[Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 + Return Duct System

Maria Karpman maria.karpman at karpmanconsulting.net
Wed Feb 27 06:45:29 PST 2013


Both the original wording of 90.1 related to fan schedule and the
interpretation that Paul kindly shared sound vague to me. I use the
following logic to decide how baseline/proposed fans should be modeled based
on mechanical ventilation arrangement in the proposed design:

 

(A)   No mechanical ventilation is provided. For example, a multi-family
building with VRF HPs in apartment units may rely on operable windows for
ventilation.

Proposed design: no mechanical ventilation should be modeled; VRF should be
modeled as cycling with load; fan power does not have to be modeled
explicitly

Baseline design: no mechanical ventilation should be modeled; PTHP should be
modeled as cycling with load; fan power does not have to be modeled
explicitly

(B)   Exhaust or supply ventilation is provided. For example, a multifamily
building with VRF HPs in apartment units, with ventilation provided by DOAS
supplying conditioned air to apartments directly or via VRFs, or
continuously running rooftop kitchen/bathroom exhaust fans pulling air
through trickle vents in living rooms / bedrooms.

Proposed design: VRF modeled as specified (cycling with load or running
continuously to deliver OA); exhaust fans / DOAS running continuously to
provide ventilation. VRF fan power does not have to be modeled explicitly if
the units cycle; fan power of continuously running systems (exhaust fans,
DOAS, and possibly VRF) has to be modeled explicitly.

Baseline design: PTHP should be modeled as running continuously, providing
the same ventilation as in the proposed design; fan power should be modeled
explicitly.

 

Thanks,

 

Maria

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James Hess
Sent: Wednesday, February 27, 2013 9:29 AM
To: Cam Fitzgerald; 'Richard Ellison'; 'Kathryn Kerns';
equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 +
Return Duct System

 

Here are just a few more clarifying points, then I have to move on, got
projects calling.

 

When I say that ducting the DOAS direct to the space results in more
ducting, I’m talking about the supply duct downstream of the unit.  The
assumption is made that the general unit supply duct already does a good job
distributing air throughout the zone so we take advantage of that and return
the DOAS to the return of the units.  Otherwise, the DOAS system duct has to
go farther into the space and do what the supply duct already present is
already doing (i.e. distributing air within the space). I don’t think you
can make the assumption that just dumping OA somewhere in the room meets the
ASHRAE 62 requirements because it would not be well mixed within the room.
If the 62 calcs were being done correctly, you would have to compensate for
the lack of mixing by increasing the amount of outside air delivered to the
space.  It’s much less expensive to just route the outside air to the
returns of the units.

 

Secondly, I don’t see how cycling the fans in the Baseline meets ASHRAE 62
ventilation requirements since the requirements are for continuous
ventilation.  Cycling the fans in the Baseline implies that you have a DOAS
present with zero fan power.  That hardly makes any sense.

 

Finally, I’m not just trying to toss rocks at Appendix G.  Obviously, there
is a problem with fan power allowances and it needs correction.  It’s up to
us energy modelers to provide feedback to the ASHRAE PRM committee and work
to change that.  Consequently, I will work on making a recommendation to the
ASHRAE PRM committee regarding this area.  I probably won’t work on that,
however, until after March 14th, due to impending project deadlines.

 

Thanks!  J

 

Regards,

 

JAH

 

James A. Hess, PE, CEM, BEMP
Energy Engineer
TME, Inc.
Little Rock, AR

Mobile: (501) 351-4667

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James Hess
Sent: Wednesday, February 27, 2013 7:43 AM
To: Cam Fitzgerald; 'Richard Ellison'; 'Kathryn Kerns';
equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 +
Return Duct System

 

10-4, here are some follow up comments.

 

In the real world, the majority of DOAS that I have seen designed have the
outside air ducted direct to the return of the units.  Therefore, the fans
must run (versus cycling) in order to adequately distribute the OA
throughout the spaces.  In an actual design, ducting the OA directing to the
spaces generally requires a separate duct system, which is expensive, and
therefore usually not done.  But, I would think that the assumption that the
DOAS dumps OA direct to the space would follow the actual design.  If your
actual design does that, then I would think you could assume cycling, and if
not, then you assume constant fan operation.  Whatever you assume in that
regard should be applied to both the Baseline and Proposed systems.
Therefore, I don’t think that the fans cycling would help all that much.
For example, if the actual design had water source heat pumps and OA ducted
to return of each unit from the DOAS, then the WSHP fans must run during
occupied hours.  The Baseline would make a similar assumption.  If the OA is
instead ducted separately to the spaces, then the OA would be independent of
the WSHPs and the WSHP fans could cycle.  Therefore, the Baseline fans could
cycle.  But if you assumed the Baseline fans are cycling, to me that implies
that a DOAS is separately providing the outside air because air is not
distributed without fans running.  The problem is there is no mention of
DOAS in Appendix G, and no fan power allowance.

 

It’s true that energy recovery from the DOAS could offset the fan energy
penalty associated with the DOAS.  However, eQuest cannot model DOAS with
energy recovery, at least not without a great deal of trickery.  I have
developed a special procedure for eQuest that I have successfully submitted
to LEED as an exceptional calc, and was approved.

 

With regards to my suggestion about treating the DOAS as a process load
being inappropriate 
 

 

If you follow the strict guidance of Appendix G to the letter, you would
assign all the DOAS fan power in the Proposed Design, and zero DOAS fan
power in the Baseline.  In the Baseline, you would only assign the 0.3
watts/CFM fan power to the HVAC units.  I’ve found that even normal FCUs
with some duct work exceed the 0.30 watts/CFM amount, so it’s highly likely
that your Proposed Design fan power will substantially exceed your Baseline
fan power.  The DOAS will be a big reason for this, but not the only reason.
Overall, you will have negative savings in the fan energy category due to
this issue.

 

I guess you have to ask yourself if that more appropriate, or if the
Baseline fan power allowance is inappropriate?

 

If you believe the former, then you will very likely have to go to the
design team and ask them for more efficient equipment in other areas (i.e.
lighting) in order to make up for the negative fan energy savings in order
to hit the overall project energy savings goal, all because we don’t have a
valid Baseline fan power allowance for system types 1 & 2 that allows for
DOAS fan power, or even a standard ducted fan coil unit.

 

The other option is to not model the DOAS fan power in either the Baseline
or Proposed, since the argument can be made that Appendix G doesn’t cover
DOAS, therefore DOAS is not modeled.  

 

I’m sure that would be viewed as inappropriate/incorrect also.  Therefore,
just do it to the exact letter of Appendix G and tell the design team here
is your savings, sorry, don’t blame me, I did not create the fan power
allowances for Appendix G.  J

 

Respectfully,    J

 

JAH

 

James A. Hess, PE, CEM, BEMP
Energy Engineer
TME, Inc.
Little Rock, AR

Mobile: (501) 351-4667

 

From: Cam Fitzgerald [mailto:cam at energyopportunities.com] 
Sent: Wednesday, February 27, 2013 6:40 AM
To: 'Richard Ellison'; James Hess; 'Kathryn Kerns';
equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 +
Return Duct System

 

Agreed
the conditioning fans can only be cycled if they are not required to
distribute the ventilation air from the DOAS.

 

From: Richard Ellison [mailto:REllison at southlandind.com] 
Sent: Wednesday, February 27, 2013 7:35 AM
To: Cam Fitzgerald; 'James Hess'; 'Kathryn Kerns';
equest-users at lists.onebuilding.org
Subject: RE: [Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 +
Return Duct System

 

I agree with your comment that “
suggestion below that the DOAS should be
modeled as a process load is inappropriate for the baseline case.”  However
I have concerns on the final portion of your comment.  Most real designs
that would be proposed don’t dump OA directly into the space and DO require
the terminal fan be running to be operating to distribute OA from the DOAS
into the building.  In this design you cannot assume the terminal unit is
cycled in the proposed case and meet the 62.1 OA requirements.

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Cam
Fitzgerald
Sent: Wednesday, February 27, 2013 7:27 AM
To: 'James Hess'; 'Kathryn Kerns'; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 +
Return Duct System

 

Good morning, all,

 

I inadvertently sent the following response only to Kathryn yesterday.

 

There is no fan power  correction available for the baseline residential
systems type 1 (PTAC) and type 2 (PTHP). The baseline systems are
standardized to establish a benchmark for demonstrating energy savings. If
the dormitory includes non-residential spaces (lounges, corridors, and other
common areas) with a total area of 20,000 sf or more, then exception (a) to
section G3.1.1 would apply and it is appropriate to used an alternate
(non-residential) system type for these areas and if the proposed systems
are fully ducted return systems with MERV 13 filters, then the adjustment
factors for those devices may be applied in the fan power calculations for
the non-residential systems.

 

In light of the suggestion below that the DOAS should be modeled as a
process load is inappropriate for the baseline case. When DOAS supply
tempered ventilation air directly to the space (e.g. not the return
airstream of the unit conditioning the space) the ability to cycle the fans
that condition the space to maintain the space temperature is acceptable
(these fans are not required to provide ventilation so they do not need to
operate continuously). In the Baseline case, the ventilation air is assumed
to be delivered through the PTAC/PTHP units so these fans must operate
continuously during occupied periods. The savings from cycling the fans in
the residential units, the energy recovery, and air-side economizer
generally compensates for the additional fan power required for the DOAS
system. Note that if the DOAS system were modeled in the baseline case, it
is possible that energy recovery would be required and some of the savings
for this measure in the Proposed design would be lost.

 

Herein lies the value of energy modeling for design decisions
is the fan
energy penalty for the DOAS system equal to or less than the potential
savings?

 

Have a great day!

 

Cam Fitzgerald

 

Energy Opportunities/a 7group company

1200 E Camping Area Rd, Wellsville, PA 17365

 

 

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of James Hess
Sent: Wednesday, February 27, 2013 12:02 AM
To: Kathryn Kerns; equest-users at lists.onebuilding.org
Subject: Re: [Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 +
Return Duct System

 

Interesting question about DOAS and the current fan power allowance for
system types 1 & 2.  I think we very much need an update to Appendix G to
account for the fan power requirements of the dedicated outdoor air systems
that are (in my opinion) required for unitary type systems such as PTACs as
well as the PSZ-ACs.  

 

I’m in the south, and so we cannot count on simply opening the windows to
meet our ASHRAE ventilation requirements, at least not anymore J (we did
exactly that through until ~ 1988 at the school in Louisiana I attending
while growing up, at which point we finally got air conditioning and
promptly shut the windows).

 

I would argue that a DOAS is necessary in the south to properly provide
outside air, while maintaining acceptable building humidity conditions, for
unitary type systems.  Anything less is negligent design, in our opinion.
However, at the present time, Appendix G assumes that PTACs (or PSZ-ACs)
will bring outside air continuously into the zones and cycle the compressor
to control room temp while ignoring humidity.  You can’t do that in real
life.  Where we have seen that done, you get condensation and eventually
mold, and eventually you get sued.

 

I think you have run into a gray area for which there is no good answer
currently.  Assuming that you are working on a LEED project, you’ll have to
hope that you run up against a LEED reviewer that will exercise good
judgment and common sense, and understand that this area is not covered by
Appendix G currently and that a reasonable allowance for DOAS fan power
should work and should be allowed, since it can be argued that DOAS are
required to meet the ASHRAE 62 ventilation requirements.

 

The issue is that the 0.3 watts/CFM is only enough fan power allowance to
account for a motel “thru the wall” type unit.  No external ductwork (supply
or return), barely a filter, and certainly no allowance for a DOAS.

 

My recommended solution would be to model the DOAS with the same fan power
in the Baseline as in the Proposed.  

 

That’s conservative in my opinion.  You aren’t taking credit for any fan
energy savings associated with the DOAS, but neither are you unnecessarily
penalizing yourself either for something that is required for the Proposed
Design.

 

Regarding AHRI fan power assumptions, I’m not sure about that question.  But
I did a quick test in eQuest.  I built a quick sample model in the DD
wizard, selected PTAC, and entered 11 EER for cooling efficiency.  I then
went to the detailed mode and the EIR conversion was the exact same that
eQuest does for any system à EIR = 0.2580 for EER =11.0.  Consequently, we
know that eQuest is using the 365 watts/KCFM conversion.  But that’s eQuest.
I don’t know what AHRI 310 uses for fan power assumption, if anything.  I
looked into this a while back and I don’t think that eQuest is on the same
page with the AHRI fan power assumptions used in the ratings.  I could be
incorrect, however.  Another thing is that the fan power assumptions used in
the AHRI ratings have absolutely nothing to do with the Appendix G fan power
requirements.  I do know that.  I think we need to let eQuest do its thing
in the wizard/detailed interface transition, and then go to the detailed
interface and change the EIR ourselves to whatever is appropriate, if we can
figure out what AHRI is doing regarding fan power assumptions.

 

Hope this helps!  J 

 

Regards,

 

JAH

 

James A. Hess, PE, CEM, BEMP
Energy Engineer
TME, Inc.
Little Rock, AR

Mobile: (501) 351-4667

 

From: equest-users-bounces at lists.onebuilding.org
[mailto:equest-users-bounces at lists.onebuilding.org] On Behalf Of Kathryn
Kerns
Sent: Monday, February 25, 2013 5:57 PM
To: equest-users at lists.onebuilding.org
Subject: [Equest-users] Baseline PTAC Fan Energy for a Proposed Merv13 +
Return Duct System

 

Everyone, I have been modeling some dormitories with a proposed HVAC system
involving a DOA with a full exhaust air HX and MERV 13 filters. The puzzle
is what to do about the baseline PTAC fan power energy? If I use 0.3 w/cfm
as Appendix G suggests, my PTAC baseline system does not account for a fully
ducted return\exhaust or MERV 13 filters. The commercial fan power equation
for systems 3-8 accounts for this, but I am not supposed to use that formula
for System 1 and 2.

 

Another puzzle is if I develop an cooling EIR value for my baseline PTAC
using the 365 w/kCFM conversion rule and equate that with 0.3 w/cfm, what
happened to the other 65 w/kCFM? 

Maybe ARI 310/380 doesn’t use 365 w/cfm fan power?  I read it and didn’t
find any statement regarding fan power requirements, but I assume it works
the same as ARI 210/240?

 

I am wondering if a compromise of 

                        PTAC fan power = CFMs *0.000365 + A
where A= PD*CFMs /4131 and PD = 1.4

might be a reasonable solution?

 

                        PTAC fan power = 0.00063 kw/cfm

 

Does anyone else have experience with this puzzle?

 

Kathryn Kerns

Systems Specialist

BCE Engineers, Inc.

| Ph: 253.922.0446 | Fx: 253.922.0896 | 

 

 

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