LAARS S8Ext - Installation Manual - Manual (Page 24)

Page 24 LAARS Heating Systems
• Standby units are used for backup or extreme load conditions only. A Standby Stage can never be a Lead Stage
• The full Standby Delay Time must always elapse regardless of what happens to system temperature. Therefore, shorter Standby
Times will result in smoother set point operation in extreme conditions. Longer Standby Times may prevent a Standby unit
from starting if the other units can eventually meet the load, or if the load decreases.
• When setting Standby Time, remember that it will be added to the Reaction Time for the first stage on the first Standby unit.
The following stages start time will rely on Pre-Purge and Reaction Time.
LAST STAGE HOLD
Adjustable 0ºF/0ºC - 30ºF/17ºC Default: 5ºF/3ºC
Button: MENU///Last Stage Hold in PID Logic only
• The Last Stage Hold prevents short cycling of the Lead Stage during low load conditions
where the system might have a load that is significantly less than the output of one stage.
LAST STAGE HOLD
0 F
[ ]
BACK SAVE
When the S8 brings on the Lead Stage, the Set Point is quickly exceeded, and the Lead Stage is turned off.
• To prolong the run time during this type of condition, use the Last Stage Hold setting.
• In heating, the S8 will allow the system temperature to exceed the Set Point by the number of degrees selected, before the
Lead Stage is turned off. In cooling, the the S8 will allow the system temperature to rise above the Set Point by the number of
degrees selected, before the Lead Stage is turned off.
• For example, with a Set Point of 160°F and a Last Stage Hold setting of 10°F, the Lead Stage boiler will remain on, at low fire,
until the Set Point reaches 170°F then turn off.
THROTTLE RANGE
Adjustable 2ºF/1ºC - 20ºF/11ºC Default: 2ºF/1ºC
Button: MENU///Throttle in OSS Logic only
• The Throttling Range sets a temperature band around the Set Point that controls when
stages will be turned on or off.
-- THROTTLE RANGE --
2 F
[ ]
BACK SAVE
• For example, in the Oversize (OSS) 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 to two full Throttling Ranges below the Set
Point, and so on, with one extra stage being turned on for every throttling below the Set Point the System temperature reaches.
• 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 Example in Heating Mode
Set Point = 180°F Throttling = 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
LEAD SETTINGS
Button: MENU///
The lead menu is to help in setting the Lead unit and type of rotation approperiate for the
system.
-- LEAD SETTINGS --
Lead Boiler AB
Rotate Mode Time
BACK SELECT