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

[Cam Fitzgerald]

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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