Page 21
Laars Sequencing Controls
PURGE TIME (
Pt
)
Default: 0 minutes
• Many units must go through a purge cycle before they are brought on line and can begin generating heating or cooling.
• The Purge Time should be set to the length of time of the unit’s purge cycle.
• The Purge Time applies to every Stage Output with ON/OFF units.
• The Purge Time applies only to the Lo stages of multiple stage units (see charts on pg. 17).
• When an ON/OFF stage, or a Lo stage, is turned on, the SC will begin counting down the Purge Time. When the Purge Time
has elapsed, then the SC will begin counting down the Reaction Time. Therefore, if the Purge Time applies to a particular Stage
Output, the minimum run time of that stage is the Purge Time plus half a Reaction Time.
• The Purge Time can be set from 0 minutes (
0.0)
to 10 minutes (
10.0
).
BOILER MINIMUM RUNTIME (
brt
)
Default: 0 minutes
• This is the minimum time any Output Stage can run. This timer begins after any Purge Time
Pt
.
• The Boiler Minimum Runtime does NOT apply when only one stage holds the load or when all the stages but one have been
turned off. In those cases, the Last Stage Hold
LSH
(see below) applies.
• The Boiler Minimum Runtime setting prevents stages from being short cycled.
• The Boiler Minimum Runtime can be set from 0 minutes (
0.0
) to 6 minutes (
6.0
).
• The System temperature may rise above Set Point during this time.
LAST STAGE HOLD (
LSH
)
Default: 10
°
F
• This
signifi cantly more output than is required to hold the load, the Output Stage will turn on, rapidly overshoot the Set Point, and
then turn off as soon as the Purge Time
Pt
and Reaction Time are satisfi ed.
• In these conditions, it may be desirable to override the Set Point or Calculated water temperature to allow the Output Stage to run
longer and therefore also be off longer before restarting.
• The Last Stage Hold provides an adjustable temperature value which is added to the Set Point or Calculated water temperature.
• When heating, the SC will keep the last Output Stage running until the System temperature exceeds the Set Point or Calculated
water temperature plus the Last Stage Hold value.
OVERSIZE SYSTEM SEQUENCING SETTINGS
THROTTLING RANGE (
thr
)
Default: 5
°
F
• The Throttling Range sets a temperature band around the Set Point or Calculated water temperature that controls when stages
will be turned on or off.
• For example, in the Set Point heating mode, no stages will be activated until the temperature falls one full Throttling Range
below the Set Point. A second stage will be activated when the temperature falls two full Throttling Ranges below the Set Point,
and so on, with one extra stage being turned on as the temperature falls additional stages below the Set Point.
• Stages will be turned off as the temperature rises toward the Set Point with one full throttling range as a differential.
• The last stage to be turned off will be allowed to exceed the Set Point by a full throttling range before it is turned off. This helps
to prevent the last stage from short cycling when the load is low or when the stage is oversized.
Throttling Range Example
Heating Application Set Point = 180
°
F Throttling Range(
thr
) = 5
°
F 4 Boiler Stages, A, B, C, and D
Falling Temperature
Rising Temperature
Temperature
Calculation
Stage Turned On
Stages On
Stage Turned Off
Stages On
185
°
F
180 + (1)
THR
None
None
A
None
180
°
F
180 - (0)THR
None
None
None
A
175
°
F
180 - (1)THR
A
A
B
A
171 to 174
°
F
---
---
A
---
A,B
170
°
F
180 - (2)THR
B
A,B
C
A,B
166 to 169
°
F
---
---
A,B
---
A,B,C
165
°
F
180 - (3)THR
C
A,B,C
D
A,B,C
161 to 165
°
F
---
---
A,B,C
None
A,B,C,D
160
°
F
180 - (4)THR
D
A,B,C,D
None
A,B,C,D
