[BLDG-SIM] [901ecb] CW Supply Temp reset

[Bill Talbert]

Mike,
Yes, that does help regarding the supply air temperature reset
question. Does anyone know whether these addenda are approved by USGBC?
The LEED reference guide specifically refers to A90.1 "without
amendments" which is an interesting choice of words, but it appears that
some of the addenda were driven by USGBC. My understanding is that
someone has submitted a CIR asking for clarification, but I haven't seen
a response yet.
 
Regarding the chilled water supply temperature reset, I will plan on
submitting a continuous maintenance proposal taking into account some of
the feedback received from 90.1ecb.
 
Also wanted to ask what the intent was for how the reset sequence is
implemented. If you are using chilled water from a central loop in your
proposed design where reset temperature is not provided, the reset would
need to occur at the building through pump/valve/mixing control schemes.
If your building includes chilled water generation, the reset could
occur at the source. For the baseline building, A90.1 requires you to
model chillers. While previous correspondence indicated that the intent
was to use minimum efficiency chiller equipment as the baseline
comparison, this seems as though there is a lot of potential for
unsupportable variations in cooling costs between the baseline and
proposed design. If the model is using purchased chilled water rates in
the proposed design and electricity rates in the baseline model, you
could see cost penalties or savings that are unwarranted depending on
how the purchased chilled water rate is structured. The best comparison
would to be to have both the baseline and proposed models use the same
chilled water source. 
Thanks,
Bill Talbert, AEI
 

>>> "Michael Rosenberg" <Michael.Rosenberg at state.or.us> 3/15/2007 11:28
AM >>>
Bill,

I will take a crack at these. In general Appendix G has tried to
eliminate options in modeling the baseline building. This is so that
users can not try various baseline options and choose the one that shows
the most savings. So when options are available in other parts of the
Standard, G has chosen what seems to make the most sense for use in the
baseline. That sometimes causes problems like the one you brought up.
Maybe we need to propose some modification language that states that if
CW reset results in unmet dehumidification loads greater than the
proposed building, the reset temperature needs to be dropped lower (but
not less than proposed), until the loads are equal. Do you want to try
putting together a continuos maintenance proposal for this?

The supply air reset issue has already been addressed in Addendum A.
Now that sections states;
" The air temperature for cooling shall be reset higher by 5°F (2.3°C)
under the minimum cooling load conditions". 
Non of the sizing language remains. This was published in the 90.1
interim addenda release, and will be in the 2007 version of the
Standard.

Hope that helped

Mike


Michael Rosenberg
Oregon Department of Energy
625 Marion St. N.E.
Salem, OR 97301-3742
Phone : (503) 373-7809
Fax: (503) 373-7806

>>> "Bill Talbert" <btalbert at aeieng.com> 03/15/07 08:42AM >>>
All,
I have a question regarding ASHRAE 90.1-2004 Section G3.1.3.8 - Chilled
Water Supply Temperature Reset. Can anyone offer some background as to
the intent of this section? Appendix G does not appear to allow any
exceptions and requires chilled water supply temperature reset based on
outdoor air drybulb (CWT = 44 to 54 deg F during outdoor drybulb range
of 80 to 60 deg F).The equivalent prescriptive path section (6.5.4.3)
exempts reset controls when VFD pump control is used or if reset
controls would cause heating, cooling, humidifying, or dehumidifying
loads to not be met and also allows reset based on return water
temperature rather than outdoor drybulb only. It appears that if you are
designing a space with sensitive humidity and/or low space temperature
requirements, supply temperature reset could result in space conditions
not being met. What was the thinking in eliminating the exceptions and
the return water temperature option and were the full range of potential
thermal comfort requirements considered? Is the intent that if you are
trying to serve an Operating Room with a VAV/Chilled water system and
you need 68 deg F/50% RH at the space, a supplemental DX cooling coil or
dehumidification system is preferred?

Also Section G3.1.3.12 - Supply Air Temperature Reset indicates that
"supply air temperature shall be reset based on zone demand from the
design temperature difference to a 10 deg F temperature difference under
minimum load conditions" and that "design air flow rates shall be sized
for the reset supply air temperature, i.e., a 10 deg F temperature
difference." This appears to require design air flows to be calculated
based on a 10 deg F temperature difference, effectively doubling air
flow rates from the standard 20 deg F temperature difference. If you
only need to reset supply air temperature down to a 10 deg F delta
during minimum load conditions, why would you size the system based on a
10 deg F delta? What is the intent of this requirement? Doesn't this
result in larger air side systems, i.e. more materials and larger fans?

I am curious how others are dealing with these requirements.

Thanks,
Bill Talbert, AEI



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