"Morning cool down" modeling

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To all,
I am modeling a building in Colorado where there are many cool
nights/mornings in the summer. My building cools to 76deg 7am-5pm with a
unocc setback to 82deg. The designer is proposing an ECM where the HVAC
will bring in 100% OA from 4am to 7am for "free pre-cooling" to get the
building to the 76deg setpoint before the occupied temperature set point
kicks in. This should save a buch of cooling energy that would be needed
to bring the building to 76deg at system start up. This will only happen
when the outside temperature is favorable. I have air side economizers
set up, but I think they will only cool the building to 82deg from
4am-7am (am I correct?).

Thanks again,
Jacob Goodman

jacob goodman's picture
Joined: 2013-09-12
Reputation: 0

Hi Jacob,
If you are scheduling the fan operation to reflect occ/unocc time you can schedule optimum start up to 6 hours before the schedule calls the fans to be on. You will want to enable night cycle control to minimize unmet load hours. This should utilize the economizer when the temperature permits, but you will always do optimum start. The excerpt from the help file is below. Hope this helps.

Takes the U-name of a schedule (TYPE=ON/OFF/FLAG) that indicates when the system's fans (supply, return, and exhaust) are on or off. If not entered the program assumes the fans are always available when needed. The schedule values have the following meaning:
* If the value is 1, the fans are on.
* If the value is 0, the fans are off but may be turned on by NIGHT-CYCLE-CTRL if zone temperatures warrant it.
* If the value is -1, the fans are not permitted to be on for any reason.
* A value of -999 defines an optimum start period of up to six hours. During this period the fan start time is delayed until the fan run time matches that needed to meet the desired zone temperatures. Notice that this decision is made on an hourly basis, whereas in the real world it is made on much smaller increments (ten minutes or less). For the hourly calculation, the number of hours needed to bring each zone on the system up or down to its set point is estimated. If the number of such hours for the majority of the zones is equal to or greater than the number of hours remaining in the start period, the fans are turned on. The target zone temperatures used in the calculation are the heating and cooling set point temperatures given in HEAT-TEMP-SCH and COOL-TEMP-SCH that correspond to the first hour following the scheduled optimum start period.

Matthew J. Baron, PE
[wendel EM]

Wendel, 140 John James Audubon Pkwy, Suite 201, Buffalo, NY 14228
p 716.688.0766 f 716.625.6825 e mbaron at wendelcompanies. com w wendelcompanies. com


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Matthew Baron's picture
Joined: 2013-07-30
Reputation: 0