Baseline Fan power calculation

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Hey eQuest users,
I have a new challanging requirement in the appendix G section G3.1.2.8,
its required the supply air-to room air cooling temperature difference is
20 degrees for each HVAC system. Now my question is how can i achieve this
if my baseline is system 8 and the chilled water leaving temperature is
specified 44 F with return delta t=12,at the same time that in my design
the indoor thermostat temp=71.6 F , and the supply cooling temp should be
51.6F, here when i set this value i got an error in eQuest stating that"
the cooling coil leaving temperatures must be at least 6 F higher than
entering water temp" and when i set the cooling supply air temp below 55 F
trying to achieve the 20 degree difference, i got this error.
I hope if any one can help,who may face this in baseline model.
Thanks.

Hala Jaber

Hala jaber
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Thanks James,but what do you mean by LAT, in addition to the fact that i
have modeled the chilled water supply temperature reset as per the
requirement the rest based on outdoor dry bulb temp using :44 F at 80 f and
above, 54 f at 60 f and below, so how can i compromise this since the
actual thermostat cooling set point is 71.6, and i don't have AHU in
baseline, its system 8.
Thanks alot

--

Hala Jaber.

Hala jaber
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>From the outside looking in and not knowing your design requirements, I would question why your indoor "Cooling" thermostat temperature setpoint is set at 71.6 F? That seems like a low and extremely precise cooling setpoint to design to? Also what is your throttling range of your thermostat? Designing to something so precise would mean that our throttling range would be zero, which is not realistic.

Also take a look at your AHU setup? Is it blow through or draw through, essentially where does your fan heat get included in the load, before or after the coil? If the system is draw through the cooling coil has to have a lower coil leaving air temperature to account for the added fan heat.
Hope that helps.

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Steven C. Tobin Jr., P.E.

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Thanks steven, but the indoor design thermostat temperature 22C , which is
not exaggerated ,since my building is office building and the recommended
design temp is 22C as per ASHRAE guidlines,the design throttling range in
my design is 0.83 K(Kelvin),so does it make it realistic?

Hala jaber
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Dear James, thank you, but i have already set the warmest rest temp as
required by section G3.1.3.12 supply air temerature reset, but still i
have the problem in the 20 degree difference between the 71.6 and the
cooling coil supply temp.
I hope if this clarify more my question.

Hala Jaber.

Hala jaber
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If it was me, I'd just set the space temperature to 75 degrees, knowing
that the difference between that and 71.6 degrees probably won't make a
huge difference in the savings between baseline and proposed.

I thought I remember seeing something from Jeff Hirsch a while back that
showed how to de-activate this particular alarm so that the model would
run??

James Hansen, PE, LEED AP, BEMP

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James,
Do you think that i can set the thermostat set point to 75 for baseline
only, i think the baseline and proposed design should have the same indoor
temp, if i can do that this will help since i dont have problem in this
small amount of saving that may differ.
thank you james.

Hala jaber
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Both models should indeed match with respect to thermostat setpoints.

With respect to model documentation, it would be totally appropriate to clearly state the design setpoint (71.6), 20 degree SAT difference mandate, and supply CHW temperature, together result in a conflict with the energy modeling software. Raising both models' set points uniformly to 75 would be a very appropriate fix given the circumstances.

That said, I would not discount the potential effects of fan heat however, as Steve suggested to investigate. The temperature flagged by this particular error (cooling coil leaving) can be directly affected by the presence and location of a fan in the airstream. If you are currently specifying draw through, the coil leaving temperature will be pushed downward from your specified LAT by the degrees the fan raises... whereas if the fan is blow through it doesn't affect the required leaving coil temperature to meet the specified LAT... This is admittedly hard to put to words, but it makes more sense if you sketch it out. If I'm not mistaken, I don't believe 90.1 cares where the baseline fans are located with respect to the coils, so long as the other specified parameters are met, and this distinction has made the difference for me in the past to avoid this particular error in the most simple fashion.

~Nick

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NICK CATON, P.E.

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Hala Jaber

I'll bet if you reduce your supply fan static pressure (to say 1") the error will go away...... the problem is fan heat! Make the AHU a blow thru unit!

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Melissa P. Crowe, LEED AP

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Thanks Melissa, will try it and let you know the result.
:-)

Hala jaber
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It works Melissa, have you any toturial for that to share with our
cooperative eQuest users , how does this solve the problem, or if you can
clarify more this.
Appreciate.
Regards,

Hala jaber
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Hala

There have been multiple discussions in
http://lists.onebuilding.org/pipermail/equest-users-onebuilding.org/

For me the best way to understand this is to draw flow diagrams thru the AHU showing temperatures at fan and coil inlets and outlets; include fan heat based on the fan kW at full load. Do this diagram for a draw thru and blow thru unit. The temps will be different. Remember that in eQuest you define cooling coil leaving temp and minimum room entering air temperatures.

The lens of energy transfer is an interesting way to look at the universe.

Good luck

Melissa

Melissa
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