[Bldg-sim] Supply temperature reset in ASHRAE baseline

[Fred Porter]

>>> "Kevin Kyte" <kkyte at robsonwoese.com> 2/29/2008 7:35 AM >>>

>So how exactly does one do G.3.1.3.12?  If a cool reset schedule were setup
>then it would reference supply air to outside air, right?  Shouldn't it be
>supply air and zone loads?

I think it's up to the analyst; the initial versions of the PRM that called for a 10F reset also specifically referenced control from the zone level, but the new text does not. Part of the reason for the change is that supply air dehumidification might be needed even if all the zones were adequately sensibly cooled. Unfortunately, other sections of 90.1 include economizer operation as "cooling," so the current language referencing reset during "the minimum cooling load conditions," provides some leeway for a baseline, but it sounds to me like when the coil is off should qualify.
 
Also the DOE-2.2 COOL-CONTROL = WARMEST setting often produces odd hourly results (all coils off and SAT = MAT,  jumping to HEAT-SET-T, etc.) I think these issues have to do with the program response to zone temp setback/up, but I'm tired of trying to figure it out. Don't even get me started on morning warmup.
 
So I think modeling an OA reset schedule (COOL-CONTROL = RESET; COOL-RESET-SCH = something) is the way to go. Typically reset SAT up to 60F at OAT below 55F. With the HEAT-SET-T = 60F, not 90F or something; that's what MAX-SUPPLY-T should be, which is the air temperature downstream of the reheat coil.
 
If the proposed design doesn't reset SAT the way the PRM baseline is written, and this proposed system has a central heating coil which is needed to maintain the SAT at high OA fractions during cold weather, the PRM seems to contradict itself. It also states that the baseline "preheat" coil will be controlled in the same manner as the proposed coil, and generally heating the primary air in a VAV AHU is done by a "preheat" coil, even if this is not what that coil is called in DOE-2, and even if the simulation controls it via a "COOL-CONTROL" statement. 
 
As a matter of principle, I would be very leery of claiming any PRM/LEED savings for a reset strategy better than the baseline definition; can the persistence of those control settings be guaranteed? Does the simulation really model both cases accurately? Is the baseline SAT reset defined precisely? 
 
Clear as mud?
 
Fred

> -----Original Message-----
> From: bldg-sim-bounces at lists.onebuilding.org 
> [mailto:bldg-sim-bounces at lists.onebuilding.org] On Behalf Of
> yizhao1 at vt.edu 
> Sent: Thursday, February 28, 2008 5:00 PM
> To: bldg-sim at lists.onebuilding.org 
> Subject: [Bldg-sim] Supply temperature reset in ASHRAE baseline
>
> Hi -
> In the baseline, ASHRAE requires a supply air temperature reset for
> system 5, packaged rooftop VAV w/ reheat (G3.1.3.12). In the air side
> system level, I can set up a schedule for cool control, but heat control
> is in grey.
> Does anyone know how to input the reset in the system? Economizer is
> inlcuded.
>
> Thanks!
>
> Ying
>
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