#1893 Clear guidance on HVAC system types

[Felicia Lee]

I’m looking for documented guidance on two HVAC related CBECC-Com items:

• Definitions for the different air system HVAC system types: VAV, PVAV, SZAC, SZHP, SZVAVAC, SZVAVHP, SPVAC, SPVHP, Exhaust, HV.

o Some of these are not defined in the user manual. While some are straight forward, there’s justification to clearly define when one should be used over another. For example, is a packaged VAV rooftop unit modeled as ‘PVAV’ when it serves multiple zones but as ‘SZVAVAC/HP’ when it serves a single zone? What are SPVAC/HP? Is a balanced ventilation system modeled as ‘HV’

• When to apply an air system versus a zone system.

o Based on conversations with CBECC-Com support, the guideline I follow is that if the HVAC system is not providing mechanical ventilation and operating continuously then a zone system should be defined, otherwise apply an air system. Is this correct? Are there other guidelines?

[Nikhil Kapur]

DR - see below:

Q. For example, is a packaged VAV rooftop unit modeled as ‘PVAV’ when it serves multiple zones but as ‘SZVAVAC/HP’ when it serves a single zone?
A. Yes. A SZVAV system has only one control zone and terminals are Type = 'Uncontrolled'; i.e. not VAV boxes.

Q.What are SPVAC/HP?
A. This type is defined in the standards (see Section 100.1), and corresponds to equipment that would fall under the requirments of Table 110.2-E.

Q. Is a balanced ventilation system modeled as ‘HV’.
A. An 'HV' system should only be used if the system provides heating and ventilation but no cooling

Here is a list of things to keep in mind:

1. ZoneSystems can only serve one zone, while an AirSystem can serve one or more zones. The single-zone AirSystems (SZAC, SZVAV, etc) can serve multiple zones, but there is only one control zone, other zones are considered 'slave zones' which receive conditioned air, but do not influence the control of the system.
2. ZoneSystems should be used when the system cycles to meet the load as opposed to running continuously (as is needed for ventilation). The current cycling control for AirSystems does not yield the same results as its corresponding ZoneSystem, so it is not recommended for this application.
3. With the exception of ZnSys Type = FPFC, ZoneSystems can only have ConstantVolume fan control.
4. ZoneSystems don't support 'add-on' components, like economizers and heat recovery.
5. ZoneSystems don't support more complex supply air temperature control strategies; coils just cycle on/off to meet loads.

[Nikhil Kapur]

Reopening for additional questions from user

[Nikhil Kapur]

9/22 User - One follow-up question: - Is a balanced ventilation system modeled as ‘HV’. An 'HV' system should only be used if the system provides heating and ventilation but no cooling.

How do you model supply or balanced ventilation only systems? Exhaust systems can only include exhaust fans correct?

The space may have a heating/cooling system, but one which doesn’t provide the ventilation for the space. When the separate system is exhaust only that’s fairly straightforward. I model a zone system for the heating/cooling (since it’s cycling) and an air system of type exhaust for the ventilation. However, I also see supply or supply/exhaust (balanced) systems for ventilation and am not sure how to handle this. Sometimes these systems are in corridors (both conditioned and unconditioned), smaller zones, and also in multifamily apartments. Typically apartment ventilation is a dedicated exhaust, but it may also be supply.

[Nikhil Kapur]

DR - What you've outlined can be modeled as the combination of a ZnSys defined as the Primary Htg/Clg system, and a separate AirSys defined as the Ventilation system.

If it is a balanced system, set the AirSys:Type = Exhaust, define both a supply and exhaust segment with their respective fans. Attached is an example: OffSml-ZnSysWithBalVent.

If it is a supply only, set the AirSys:Type = HV, define both a supply and relief segment (this is a 'dummy' segment to full fill model structure needs), with a supply fans. In this case you will also need an OACtrl and TrmlUnit object. Attached is an example: OffSml-ZnSysWithSupVent.