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

[Varkie Thomas]

When selecting a residential unit, I would undersize the cooling capacity
and oversize the heating capacity. The energy use with decreasing cooling
capacity is not directly proportional (about 1 kw/ton at 100% capacity to 3
kw/ton at 20% capacity).  The energy use of a heating system tends to be
directly proportional to the heating load (up to 90% efficiency).  This is
also true for commercial buildings.  Residential unit control is usually
ON-OFF.  It is more comfortable to let the fan and cooling system run
continuously and let the temperature rise a few degrees under extreme
conditions.  150 years ago there was no air conditioning and mankind
survived. 

 

From: bldg-sim-bounces at lists.onebuilding.org
[mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of Robby Oylear
Sent: Thursday, December 5, 2013 10:02 AM
To: Chip Barnaby
Cc: Patrick Bivona; bldg-sim at lists.onebuilding.org
Subject: Re: [Bldg-sim] Conservative design cooling load calculations vs
baseline sizing

 

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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