that it is oversized 
>by a prescriptive 25% for heating and 15% for cooling.  ASHRAE 90.1 
>doesn't strictly define what a "sizing run" is, but it would be 
>considered standard practice to utilize the peak lighting and plug 
>loads during your sizing calculations (i.e. don't assume any 
>diversity on the lights and use the maximum anticipated coincident 
>plug load value).
>
>-Robby
>
>
>On Thu, Dec 5, 2013 at 6:43 AM, Patrick Bivona 
><<mailto:patrick.bivona at gmail.com>patrick.bivona at gmail.com> wrote:
>Hi Mike,
>
>I can't say I'm frustrated yet. Just going through the learning 
>curve. I'm sure frustration will come in time!
>
>I was musing about the potential imbalance in sizing approach 
>between the proposed design and the baseline design. If we assume 
>for a second that engineers also use ASHRAE design days when sizing 
>equipment for the proposed design, there is still a potentially 
>significant difference between schedules used for sizing and normal 
>operation schedules. Is the 1.15 sizing factor for baseline cooling 
>enough to cover such imbalance? I don't know enough to say so yet.
>
>Patrick
>
>
>
>
>On 5 December 2013 20:19, Michael tillou 
><<mailto:michael.tillou at gmail.com>michael.tillou at gmail.com> wrote:
>Hi Patrick,
>
>I just wanted to clarify for you that Appendix G (G3.1.2.2.1) 
>requires the simulation of sizing runs for equipment selection based 
>on either the peak from the weather file or ASHRAE 99.6% heating and 
>1% cooling design temperatures.  As far as I know this is not 
>something USGBC or GBCI has written a clarifying rule on for LEED.
>
>As far as being frustrated over engineers doing overly conservative 
>load calculations, welcome to life as an energy analyst.  The best 
>you can hope for is to use simulation to show them and the building 
>owner that they are over engineering the system and try to get some 
>concession.  Ultimately at the end of the day it's the engineer who 
>is stamping the work and taking on the liability that has the final say.
>
>Mike.
>
>Sent from my iPad
>
>On Dec 4, 2013, at 11:17 PM, Patrick Bivona 
><<mailto:patrick.bivona at gmail.com>patrick.bivona at gmail.com> wrote:
>
> > Hi,
> >
> > When sizing cooling equipment for apartments, the design team on 
> my project used a worst case scenario approach of maxing out all 
> internal loads. For instance, lighting would be on during the day, 
> with full occupancy and all plug equipment running, while the sun 
> is hitting the windows. Adjacent rooms would be considered without cooling.
> > For LEED, the sizing for the baseline is based on schedules 
> representing typical operations for occupancy, lighting, plug, etc. 
> That leads the baseline to size equipment that has smaller capacity 
> than what considered for the proposed design.
> > There's an extra dimension, in the fact the design team sized 
> equipment for each individual room, reaching their peak load at 
> different times of the day. So the total cooling capacity is the 
> sum of the worst possible scenarios. The model currently uses a 
> single zone per apartment, with a not so worst case scenario.
> > How do I solve this conundrum? Am I missing something?
> > Thanks,Patrick
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---------------------------------------------------------
Chip Barnaby, BEMP     cbarnaby at wrightsoft.com
Vice President of Research
Wrightsoft Corp.            781-862-8719 x118 voice
131 Hartwell Ave            781-861-2058 fax
Lexington, MA 02421      www.wrightsoft.com
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