CBECC-Com Issue - HVAC

[GoogleCodeExporter]

I have a question about the compliance calc software.  Is it still the case 
that split system HPs and VRF systems cannot be modeled as such, with actual 
efficiencies, and instead must be modeled as minimum efficiency split system 
HPs because the modeling software has not been approved for modeling of these 
systems?  If so, is the certification process or such being completed at this 
time?

Having a hard time also figuring out when I do and don't need an economizer.   
I understand at what capacities it is required and such, but not sure if the 
non-res manual Table 4-17 applies to these units (VRF and HP), when the split 
system unit efficiency exceeds the minimum efficiency by the specified 
percentage.  Does the minimum efficiency requirements only apply when modeling 
the units?  Can I exclude economizer per this exception when doing forms 
manually?

Also is there a publication that defines these limitations in more detail?

Original issue reported on code.google.com by f.le...@gmail.com on 2 Dec 2014 at 2:15

[GoogleCodeExporter]

Roger may have guidance on the first question.

Regarding when an economizer is required: first, the economizer requirement is 
a prescriptive requirement, so it is never mandatory for the proposed case. The 
standard design (baseline) will have an integrated economizer for systems with 
cooling capacity above 54 kBtu/h when the system provides outside air 
ventilation as well as cooling.

Some systems that have no ducts (PTAC, Four-pipe fan coil, VRF) by definition 
cannot have an airside economizer and therefore won't have an economizer in the 
performance method (unless, in the case of VRF systems, their performance is 
approximated by a packaged single zone or PVAV unit that delivers outside air 
in the model. Table 4-17 does not apply to CBECC or the performance method, 
since all tradeoffs are done in the software. 

Assuming that CBECC will at some point have a VRF model that models the 
performance of these systems, IF and WHEN that is available, the VRF model 
would not include an economizer. The standard design likely would. The VRF 
system would require a separate dedicated outside air system that provides the 
necessary ventilation requirements for the zones served by the VRF system. This 
system would likely use 100% outside air, so would not need an economizer.

I will see if I can document this more clearly in the ACM Reference Manual - 
that airside economizers do not apply to some types of systems.

Original comment by JohnJArent on 3 Dec 2014 at 11:02

• Changed state: Discussion

[GoogleCodeExporter]

CBECC-Com can model split system heat pumps, but cannot model VRF systems as 
yet.  There is an Exceptional Design compliance procedure that can be followed 
which allows use of the VRF modeling capabilities in EnergyPlus.  See 
http://bees.archenergy.com/faq_software.html#excep for additional information.

The economizer requirement is a prescriptive requirement, so it is never 
mandatory for the proposed case. The standard design (baseline) will have an 
integrated economizer for systems with cooling capacity above 54 kBtu/h when 
the system provides outside air ventilation as well as cooling.

Some systems that have no ducts (PTAC, Four-pipe fan coil, VRF) by definition 
cannot have an airside economizer and therefore won't have an economizer in the 
performance method (unless, in the case of VRF systems, their performance is 
approximated by a packaged single zone or PVAV unit that delivers outside air 
in the model). Table 4-17 does not apply to CBECC or the performance method, 
since all tradeoffs are done in the software.

When VRF systems are included in CBECC-Com, the VRF model would not include an 
economizer. The standard design will.  The VRF system will likely require a 
separate dedicated outside air system that provides the necessary ventilation 
requirements for the zones served by the VRF system.  This system would likely 
use 100% outside air, so would not need an economizer.

Original comment by rhedr...@archenergy.com on 3 Dec 2014 at 11:10

• Changed state: WontFix