[Equest-users] Baseline PTAC/PTHP calc

Fri Apr 1 12:10:47 PDT 2016

Hi Steve,

I've used both "averaging" approaches and "per system" calculations with
success for per-zone baseline system types.  I would reason project size
/ sheer system quantities into my decision to go one way or the other.
I think it's key in any case to be consistent in your
documentation/calculations. 

Note the GBCI LEED spreadsheet has support for (paraphrasing) "typical
of ## systems" baked in.  At some point, it would be silly from all
fronts (review and documentation) to separately document tons of nearly
identical units in separate columns.  If the reviewer should wish to
spot check a couple individual cases, you can always provide complete
SIM reports for the systems.  If going along the "averaged" route, I'd
use some of that time saved to clearly show my work for coming up with
averaged system inputs as supporting documentation (and reference that
upload in the Section 1.4 workbook alongside your "typical" system so
that's clear, for context).

I also don't think there's precedent for forcing any particular doe-2
system type - you should be free to choose any system that best
approximates what's required.

Regarding # of systems per apartment:  Whatever you do... don't let any
of that "real world common design sense" muddy the waters for
interpreting baseline appendix G requirements!  

In all seriousness though, if there exists an actual design for
reference showing a given apartment is actually multiple systems/zones,
the onus falls on the modeler to determine those separate systems/zones
(& even apartments) may be reasonably blocked together for having
similar system properties & load profiles.  If you aren't comfortable
making such assertions (and dealing with the consequential extra legwork
for determining/documenting things like "average system size" for
efficiency calculations later), then you can always fall back to drawing
up zoning to reflect as many systems/zones as will actually exist.  I
try to weigh such end-to-end considerations at the outset of model zone
mapping, where applicable.

~Nick

------------------------------------------------------------------------
------------------------------------------------------

Nick Caton, P.E.

  Senior Energy Engineer
  Energy and Sustainability Services
  North America Operations
  Schneider Electric

D  913.564.6361 
M  785.410.3317 
E  nicholas.caton at schneider-electric.com
<mailto:nicholas.caton at schneider-electric.com> 
F  913.564.6380

15200 Santa Fe Trail Drive
Suite 204
Lenexa, KS 66219
United States

From: Equest-users [mailto:equest-users-bounces at lists.onebuilding.org]
On Behalf Of Steve Jacobs
Sent: Thursday, March 31, 2016 12:07 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Baseline PTAC/PTHP calc

I'm starting to realize that there is a fairly large range of approaches
when calculating the PTAC EER value for a baseline model. A simplified
approach I've always used is to get the baseline EER is to take the
total cooling capacity of all the apartments, and then divided that by
the total number of units. Then use this average value to calculate EER.
I've used this approach on a lot of LEED projects and never had a
problem. I'm curious if others are calculating in on a unit by unit
basis. My approach does take into account that top floor and corner
units will probably need bigger and less efficient units. But
calculating it on a unit by unit basis could also become very tedious,
where a small change in the baseline model could necessitate the
re-calculation of every system's EIR. Also, this all assumes one PTAC
unit per apartment. But in a real world design you could easily have one
in the living room and a different unit in the bedroom.

I'm just curious what approach people use on this calculation.

Also, I've found the controls on the actual PTAC unit a little
restrictive so I've been using PSZ to model the PTAC systems at times
and changing the performance curves.

-        Steve

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