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

[Robby Oylear]

Chip,

What you say makes sense to a point.  I would argue that most Owner's would
expect the system to work under the very conditions you describe as
excessively conservative and if they don't the contractor and engineer will
be hearing about it down the line.  This is something that should be
discussed on a project by project basis, because when presented properly
the Owner can see considerable savings from such a design as long as they
understand the risk they are taking on by accepting the reduced system size.

-Robby


On Thu, Dec 5, 2013 at 7:34 AM, Chip Barnaby <cbarnaby at wrightsoft.com>wrote:

>  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 <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 <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 <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
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