Figure 42. Optimal Start Time Line; Table 19. Optimal Start Time Increments (In Minutes) - McQuay MicroTech II Operation Manual

Figure 42. Optimal Start Time Line

When the Scheduled Start-Up Time minus the Optimal Start Begin Recirculate Period occurs, the CSM enters the
Recirculate operating state and starts the cooling load pump(s). The Optimal Start Recirculation Period variable defines the
length of time the CSM remains in the Recirculate State. At the end of the recirculation period, the CSM samples the
chilled water return temp (CWRT) and outside air temp (OAT).
The exact time at which the CSM enables the chiller system is determined by calculating the optimal start time increment.
The time increment is calculated using the following equation:
OptimalStartTimeInc = (OAT–System Setpt)*RunTimePerDegreeOAT + (CWRT–System Setpt)*RunTimePerDegreeCWRT
The optimal start time increment is limited to a maximum of 240 minutes.
When the Scheduled Start-Up Time minus the Optimal Start Time Increment occurs, the CSM enters the On operating state
and starts the chillers and cooling load pump(s). Optimal Start keeps the CSM ON until the Scheduled Start-Up Time. If
the supply temperature reaches the System Setpoint before the scheduled start-up time, the system continues to operate; it
does not shut down and then start up again.
A table of optimal start time increments in Table 19 shows some temp/time combinations for the default values (0.5
Min/degree) of Run Time Per Degree OAT and Run Time Per Degree CWR. This table was calculated assuming a System
Setpoint of 45 °F. For any combination of CWRT and OAT, a particular time increment is used. Notice that as the return
water or OAT increases, the optimal start time increment increases.

Table 19. Optimal Start Time Increments (in Minutes)

Outdoor Air Temperature
(OAT)
50°F (10°C)
60°F (15°C)
70°F (21°C)
80°F (26°C)
90°F (32°C)
100°F (38°C)
Note: Calculations in this table used the default optimal start settings and a System Setpoint = 45°F (7.2°C)
For example, if the return water temperature is 83°F (28°C) and the OAT is 87°F (31°C), the optimal start time increment
would be 40 minutes;
Optimal Start Time Increment = (87 – 45) * 0.5 + (83 – 45) * 0.5 = 40 minutes
If the OAT were 106°F (41°C) instead of 87°F (31°C), the optimal start time increment would be 49.5 minutes
Optimal Start Time Increment = (106 – 45) * 0.5 + (83 – 45) * 0.5 = 49.5 minutes
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Optimal Start Begin Recirculate Period
CSM samples temperatures and
calculates optimal start time
Optimal Start
Recirculation Period
Today's Optimal Start Time
Chilled Water Return Temperature (CWRT)
50°F(10°C) 60°F(15°C)
5 10
10 15
15 20
20 25
25 30
30 35
Scheduled start-up time
Optimal start window
time
Optimal start
time increment
70°F(21°C) 80°F(26°C)
15 20
20 25
25 30
30 35
35 40
40 45
90°F(32°C)
25
30
35
40
45
50
OM 780-2