[Equest-users] Modelling thermal zone with multiple AHUs

Nicholas Caton ncaton at catonenergy.com
Wed Mar 4 22:48:28 PST 2015

Hi Anura,

If the individual systems do not act independently, or otherwise are not
expected to handle substantially different load profiles through the year
(consider beyond internal loads: do some zones have differing skin loads?),
then it’s probably safe to combine systems.

In my experience, I would however caution this particular approach of
combining systems to streamline your model development has potential to
backfire.  The time you save in the short term with inputs could be lost to
processing those system inputs (creating & documenting those
heating/cooling/airflow capacity sums and weighting associated unitary
efficiencies) and perhaps also in troubleshooting/re-constructing the model
if you find out later the assumption of identical system behavior was
off-base.  On the other hand, the extra time you would spend setting up
each individual system could be better invested elsewhere in the modeling
process to create a better final product (or towards getting home on time)!

In my mind, this sort of “how detailed is necessary” decision is a matter
of risk:reward.  It’s a regular category of judgments that pop up all the
time in new work.  Being able to recognize and explore opportunities for
acceptable approximations is a defining trait for experienced modelers.

Insofar as LEED reviewers are concerned, I have anecdotally combined
systems (documenting them as such with supplemental language) without
incident in past projects.

Kind regards,



*Caton Energy Consulting*
  1150 N. 192nd St., #4-202

  Shoreline, WA 98133
  office:  785.410.3317


*From:* Equest-users [mailto:equest-users-bounces at lists.onebuilding.org] *On
Behalf Of *Anura Perera
*Sent:* Wednesday, March 04, 2015 4:01 PM
*To:* equest-users at lists.onebuilding.org
*Subject:* [Equest-users] Modelling thermal zone with multiple AHUs

Dear All,

I am modelling a building having thermal blocks with multiple thermal
zones. Each thermal zone has an AHU. The floor level internal loads are
evenly distributed. As such I am planning to model the thermal block
considering all AHUs in the thermal block as lumped into one large AHU with
total capacity of all individual AHUs of zones in the block.

Will this be an acceptable approach for LEED reviewers?

Any experience to share please?

Thanks in advance

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