Baseline PTAC Fan Energy for a Proposed Merv13 + Return Duct System

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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

Kathryn Kerns's picture
Joined: 2011-09-30
Reputation: 0

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! :)



James A. Hess, PE, CEM, BEMP

James Hess's picture
Joined: 2011-09-30
Reputation: 601

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

Have a great day!

Cam Fitzgerald

Cam Fitzgerald's picture
Joined: 2011-09-30
Reputation: 0

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.

R I C H A R D E L L I S O N BEAP, BEP, BEMP, BESA, CBEP, CDSM, CEA, CEM, CMVP, CTAB, DGCP, HBDP, LEED AP, PE Manager of Energy and Simulation Southland Industries 22340 Dresden Street, Suite 177 Dulles, VA 20166 Office: 703.834.5570 Direct: 703.
Richard-Ellison's picture
Joined: 2011-09-30
Reputation: 1

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

Cam Fitzgerald's picture
Joined: 2011-09-30
Reputation: 0


Attached is an official interpretation I requested from ASHRAE concerning this
issue. In my understanding, the supply fans that do not provide ventilation are
not required to run continuously.

That being said, it is implied that the ventilation air is somehow distributed
throughout the occupied space in order to provide air to all the rooms. I was
modeling an apartment building with two PTAC systems in each apartment of 4
rooms. The ventilation air was introduced via an outlet over the apartment
entry door. It would not be possible to distribute this air to the bedrooms or
living room without ductwork. So in this case the PTAC units (2) in each
apartment need to run in order to distribute this air, especially if the
interior doors are closed.

Of course, in the real world the occupants would not leave the PTAC fans running
continuously 24 X 7 to distribute this air.

Has anyone submitted a model to LEED cycling the PTAC fans when ventilation air
is provided by another system?

Paul Diglio, CEM, CBCP

Paul Diglio's picture
Joined: 2011-09-30
Reputation: 400

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, :)


James A. Hess, PE, CEM, BEMP

James Hess's picture
Joined: 2011-09-30
Reputation: 601

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! :)



James A. Hess, PE, CEM, BEMP

James Hess's picture
Joined: 2011-09-30
Reputation: 601

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

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

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

(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



Maria Karpman's picture
Joined: 2011-09-30
Reputation: 0

My apologies, it was early and I was distracted. It should state that
cycling the fans in the Proposed is acceptable under these conditions;
Appendix G clearly requires the fans to operate continuously since the units
supply the ventilation air.

Cam Fitzgerald's picture
Joined: 2011-09-30
Reputation: 0

I have been following this because I have some related issues with
another DOE-2 based program, EnergyPro, regarding fan power and energy.
I am wondering if my concerns are related, so I will just put them out
there and if not, not.

EnergyPro asks for Fan Brake Horsepower. That fan BHP is then converted
to fan motor Watts by dividing by motor and drive efficiency. The issue
is that getting FAN BHP is difficult. What is usually available from
the manufacturers is FAN MOTOR AMPS, which is typically more than the
motor ever draws, for electrical conservatism reasons (don't want to
undersize the wires). So getting FAN BHP (essentially at the fan shaft)
is not easy. But using Fan Motor Amps gives a much higher number.

So, is this germane to the discussion here, that is, would getting fan
BHP be helpful? The theoretical Fan Horsepower equation, Fan BHP = (cfm
x static press "w.c.) / (6356 x fan efficiency), can get Fan BHP, given
fan efficiency, but again fan efficiency is not easy to get. I am
wondering if using Fan BHP is what EQuest needs, not Fan Motor Watts and
the lower Fan BHP would provide better compliance, in accordance with
the basis of the program. When you say "we know that eQuest is using the
365 watts/KCFM conversion" is that Fan Shaft Watts or Fan Motor Watts?
Does EQuest subsequently divide that by the drive and motor efficiency
to get Fan Motor Watts? If so, the .365 W/CFM may be sufficient. Fan
Motor efficiencies can be pretty dismal, I have been told. If EQuest
does this calculation, what Fan Motor Efficiency is used?

Regarding ratings and what interior fan power and energy is accounted
for in the ratings, well, if you find out anything on that, I'd love to
hear it. I know that indoor fans are included in the ratings, but how
much power and energy? I do not know. I have toyed with calling AHRI
and asking them, and I think that is what is needed, but I haven't done
that. Yet.

Robert Wichert P.Eng. LEED AP BD&C

RobertWichert's picture
Joined: 2011-10-02
Reputation: 201


I believe the attached document covers what eQuest is doing with regards to EER entered into the wizard, and then translated into EIR for the detailed interface.

This shows that eQuest is assuming the AHRI Fan Power to 365 watts/KCFM. We verified this by implementing the calc in a spreadsheet and then comparing the eQuest results to it. I'm not entirely sure, however, that the 365 number is what AHRI is currently using for all systems, whereas eQuest assumes that conversion applies to any DX equipment in the wizard (i.e. split DX, PRTU, PTAC, etc.).

With regards to fan power values, you are absolutely correct, BHP is difficult if not impossible to get for unitary type equipment, and estimating power using amps is problematic, I agree. For example, we have tried calculating fan power for FCUs from submittals using amps and volts, and the result was ~ 0.50 watts/CFM, or >> 0.30 watts/CFM.

In my own house, I have a high efficiency unit with a variable speed ECM fan motor; I have measured the fan power to be ~ 0.40 watts/CFM, which is for a system with supply duct, but no return duct (i.e. through the wall return). So at least that gives me some frame of reference.

Within eQuest, you can either just use the defaults for TSP and efficiency, and let eQuest calculate fan power, or just calculate the kW/CFM yourself and input, which in that case, eQuest ignores the TSP and efficiency inputs. The resulting kW, through either method, is assumed to be the motor input kW.

Below is a screen capture with default inputs for PTACs, which interestingly result in ~0.18 watts/CFM (if I did the calc correctly). That's quite a bit lower than the 0.30 watts/CFM value. I didn't realize that the default PTAC value in eQuest was that low.

If you don't know or cannot calculate fan power from submittal data, you can consider just using the defaults. The might actually help with the original problem of absorbing the fan power of the DOAS into the Baseline. In other words, Proposed is modeled @ default value of 0.18 watts/CFM for main PTAC units + DOAS fan power. Baseline is modeled at 0.30 watts/CFM which is understood to include DOAS fan power.

Anybody know if the default eQuest PTAC fan power values would be acceptable? I don't see how anybody except the product engineers at the companies that make PTACs would know otherwise because the fan power wattage values are simply not available. You would have to break out the Kill-a-watt measurement device to verify this. I have that device, and I would verify if I had access to a PTAC.

The other alternative is to just make the fan power the same between Baseline and Proposed for the main units (PTACs for example) --> 0.30 watts/CFM. In this regard, you are being conservative in that you aren't taking credit for any fan power savings, but they are not hurting you either.

[cid:image001.png at 01CE14D1.92C61AE0]



James A. Hess, PE, CEM, BEMP

James Hess's picture
Joined: 2011-09-30
Reputation: 601


I appreciate your response, and had additional question.

Regarding scenario B, you are saying that the DOAS fan power must be included in the Proposed Design. Agree.

For the Baseline, I see what you are saying. The Baseline fans must be modeled explicitly because they run continuously while the compressors cycle.

However, my question is what fan power allowance do you use for the Baseline systems, and do you include any additional allowance to account for a Baseline DOAS?

Thanks! :)



James A. Hess, PE, CEM, BEMP

James Hess's picture
Joined: 2011-09-30
Reputation: 601


The baseline HVAC fan power budget for Systems 1 & 2 is fixed at 0.3 W/CFM.
The only difference between the baseline for Option A versus B below is that
with (A) the baseline fans cycle with load, while with (B) the baseline fans
run continuously. To rephrase, there is no additional baseline allowance in
(B) for DOAS or exhaust fans that pull air in through trickle-vents (if
proposed design relies on exhaust ventilation to provide fresh air to
occupants). In this respect the baseline Systems 1&2 are no different from
any other baseline system type.



Maria Karpman's picture
Joined: 2011-09-30
Reputation: 0