I have a packaged multi-zone VAV proposed system that has a 25 ton load and is scheduled with 10,000 cfm supply air and 4550 cfm outside air (45.5% OA). This system has significantly fewer operating hours that the majority of the building (which is type 5 for the baseline) so for the baseline building it was split off as a separate system type 3. When I run the baseline system based on a 20 degree rise, and then override the ventilation to match the proposed, it comes out at 5,888 cfm supply air and 4550 cfm outside air (77.3% OA). The tonage would require it to be a 20-25 ton unit at 8,000-10,000 cfm supply air in reality which would bring the OA fraction down to 57% or less.
The proposed system does have heat recovery although it is not required, and the baseline system already has demand control ventilation . I'm guessing I have to model the baseline unit with heat recovery, right? It doesn't meet any of the exceptions listed under G3.1.2.10.
Or should this baseline system be another type 5 packaged VAV system?
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
August 14, 2014 - 1:15 am
This touches on the zone required minimum outdoor air (ODA_min) specified in the proposed design. This specification must be the same for both proposed and baseline models. It does not, in my humble opinion, mean that those rates are not exceeded at different points in time by either model. In fact, due to the models having different HVAC systems often including economizers and so on, it makes sense that the actual ODA rates will differ between models at any hour in the simulation.
The exception to the ODA_min specification being the same for both models is the demand controlled ventilation (DVC). If the proposed model has DVC, then the baseline must also have DVC, but with the ODA_min set to those in ASHRAE 62.1 (which implies that the DVC for the baseline controls on occupant sensor, which does not have to be the case for the proposed model, for example, which may model CO2 based control).
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
August 14, 2014 - 8:42 am
The proposed system is a multi-zone VAV unit with heat recovery, but it does not have CO2 controls for ventilation. Per G3.1.1 exception b, the baseline system a type 3 single zone unit. Is this correct or would it be better to model it as another type 5 multi-zone VAV system?
Jean Marais
b.i.g. Bechtold DesignBuilder Expert832 thumbs up
August 14, 2014 - 9:37 am
I would model a 3-PSZ-AC for each zone to which the G3.1.1 exception b applies. If the proposed system has no DCV, then why have you modelled it so in the baseline as stated in your last post?
The reason for splitting off "different load" zones from the system 5, is that it becomes impossible to optimally cool and heat both zone types. Using an extra system 5 may work if all the zones connected to it have the same load profiles, but is allowed only under exception a), i.e. if the sum of your "different load" zones is more than 1900 m2.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
August 14, 2014 - 9:58 am
You are in good hands with Jean.
One thing I would say that I often seen design engineers struggle with is the attempt to design the baseline system. Often the baseline system cannot be designed or the end result does not make design sense. It is what it is.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
August 14, 2014 - 10:35 am
The proposed system I am talking about covers 11,000 sqft (entire building is 82,000 sqft) and is sized at 25 Tons and 400 CFM/Ton for a total of 10,000 CFM supply air including 4550 CFM of outside air (about 46% OA). It contains 11 zones, all of which are simultaneously occupied and only for a few hours per week while the majority of the building is occupied 70+ hours per week. This proposed system requires demand control ventilation due to the occupant density in several of its zones, but not heat recovery since the OA fraction is less than 70%. However, the proposed design does have heat recovery, thus the demand control ventilation is not required based on 6.4.3.9 exception a. And just for clarification, I am referring to 90.1-2007.
For the baseline building, I took exception b and split this system off as a single type 3 system. Now when it's auto-sized based on a 20 degree rise, it comes out at 5,888 CFM supply air. Correcting the ventilation to match the proposed case (4550 CFM outside air), it comes to 77% outside air. Since this outside air fraction is above 70%, the baseline system must have heat recovery modeled per G3.1.2.10. And it's required to have Demand Control Ventilation based on the occupant density mentioned in the proposed case. However, since Heat Recovery is required, Demand Control Ventilation is not required. Do I have all that right?
As for the proper baseline system type for this part of the building, is a single zone type 3 system ok? Do I really need to split it into 11 single zone type 3 systems? Or would it be better to make it a single type 5 multi-zone system even though it covers less than 20,000 sqft of the building? I've been told before the 20,000 sqft rule is not an absolute rule. What would best practice in the eyes of a reviewer?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
August 14, 2014 - 11:12 am
All sounds right in first two paragraphs.
Regarding the zones you still need to follow the rules for creating thermal blocks from Table G3.1-7,8 or 9. In general you do not have a choice when modeling the exceptions to G3.1.1, they are required. Exception a only applies to the examples given (residential/non-residential and fuel source) and the 20,000 sf is pretty absolute.
Best practice to the reviewer is to follow the rules and principles of Appendix G. If you have a doubt about what is the best way to do something then it is best practice to do that which produces the most conservative results.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
August 14, 2014 - 11:28 am
I guess I am not clear on the difference between a thermal block, a zone and system when it comes to appendix G. For the Proposed case, if I have a designed VAV system with one RTU serving 11 zones, and each zone is unique, is that one thermal block or 11 thermal blocks and for the purposes of a LEED energy model, do I model it as 1 RTU with 11 zones or 11 RTUs each with one zone?
It sounds like what you're telling me is that since my exception b system serves 11 zones in reality, it must be modeled as 11 individual type 3 systems for the baseline building, otherwise it must be included with the primary type 5 system as ab additional 11 zones.
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
August 14, 2014 - 12:03 pm
The three terms are all in the definitions in the front of the Standard. Zone and system relate to the HVAC system. Thermal blocks are modeling constructs that enable you to simulate groups of zones to simplify the energy model where the grouping of zones would have little to no impact on the modeling results.
Table G3.1-7 tells you what to do. You do model each zone as a thermal block unless the exceptions listed apply. If any of the exceptions apply then you can group like zones into thermal blocks. G3.1.1 requires a system per block for systems 1-4 and a system per floor for systems 5-8. Since I do not know whether any of the Table G3.1-7 exceptions apply I cannot tell you how many thermal blocks you should model. You certainly can model each zone with a separate system 3 but you should not model a single system unless it can be considered one thermal block and you never model a system 3 as a multi-zone system since it is a packaged single zone (PSZ) system.
If G3.1.1 exception b applies then you must model the system 3 and it should not be included in the system 5.
So you will have somewhere between 1 and 11 thermal blocks depending on the exceptions in Table G3.1-7.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
August 14, 2014 - 2:34 pm
Thanks, I think I get it now, but please bear with me as I wrap my brain around this...
Let's say my design is a single story non-residential building less than 25,000 sqft. It's cooled and gas-heated by one multizone VAV rooftop unit serving 10 zones. Based on G3.1.1A, the baseline system would be System 3-PSZ-AC. Let's say each of these zones has a different space usage, glass exposure, etc.. so none of them can be combined into thermal blocks. So now my baseline building would be ten System 3s being compared to my one proposed system, correct?
Now here's a more complicated scenario. Let's say the designed building is 50,000 sqft with two RTU VAV systems, each serving 10 zones. RTU-1 covers 40,000 sqft which is occupied seven days per week, and RTU-2 covers 10,000 sqft, occupied only one day per week. The primary baseline system would be type 5, packaged VAV with reheat. Since the zones served by RTU-2 are only occupied one day per week, I take exception b to G3.1.1 and split those zones off from the primary baseline system and make them into one to ten separate System 3s, depending on whether or not I can combine any zones into thermal blocks, right? It turns out each zone served by RTU-2 has different space usages and glass exposures/orientations, thus I must split it up into ten separate System 3s, correct? And assuming the zones served by primary system 5 cannot be combined into any thermal blocks either, that system would have 10 zones, correct?
So my baseline building would result in 11 systems (ten single zone packaged units and one VAV packaged unit with ten zones) compared to my proposed design of two VAV packaged units, each with ten zones. Right?
Marcus Sheffer
LEED Fellow7group / Energy Opportunities
LEEDuser Expert
5867 thumbs up
August 14, 2014 - 2:53 pm
First question - yes.
Second paragraph - yes, yes, yes
Last question - yes
Sounds like you got it.
Matt Scott
EngineerN.E. Fisher & Associates, Inc.
18 thumbs up
August 14, 2014 - 2:59 pm
Awesome. Thanks so much for your time.