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Too many AHUs in the proposed model [beginner question]

asked 2021-08-05 02:28:18 -0600

Kirill Fedtsov's avatar

updated 2021-08-17 11:15:08 -0600

I am beginning to create a proposed model of a medical ward for the purpose of LEED certification. The building has over 150 AHUs of the similar type (CAV and 100% OA). Do I have to create each AHU in the proposed model or can I somehow approximate them into few? I can't seem to find what Appenndix G says about that. I am using DesignBuilder.

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answered 2021-08-05 11:53:24 -0600

You can combine HVAC zones and systems into thermal blocks if they have similar usage, exterior orientation and system type. See ASHRAE 90.1-2013 Table G3.1 No. 7:

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This is the point I can't quite wrap my head around. If it says that different HVAC zone (i.e. with similar AHUs and everything else) can be combined into single thermal zone, does this mean that the thermal zone can be modeled with HVAC equipment that consists of combined characteristics of those HVAC zones equipment. For example, two similar HVAC zones are served by two AHUs (one supplies 1000 cfm and the other supplies 1500 cfm); does this mean that I can group those HVAC zones into one thermal zone and model it with a single AHU unit that supplies 2500cfm with equivalent energy consumptn?

Kirill Fedtsov's avatar Kirill Fedtsov  ( 2021-08-05 12:18:37 -0600 )edit

Yes in that case you can combine additive attributes of the systems.

ericringold's avatar ericringold  ( 2021-08-05 13:07:03 -0600 )edit

we commonly use this section to combine zones along a single thermal face. example is a row of closed offices with their own VAV boxes will get combined into a single thermal zone. (there an alternative option to use zone multipliers, but we find that can get messy/confusing at times)

dradair's avatar dradair  ( 2021-08-05 17:15:24 -0600 )edit

answered 2021-08-05 08:27:58 -0600

updated 2021-08-05 08:29:10 -0600

If there are repeating floor plates of same/similar lay out, then a floor multiplier could be used. This is especially helpful in a building/site with multiple of the same floors (same envelope, same interior loads, same schedules. same HVAC).

For the unique floor plate or areas, those need to be modeled individually.

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Do I understand correctly that if each floor is unique I would literally have to create 150 separate airloops to model 150 separate AHUs? And there's no other way withing 90.1 methodology?

Kirill Fedtsov's avatar Kirill Fedtsov  ( 2021-08-05 10:39:18 -0600 )edit

If understanding you situation properky, then 'yes'

that is a bunch of systems / huge model. I would give thought to determine a meaningful way to split that model up such that simulation run times don't become unacceptably long (many hours, if not days...)

dradair's avatar dradair  ( 2021-08-05 11:00:06 -0600 )edit

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Asked: 2021-08-05 02:28:18 -0600

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Last updated: Nov 15 '21