[BLDG-SIM] CW Supply Temp reset

[Bill Talbert]

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




===========================
You received this e-mail because you are subscribed 
to the BLDG-SIM at GARD.COM mailing list.  To unsubscribe 
from this mailing list send a blank message to 
BLDG-SIM-UNSUBSCRIBE at GARD.COM
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.onebuilding.org/pipermail/bldg-sim-onebuilding.org/attachments/20070315/7d4b5a37/attachment.htm>