[Equest-users] Thermal Blocks

[Jeremy Poling]

Maybe this helps the definition debate (apologies for the length, but some things require more words to described).  Section 2 in ASHRAE 90.1-2007 gives the following definitions:
 
Space: an enclosed space within a building. (It goes on to define the different types of spaces, conditioned, semi-heated, etc., but those don't seem to be of concern for this discussion).
 
Zone: a space or group of spaces within a building with heating and cooling requirements that are sufficiently similar so that desired conditions (e.g. temperature) can be maintained throughout using a single sensor (e.g., thermostat or temperature sensor).
 
Thermal Block: a collection of one or more HVAC zones grouped together for simulation purposes.  Spaces need not be contiguous to be combined within a single thermal block.
 
Appendix G goes on to set up rules as to how to determine a thermal block in Table G3.1(7):
 
Where HVAC zones are defined on HVAC design drawings, each HVAC zone shall be modeled as a separate thermal block.  Exception: Different HVAC zones may be combined to create a single thermal block or identical thermal blocks to which multipliers are applied, provided that all of the following conditions are met:
a. The space use classification is the same throughout the thermal block.
b. All HCAV zones in the thermal block that are adjacent to glazed exterior walls face the same orientation or their orientations vary by less than 45 degrees.
c. All of the zones are served by the same HVAC system or by the same kind of HVAC system.
 
 
The Users Manual has additional explanation on how to put together thermal blocks.  For the purposes of the LEED reviewers comment, don't forget that they are not assessing your use of the software you chose for the model, they are assessing whether you complied with Appendix G correctly.  The specific comment issued is related to G3.1.1 of ASHRAE 90.1-2007 specifically.  I couldn't tell from the screen capture if this was the case, but based on the comment I would assume that the reviewer agrees with you that your baseline system is one of the systems numbered 1-4.  They are expecting to see a separate system modeled per thermal block.
 
To address eQuest definitions, the DOE-2.2 Volume 2: Definitions classifies the "SPACE" keyword as an Envelope Component and more specifically, something the LOADS portion of the software uses to determine the loads to impose on a system.  the "ZONE" keyword is defined as an HVAC Component used by the HVAC portion of the software to determine the system and plant operating responses to loads in the building.  Very specifically, the ZONE keyword definition states:
 
"Each zone is associated with one and only one SYSTEM command, which specifies the system to which the zone belongs. This applies to conditioned zones as well as unconditioned zones and plenum zones. Note that there must be a one-to-one match-up between the zones specified here and the spaces specified in the LOADS program. That is, for each SPACE command in LOADS there will be a corresponding ZONE command to represent a physically identical portion of the building."

So you can see from the definitions in eQuest and in ASHRAE 90.1-2007 that the software is really not concerned with how many spaces are in a zone.  It relies on the modeler and HVAC designer to ensure that they have appropriately zoned the building.
 
In addition, I wouldn't interpret the reviewers comment as questioning your zoning at all: they specifically are asking that the model include 1 system per thermal block.  Unless you are using the exception (and it is an exception) that allows multiple zones to be combined into the same thermal block, this means that you will have one eQuest zone per system.  
 
For my personal interpretation: all of the wording in ASHRAE 90.1-2007 in G3.1.1 and Table G3.1 really points to systems 1-4 being single-zone systems and systems 5-8 being multi-zone systems.  If you have a building that is non-residential, 3 floors or less, and less than 25,000 SF in real life, it would seem more probable that this building has single-zone systems and that larger buildings have multi-zone systems.  The system-per-floor requirement seems to match my experience as to how most mid-rise and high-rise office buildings are designed: one air handling unit per floor serving a primary duct system that terminal units in the tenant space connect to.
 
Jeremy R. Poling, PE, LEED AP+BDC


________________________________

From: equest-users-bounces at lists.onebuilding.org on behalf of Paul Diglio
Sent: Wed 5/4/2011 4:46 PM
To: Jeff Ross-Bain; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Thermal Blocks


Jeff:

I've seen some of the other e-mails from the forum and I do not agree with the terminology other people are using, i.e. space v. zone v. thermal block.  Some of these replies are exceedingly long and contradictory.

Let's leave 'thermal block' out of the discussion.   Take one floor of an office building.  The baseline would usually require one HVAC unit for this floor per 90.1.  I would create zones in the wizard for each large office space, the restroom, the utility rooms, the corridor and elevator lobby.  I would use the same procedure that you used below to add these zones to the HVAC system.  Having the separate zones allows you to adjust the airflow and temperatures to the individual zones and also to find out where unmet hours might be occurring.

As an example of my understanding of a thermal block, say the office building has 8 floors and each elevator lobby is supplied by the same common HVAC system.  I would zone each lobby as described above and add them to the elevator lobby HVAC system.  This is a thermal block because it is a collection of spaces that share the same system and space characteristics.

Paul Diglio




________________________________

From: Jeff Ross-Bain <jeff at rbgb.com>
To: Paul Diglio <paul.diglio at sbcglobal.net>
Sent: Wed, May 4, 2011 9:39:19 AM
Subject: RE: [Equest-users] Thermal Blocks



Hi Paul,

So you establish individual zones in the building footprint window (2) and discriminate between corridors, classrooms... but what about all the little closets, odd offices here and there, rest rooms, mechanical rooms, meeting rooms, etc.? It seems that the zoning patterns would then become a nightmare. In your Zone Group Definitions window (14) you would then assign the same system to each of those zones? Here is a screen shot of a zoning pattern. I have used this approach for years:

 

 

 

 

 

Jeffrey G Ross-Bain, PE, LEED AP, BEMP

404-228-2893 office 

404-408-2577 mobile
www.rbgb.com <http://www.rbgb.com/> 

________________________________

From: Paul Diglio [mailto:paul.diglio at sbcglobal.net] 
Sent: Wednesday, May 04, 2011 9:21 AM
To: Jeff Ross-Bain; equest-users
Subject: Re: [Equest-users] Thermal Blocks

 

Jeff:

ASHRAE 90.1 allow spaces of similar thermal characteristics to be combined in thermal blocks.  I do not include what you call support spaces because the load and occupancy is much different than, say a classroom.  I would define the corridor as a separate zone within the thermal block but served by the same HVAC system.  This allows you to have a different set-point, airflow, occupancy and schedules for the corridor.

You do not have to define a separate shell for each block, you can define the block by custom zones in the wizard.

I divide up a thermal block into as many reasonable zones as possible.  This allows me to find out which area of the thermal block is causing any unmet heating or cooling hours.  It also allows me to change the HVAC system control zone to that zone which is the hardest to satisfy in the summer and winter.  These zones can be served by a single HVAC system.

For a proposed model that needs to conform to an actual design, defining multiple zones per thermal block allows you to balance the airflow to zones that have unmet hours without increasing the overall airflow of the HVAC system.  I define the zonal airflow in the Zone tab.  I reduce the airflow to the zones without any unmet hours and increase the airflow to those zones that have unmet hours.  I do not define the total fan CFM in the Airside Systems tab.

What were the comments from the LEED reviewer?

Paul Diglio

 

 

________________________________

From: Jeff Ross-Bain <jeff at rbgb.com>
To: equest-users <equest-users at lists.onebuilding.org>
Sent: Wed, May 4, 2011 9:05:15 AM
Subject: [Equest-users] Thermal Blocks

Hi All,

 

Are there any additional discussions out there regarding thermal blocks besides what is in ASHRAE Standard 90.1 and the User's Guide? I have received comments back from a LEED reviewer that seems to reflect an interpretation of that concept which I have not seen before.

 

In a nutshell, does each thermal block require a separate shell with individual HVAC system? I understand the concept of having individual thermal blocks or combining these if they are similar space use categories. I am assuming then that the similar space use categories (say a college classroom building) can also include support spaces such as corridors, storage spaces, faculty offices, etc. 

 

So if several thermal blocks are combined into one, can they not be served by a single, similar type of HVAC system?

 

I hope that I have made myself clear and many thanks for your thoughts and comments.

 

Regards,

 

Jeffrey G Ross-Bain, PE, LEED AP, BEMP

404-228-2893 office 

404-408-2577 mobile
www.rbgb.com <http://www.rbgb.com/> 

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