CBECC-COM - How do you model Make Up Air Units (MAU)?

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For situations where you have kitchen exhaust hoods covering your cook line, how do you properly define a MAU in CBECC-COM? I'm hoping someone has done this before with success who can help guide me (and others like me) on how to make this work.

I tried the following methods to no avail... granted i might be way off base with these approaches.

1 - I had an HVAC zone that contained my cook line. This zone was served by an HVAC unit for comfort cooling. The cook line exhaust was specified as process exhaust (no problem there). Unfortunately i couldn't specify a second HVAC system to process the makeup air for the cook line since the comfort cooling HVAC system was already assigned to the thermal zone.

2 - I broke out the area of the cook line into its own space and thermal zone hoping to solve the above issue. I was able to specify the MAU and assign it to the cook line thermal zone. The model ran and I found that we were getting destroyed in energy usage. Apparently the baseline (standard) model was not operating in at all the same way. I suspect it was only bringing in and treating a token amount of OA associated with the zone default required OA. In the meantime my proposed model was conditioning 7000+ CFM of process OA for the makeup of the process exhaust.

Any help/suggestions would be greatly appreciated.

Thanks, JO

asked 2015-11-16 16:19:51 -0500

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Comments

@JO, at the thermal zone you can set a primary htg/clg system and a separate ventilation system. Doing your first option with this may yield somewhat promising results. There is an issue with CBECC-Com at the moment where air systems are given priority position one over zone systems so if the kitchen is conditioned by a zone system and has a MAU then the MAU will try to satisfy load before the zone system kicks on. I believe this is being fixed in the next iteration though.

pflaumingo (2015-11-17 13:21:03 -0500) edit

Thanks for the comment. I'll experiment with this method and report back.

Anonymous (2015-11-18 07:41:47 -0500) edit

I revisited the model where the cookline was not broken out into its own space and thermal zone. I assigned systems to the kitchen thermal zone as follows:

AircCondgSys = HP-5; VentSys = MAU-1; ExhaustSys = PCU-1 (the cookline exhaust system)

Upon attempting to run the simulation I encountered the following error:

Error: The VentilationSystem and PrimaryAirConditioning system of ThermalZone 'HP-5 ThermalZone' are separate air systems. Only one air system can be referenced by a ThermalZone. (53:'ThermalZone-General.rule' line 453)

Did I interpret your suggestion correctly?

Anonymous (2015-11-18 08:44:08 -0500) edit

@JO, I'm guessing HP-5 is an air system, not a zone system? Do you have a quick description of the mechanical systems?

pflaumingo (2015-11-18 10:00:25 -0500) edit

Correct. HP-5 is an air system. It's a heat pump split system (remote condenser) that serves the comfort cooling needs of the kitchen prep area. HP-5 is a packaged single zone hp and also features ventilation air and is equipped with an economizer.

Could you describe the difference between an "air system" and a "zone system"? When would one be used over the other?

Anonymous (2015-11-18 10:11:23 -0500) edit

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After working with this for a while I ended up going back to my second method where the cook line was a separate thermal zone. This approach seemed necessary due to Error 53 (in comments section above).

The assignments were as follows: AirCondgSys = MAU-1; VentSys = MAU-1; ExhaustSys = PCU-1

At the space level I made sure that the "process exhaust" box was checked. I also made sure that Ventilation Air quantity was updated to match the MAU-1 supply air. This was the crucial step I was missing in my original study. It turns out that for systems serving kitchens that have process exhaust; the Baseline outside air value tracks the value entered for the proposed design. (It's actually more complex than this; see the attached screen clip from the ACM.)C:\fakepath\Untitled.png

This methodology resulted in the baseline system energy increasing enough to not penalize me for my MAU.