Fixing unmet hours / outside of throttling range warnings

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Good Morning, eQUESTers,

I'm working on my design case model. Here's my ATTN report:

**WARNING**********************************************************************
SYSTEM EL1 Sys1 (PSZ) (G.NNW1) may have inadequate cooling capability
Check COOLING-CAPACITY and MIN-SUPPLY-T for consistency

**WARNING**********************************************************************
SYSTEM EL1 Sys2 (PSZ) (G.WSW2) may have inadequate cooling capability
Check COOLING-CAPACITY and MIN-SUPPLY-T for consistency

**WARNING**********************************************************************
SYSTEM EL1 Sys3 (PSZ) (G.N3) may have inadequate cooling capability
Check COOLING-CAPACITY and MIN-SUPPLY-T for consistency

**WARNING**********************************************************************
SYSTEM EL1 Sys4 (PSZ) (G.S4) may have inadequate cooling capability
Check COOLING-CAPACITY and MIN-SUPPLY-T for consistency

**WARNING**********************************************************************
SYSTEM EL1 Sys5 (PSZ) (G.N5) may have inadequate cooling capability
Check COOLING-CAPACITY and MIN-SUPPLY-T for consistency

**WARNING**********************************************************************
SYSTEM EL1 Sys6 (PSZ) (G.E6) may have inadequate cooling capability
Check COOLING-CAPACITY and MIN-SUPPLY-T for consistency

**WARNING**********************************************************************
SYSTEM EL1 Sys7 (PSZ) (G.C7) has zero outside air for design calculations

This last one I can answer as this system is a R/A only computer room cooler. For the others:

COOLING-CAPACITY comes directly from the HVAC designer and is the capacity of the installed units. I've tried adjusting MIN-SUPPLY-T from 55 to 50 (and this seems to jive with information the coil manufacturer provides) to get a larger delta-T and it only seemed to make things worse somehow. Minimum OSA is 20%, but again, this is a design condition. These units do have enthalpy-controlled economizers.

I'm also getting WAY too many hours outside of throttling range - 15%. My baseline model only has a 3%. I'm way out of whack with Appendix G requirements.

Any suggestions as to settings to modify to clear these? Thank you!

MARK DARRALL, AIA, LEED AP BD+C, NCARB

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Hi Mark,

It does not appear that you got any response to your email. If you didn't,
it may be because this question has been asked and answered so many times,
and recently, too.

The first thing I'd suggest is to go onto the onebuilding.org site and find
out how to search threads. In addition to, or instead of, doing that just
type in something like "hours out of throttling range" in the Google search
line.

Once you go through that information if you still have questions, as them
then. Please note though that if you do have questions still, it will be
best to zip up your .inp and .pd2 and include them, too, so people can see
what you've got.

Best,

Carol

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Mark:

Either spread your thermostat throttling range, or incrase your total supply
airflow.

John R. Aulbach, PE, CEM

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Mark:

Since you are using the as-designed HVAC system, you cannot increase the airflow
and still be compliant with the design.

I have had some success in reducing unmet hours by increasing the thermostat
throttling range as long as the increased range is consistent with the Owner's
Project Requirements for space temperature limits. I also have had some success
by choosing a different control zone in the Airside Systems Basic tab. This
depends if you have more than one zone per system. Look at the SS-R Zone
Performance Summary reports to see which zone(s) is giving you trouble.

If you have multiple zones per system and increasing the throttling range or
changing the control zone does not work, try imputing the airflow on the Zone
level instead of the System level. If you input the supply CFM in the System
tab, return that to default and divide up the design airflow in the zones. I
would reduce the CFM in those zones that have 0 unmet hours and increase the CFM
in those zones that have high unmet hours. The SS-R report will show how eQuest
has allocated the supply CFM.

Paul Diglio

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Mark,

Your question seems to be less "how do I make these go away" and more "why is eQuest making it look like my engineer is wrong", correct? The way you phrsed your question makes this sound like a design model question and not a question about a baseline model.

John Aulbach's advice is a good place to start: see if those items match the mechanical design. At that point, I would then take the time to discuss the rest of the system inputs with the mechanical engineer to make certain the system is described the way he intended. In addition, check the basic load conditions that he assumed compared to what is in eQuest: Occupancy, lighting, miscellaneous loads. If you have a higher occupancy in eQuest than the engineer did when he selected the coils, that could cause the warnings you are seeing and the hours outside the throttling range if the discrepancy was large enough. Make sure to confirm that the right system type was selected in eQuest - take a look in the help file for the system type. A system in the model set to be a single-zone system will have large numbers of hours outside the throttling range if it has multiple zones assigned to it. If you have every room setup in eQuest, but the engineer is designing single-zone systems and grouping rooms into common thermal zones, that may very well be the cause of your problem.

Another tip: look at the SS-K and SS-O reports to get an idea of where the system (SS-K) temperatures and zone (SS-O) temperatures are at. The SS-O report logs 0 hours in a temperature band when the fans are not on. Those two reports will give you an idea of how far outside of the temperature zone the system is operating. The SS-R report will show you more detailed information on hours where the space temperatures are not being met.

When you do discuss this, make sure you approach it from a "we have a difference here that I want to make sure we figure out together" approach: both eQuest and the engineer's load calculations take into account a large number of variables. While each variable has a greater or lesser impact on the results, they all do affect the calculations.

Good luck with the tweaking of the model!

Jeremy R. Poling, PE, LEED AP+BDC

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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

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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

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Jeff,

I too would be curious what the comment was verbatim, but I'm going to go out on a limb here and suggest something before that anyway. I received a comment that I considered odd regarding how many systems were required.

ASHRAE 90.1-2007 clarified some requirements around system sizes, since there can be efficiencies from selecting more or less equipment for the baseline. Section G3.1.1 was revised to require that baseline systems 1-4 be modeled with one system per thermal block and baseline systems 5-8 be modeled with one system per floor. It does go on to allow grouping of floors with identical thermal blocks for modeling purposes.

This won't require a separate shell to model. If your model is in Detailed mode right now, you'll need to re-assign zones to match. If you somehow managed to keep the model in the Wizard mode (or for future models when you start out), you can select how eQuest assigns zones to systems (System per zone, system per floor, system per shell, or system per site). For baseline 1-4 you would use system per zone and for baseline 5-8 you would use system per floor. This is on screen 1 of the HVAC System Definition portion of the wizard.

I just checked with a basic, "default" model in the wizard, and eQuest will group floors onto the same system if you tell it to use floor multipliers and will create a separate system for each floor if you tell it not to. I did a quick inspection of the reports (someone who knows more should correct me if I am wrong on this) and it looks like the floor multiplier can only be separated out by straight division, so you would want to make sure you can justify to the reviewer that you have identical thermal blocks for cases where they are combined but G3.1.1 says you should have separate systems.

Hope this helps - I made a big assumption, but hopefully this post will help someone else before they receive a comment back from a LEED review.

Jeremy R. Poling, PE, LEED AP+BDC

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I?m also curious to hear Jeff?s LEED reviewer interpretation of thermal blocks. I?ll add some general comments.

I believe it is incorrect use of terminology to talk about ?multiple zones per thermal block? in eQUEST.

The words space and zone both have specific meanings in eQUEST. For each space, there is a single corresponding zone.

Multiple zones can be applied to a single system. (Unfortunately, multiple systems cannot be applied to a single zone, but that is another thread.)

The term thermal block does not have a precise meaning in eQUEST, but I would not describe the group of zones assigned to a single system as a thermal block.

If you wanted to try to model the exact intent of a proposed design, you would model each thermally-unique area as its own space/zone. A generalization is that a zone is an area served by a single thermostat.

To simplify a model, similar zones can be combined into thermal blocks per the guidelines in ASHRAE Standard 90.1.

Modelers have to make judgments calls on if/how to combine zones into thermal blocks.

In eQUEST, the decision to combine zones into thermal blocks really should be done at the wizard input level, when the zones are defined. You combine multiple zones per the design intent into a single zone definition for simulation purposes.

You don?t have multiple zones in a thermal block. Each thermal block is defined/modeled as a single eQUEST zone.

You could still assign multiple thermal blocks/zones to a single system, but I would be wary of over-simplifying a proposed design.

Regards,

Bill

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Here is the comment - it could be that I drew a blank - the comments so far
have helped to clear up the issue. I initially interpreted the comment to
imply a separate shell for each thermal block.

According to the SV-A report, the Baseline Case was not modeled with a
separate PSZ-AC serving each thermal block as requested.

However, I do appreciate the comments regarding thermal blocks and they have
been very helpful.

Thanks for the response,

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

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Hi Jeff!

It sounds like it's simply a quantity of systems issue then.

90.1 & LEED reviewers would have you use one system per "thermal block"
for packaged baseline systems (#1-4). I like Bill's clarification, and
agree "thermal block" (90.1's term) is too fuzzy in the context of an
eQuest project, so I'll continue using eQuest terminology.

Re-worded specifically for eQuest: I'd generally expect each baseline
system to have either a single zone, or a "floor-level" zone & its
corresponding plenum zone, if I were looking over your shoulder at the
HVAC component tree. You probably shouldn't have any more zones than
that grouped under any one baseline system.

The SV-A reports list the zones under each system - same as the
component tree under the HVAC tab. Scan through those listings, and I
expect you'll see what triggered this comment from your reviewer, if it
isn't already apparent ;).

~Nick

NICK CATON, E.I.T.

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Bill:

You say that it is incorrect use of terminology to talk about multiple zones per
thermal block then go on to say similar zones can be combined into thermal
blocks and yet later you say you don't have multiple zones in a thermal block.

What is the difference between 'multiple' and 'similar'?

You also write that space and zone both have specific meanings in eQuest then
later you sayyou would model each thermally-unique area as its own space/zone.
I don't understand your comments.

Paul Diglio

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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

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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

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Jeremy:

Good explanation, thank you.

I model per the 90.1 zone definition. If a zone is too large one thermostat
cannot control it and we get unmet hours. I zone the building for modeling the
same way it would be zoned for airflow or VAV terminals, if specified. I find
that I can provide a better model if I create many zones and combine them. This
way I can adjust the airflow to each zone, choose the control zone, vary the
ventilation air, change the set-point and schedule all while using just one HVAC
system to feed all the zones as in a typical design.

I don't use thermal blocks too much, just for a situation like I mentioned in my
previous e-mail, such as elevator lobbies on different floors served by the same
HVAC system. I do not understand the logic behind using large thermal blocks if
the proposed design does not incorporate the same zone/block pattern. I have
reviewed models where the simulator created a thermal block consisting of an
entire floor with glazing in three or four orientations, which does not comply
with the 90.1 Appendix G requirements that zones in a thermal block adjacent to
glazed exterior walls face the same orientation or the orientations vary by less
than 45 degrees.

Paul Diglio

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Jeff

Your query has generated a lot of discussion. Please attach the following
for your benefit--

1. Screenshot of your baseline equest zoning
2. Architectural floor plan

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See here for a very good discussion of thermal blocks. We all use them all
the time we just don't understand the terminology. It's not hard to
understand why we have trouble with the terminology, too many words that
sound almost the same that mean something just slightly different.

www1.resnet.us/comments/comnet/100714_COMNET.pdf

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Okay,

Thanks for all of the feedback - great discussions. Here is a copy of a
typical floor (the middle floor of three). The ground floor has a small
library and student center. I have also included a screen shot of the zoning
pattern for the west side which is for one half of the building as well as a
copy of the zoning for the east side first floor which has the library and
student center. The east side second and third floors are similar to the
west side. The building has several fume hoods.

Regards,

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

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Ah, the difference a setting can make! After playing with throttling ranges and sizing factors and getting only tiny improvements, looking at the zone temperature reports got me thinking to what was happening overnight. I then realized the AHU fans were set to never come on on night cycle, so the building was overheating at night!

Setting night cycle control to "Cycle on First" took care of it all! I now have unmet hours under 150 hours for both models and the design case unmet hours are fewer than baseline. I think I got it!

Thanks for all the help!

MARK DARRALL, AIA, LEED AP BD+C, NCARB

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Glad you found your fix. Appendix G section G3.1.2.4 requires modeling the systems to be "cycled on to meet heating and cooling loads during unoccupied hours".
For some system types, 'Cycle on Any' is more appropriate.

Paul Riemer, PE, LEED AP

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Thanks, Paul. That statement in Appendix G was one of the things that got me to thinking about that.

These systems are PSZs -one thermal zone per unit, basically 3T and 4T residential furnaces / DX systems.

MARK DARRALL, AIA, LEED AP BD+C, NCARB

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