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Chapter 5
Calculate OAHtFct:
OAHtFct = HtStrLmt ÷ (OALRef) – (OA point where base prestart
heat should equal Heating Start Limit)
OAHtFct = 90 minutes ÷ (50 °F – 0 °F)
OAHtFct = 1.8
Calculate OAClFct:
OAClFct = ClStrLmt ÷ (OA point where base prestart cool should
equal Cooling Start Limit – OAHRef)
OAClFct = 120 minutes ÷ (90 °F – 60 °F)
OAClFct = 4.0
Figure–5.115 illustrates the relationship of the calculated heating and
cooling factors to the outside air temperature.
140
120
Example:
OAClFct = 4.0
OA Temp = 80.0F
Cool Prestart Time = 80 Minutes
Before Occupancy Time
Minutes Before Scheduled Occupancy Time
100
80
60
40
20
0
110.0
100.0
OAC
lFct =
90.0
4.0
80.0
Cooling Start Limit
(120 minutes)
70.0
60.0
OAHRef
50.0
OALRef
40.0
Heating Start Limit
(90 minutes)
O
t
tFc
AH
30.0
.8
=1
20.0
10.0
Example:
OAHtFct = 1.8
OA Temp = 20.0F
Heat Prestart Time = 54 Minutes
Before Occupancy Time
0.0
-10.0
-20.0
140
120
100
80
60
40
Minutes Before Scheduled Occupancy Time
20
0
Figure–5.115 Outside Air Mode — Example Base Prestart Heating / Cooling.
Select the ZnHtFct and ZnClFct factors utilized for calculation of the prestart
offsets, as follows:
1. Determine the number of minutes required for increasing the zone
temperature one degree under the worst-case heating conditions (i.e. at
100 % heating capacity). Conversely, determine the number of minutes
required for decreasing the zone temperature one degree under the
worst-case cooling conditions (i.e. at 100% cooling capacity).
For this example, the zone temperature will increase one degree in a
10 minute period during heating control (100% heating capacity when
outside air is at 0 °F), and will decrease one degree in a 20 minute
period during cooling control (100 % cooling capacity when outside air is
at 90 °F).
396 WorkPlace Tech Tool 4.0 Engineering Guide
F-27254