This is not exactly an eQUEST question but y'all have been so helpful in the
past so I'm hoping you can help here too. I'm looking for a little
clarification. It was my understanding if you had a DOAS system that heated
and cooled the outside air before delivering it to the floor mechanical room
to be combined with return air and further conditioned by the AHU in your
Proposed building that this system would NOT be modeled separately in the
Baseline. I cannot find anything in 90.1 that would regulate the DOAS
system and I'm not sure how you would model it. (I understand the dummy
zone concept for modeling DOAS but not sure that would apply to the
Baseline.)
Should it be separate system in the Baseline and would it be VAV or constant
volume? And if it is a separate system what section of 90.1 would you
reference for this?
Thanks in advance.
Hi Susan!
Whether or not a baseline model gets a separate system for a proposed
design featuring a DOAS is dependent on a few things, including how you
decided to approach modeling the DOAS in the proposed model.
- A clarification first: Type and quantity of baseline systems
is determined first and foremost per Appendix G.3.1.1. You generally
have either one system per floor or one system per zone, depending on
the system type. You never start with "one system per proposed system"
except by coincidence.
- If you use a dummy zone approach (not integrating the DOAS
into the proposed systems), and you have one baseline system serving
only the dummy zone per G.3.1.1 (or its exceptions), and you have
specified the ventilation requirements at that dummy zone (not
indirectly from the actual zones - using OA-FROM-SYSTEM for the proposed
systems)... then you'll end up with a distinct baseline system handling
the ventilation requirements analogous to the proposed DOAS.
- In many/most cases (partially dependant on your preferred
strategy for handling DOAS's in your proposed models), you will not have
a dedicated baseline system handling the ventilation air. That's okay.
NICK CATON, E.I.T.
Nick:
If you do not use the OA From System how will eQuest calculate the CFM required
by varying occupancy?? In addition, since the DOAS system might not provide the
supply air temperatures required eQuest will not correctly calculate the energy
required to condition the OA.
Paul Diglio
Hi Paul!
1. You could specify the DOAS as part of the systems it?s serving (breaking out the fan energies/ERV components/preconditioning and so forth), then the OA and scheduling specified at the zonal level carries through.
2. If the DOAS has preconditioning/ERV elements maintaining specific temperature setpoints, then those could be specified at the systems being served. This may not be technically feasible for every system type.
I?ve used both approaches in the past: OA-FROM-SYSTEM using dummy zone(s) and integrating DOAS/ERV into the proposed systems served. I believe both are viable approaches, with the first benefiting from more nuanced control over system behavior (fans, specifically), and the latter being much easier/simpler to set up and troubleshoot.
NICK CATON, E.I.T.
Nick,
So if my baseline is system 8 and I'm using 1 system per floor as indicated
in G.3.1.1, I don't really see an exception that would allow the DOAS. Am I
missing something that would actually allow it?
Thanks,
Susan
Susan,
My short answer is no, there is nothing in Appendix G that requires you
or allows you to model a DOAS system that preconditions outside air. As
you and Nick said, first use G3.1.1 for the system type. Then refer to
G3.1.2.10 to determine if exhaust air energy recovery is required. Keep
minimum outside air requirements identical between the baseline and
proposed per G3.1.2.5. If the DOAS penalizes your proposed design from
an energy perspective, check Table 6.5.3.1.1B for potential adjustments
to your baseline fan pressure drop, per the note to Table G3.1.2.9.
Regards,
Bill
William Bishop, PE, BEMP, LEED(r) AP
If this is for a LEED project, Table 3.1, p28 of the USGBC ?AEMG? says no
separate outside air handling unit for baseline.
Perhaps the 90.1 User?s Manual may also clarify, I don?t have 2007 UM in
front of me.
David
*
*
David S. Eldridge, Jr., P.E., LEED AP BD+C, BEMP, HBDP
*
*
*From:* equest-users-bounces at lists.onebuilding.org [mailto:
equest-users-bounces at lists.onebuilding.org] *On Behalf Of *Bishop, Bill
*Sent:* Thursday, January 27, 2011 10:35 AM
*To:* Susan F; Nick Caton
*Cc:* equest-users at lists.onebuilding.org
*Subject:* Re: [Equest-users] DOAS in baseline
Susan,
My short answer is no, there is nothing in Appendix G that requires you or
allows you to model a DOAS system that preconditions outside air. As you and
Nick said, first use G3.1.1 for the system type. Then refer to G3.1.2.10 to
determine if exhaust air energy recovery is required. Keep minimum outside
air requirements identical between the baseline and proposed per G3.1.2.5.
If the DOAS penalizes your proposed design from an energy perspective, check
Table 6.5.3.1.1B for potential adjustments to your baseline fan pressure
drop, per the note to Table G3.1.2.9.
Regards,
Bill
Susan et al:
"Where it's not allowed" is found simply where the quantity of systems
is mandated: 3.1.1. Model reviewers will actively challenge baseline
system quantities & exceptions taken if they suspect this isn't being
followed to the letter.
You should probably inform your colleagues that System 8 is VAV and
would not be CV in any case. You might leave it at that for your own
sanity, but this segways into a shaky possibility...
I mentioned 3.1.1 has exceptions - some of these would dictate the
baseline have a single-zone unit of type 3 or 4, which would be (!)
constant volume, if the zone is significantly different from the rest of
the model - read those exceptions carefully. I have never tried this,
but you might explore applying one of these exceptions (which were
almost certainly not intended for artificial dummy zones in eQuest/DOE2
models) to force an analogous type 3 or 4 baseline system to a DOAS
dummy zone... I would personally not advise this approach but I'm
bringing it up so you might consider all angles if you're having to
argue your case.
It's totally possible the proposed DOAS design will ultimately cause a
net performance decrease - if so, the most you can do is focus on
ensuring the proposed system is designed well and is being modeled
correctly.
Proposed: You should carefully explore the DOAS control sequences to be
sure you're addressing its setpoints, efficiencies and when it's VAV
fan(s) do and don't run. Much of this is often buried in submittal
information and cryptic control sequences that only the designer may
understand. If it's a challenge (and it usually is) I'd encourage you
make a quick "fill in the blank" for all inputs and have your colleagues
research & fill it out for you.
Baseline: Do explore fan energy bump-ups per Bill below.
I'm going to tangent a bit - I hope this helps you, but this is
partially a form of selfish venting also =):
Remind your colleagues that the baseline is what it is: an arbitrary,
imaginary system that may not be a realistic design, but is supposed to
represent the minimum requirements of the 90.1 standard. Nobody can
claim a design is "efficient" or suggest it's "inefficient" without a
reference, and 90.1 baseline models are (often) a poor reference in this
context, as they are supposed to be arbitrary to an extent.
Unfortunately, some entities adopting & enforcing the USGBC/LEED rating
system and the finger-pointing culture that occasionally surfaces
through the submission process have collectively done a lot to suggest
the contrary: that such baseline models fundamentally represent some
form of real-world design (rather than a set of minimum requirements in
an energy standard), and so may be appropriately used to accurately
judge the value of a building's designers. Some designers & LEED
project managers will, in turn, "pass the buck" and judge the value of
the person doing the energy modeling when the numbers aren't favorable.
No (practical) energy standard could account for all the variables and
design constraints of real-world building design - there will inevitably
be disconnects where "good" real-world design decisions are not rewarded
(even punished) in a rating system using a baseline based on an energy
standard.
People shouldn't have lose sleep over this, but they do. We as energy
modelers sometimes have a duty to both inform and set an example in this
industry's culture.
~Nick
NICK CATON, E.I.T.
Nick:
The problems I have experienced with using a dummy zone, without OA From Units,
for the DOAS, as confirmed by different hourly reports I input are as follows:
1. The dummy zone does not equal the size of the conditioned space served and
heat loss/gain is not accurate.
2. The return air temperature is not indicative of what is being returned to
the DOAS when a ERV is used.
3. The fan kW of a variable speed DOAS does not track correctly.
4. The dummy zone occupancy level and schedule do not equal that of all the
systems served.
When using an OA From System, if the system served is not running, eQuest
assumes no outside air is being supplied. The designer needs to specify OA
isolation dampers on the systems served.
I recently modeled two projects with DOAS that was conditioning one zone
(Corridors) and supplying ventilation air to the other systems. This design
results in more accurate modeling, a dummy zone is not needed, but the same
problem of incorrect return air temperatures for an ERV, since only the heat
loss or gain from the corridor is included.
DOAS is becoming more and more popular, due in large part to LEED 62.1
requirements, and hopefully the eQuest developers will refine the program to
accurately model this system
Paul Diglio
Paul,
I agree ? using a dummy zone approach but without using OA-FROM-SYSTEM is probably inadvisable and difficult at best? certainly not preferable ? I was drawn to attempt this once where circumstances prevented me from using OA-FROM-SYSTEM.
I haven?t considered this particular approach in a long while (and hope to not have to), but here?s a few suggestions if you or others are trying to tweak/correct this approach and are encountering those issues listed:
1. Dummy space volume can be adjusted, and a dummy zone (in my mind) should have zero heat transfer ? deleting the offending space surfaces may do the trick ;).
2. The system exhaust at the ERV components should match the return air stream temperature ? perhaps add an internal load using the fractional schedule used for ventilation (remember this isn?t using OA-FROM-SYSTEM) and assigned to a dummy meter (free) to bump the RA into the right range?
3. ERV-FAN-KW-FFLOW produces a figure that considers both the system?s MIN-OUTSIDE-AIR ratio and the hourly ventilation flow. It?s possible the hourly flows (re: ventilation schedule), MIN-OUTSIDE-AIR ratio and/or the fan curve used may be throwing the hourly expected power draw off? you?d have to investigate further.
4. This point ? achieving the correct hourly ventilation requirement - is why in the end, this method avoiding OA-FROM-SYSTEM is such a pain to construct and document.
I also hope DOE2/eQuest may someday have a ?parent DOAS? option for airside systems? I?m not privy to exactly how development is funded, but I speculate this issue might fall into the category of ?a really big R&D windfall needs to navigate through the state of California.? (i.e. not anytime soon )
NICK CATON, E.I.T.