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

[James Hess]

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.  :)

Respectfully,    :)

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<mailto: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> [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<mailto: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> [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<mailto: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 :) (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 --> 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!  :)

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> [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<mailto: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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