LAARS M4-Ext - Installation Manual - Manual (Page 20)

Page 20 LAARS Heating Systems
SETBACK
Adjustable 0°F to 75°F Default: 0°F
Button: MENU/<System Settings>/<More Settings>/Setback
•
The Setback feature can be used to provide the M4 with a lower temperature Set Point
when less load is required.
• The lower Set Point will appear on the main display indicating this condition.
• For an example, if the calculated temperature is 180°F and the Setback is 20°F, then
when in Setback, the M4 will hold a Set Point of 160°F.
• A typical use for Setback is to provide less system temperature to a building during the
night or on the weekends when building is not occupied, but heat is still required.
BACK SAVE
-------------- SETBACK -----------
0
F
[ ]
• The amount of Setback selected is subtracted from the Set Point when a Setback External Signal is received or the Night Time
schedule setting started.
• If External Signal is selected as a Setback Mode (See Startup menu), the Setback will not be activated unless a dry contact
signal source is wired into the Shutdown/Setback terminals (31 and 32) and the M4 receives a SHORT signal.
• If Day/Night Schedules is selected as a Setback Mode (See Startup menu), the Setback will be activated only when Night
Schedule time has started.
CAUTION
When using Soft-Off and Last Stage Hold, the last boiler stage will not turn off
until both parameters have elapsed. In this case, Soft-Off will start after the Last
Stage Hold.
LAST STAGE HOLD
Adjustable 0°F to 30°F Default: 0°F
Button: MENU/<System Settings>/<More Settings>/Last Stg Hold
•
The Last Stage Hold prevents short cycling of the Lead Stage during low load periods.
• In low load conditions, the system might require only 5% of the output of one Stage.
When the M4 brings on the Lead Stage, the Set Point is quickly exceeded, and the M4
turns the Lead Stage off.
• To prolong the run time during this type of condition, use the Last Stage Hold setting.
• The M4 will let the system temperature exceed 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 modulation, until the Set Point reaches 170°F.
• From an efficiency standpoint, it is better to overshoot slightly than to short cycle a
boiler.
BACK SAVE
------ LAST STAGE HOLD ------
0
F
[ ]
WARNING
The temperature limits set
on the boilers must be higher
than the M4 Set Point. Read
the section at left for details
that will prevent erratic system
operation.
Avoiding Conflicting Boiler Limits
•
The temperature limits set on the boilers MUST be set considerably higher than the M4’s
Set Point for the reasons detailed below.
• The M4 sensor is located in a common header some distance from the boilers.
• As the temperature rises in the header and before reaching the sensor location, energy is
dissipated. Therefore, the temperature in the header could be lower than that registered
by boiler sensors.
• In addition to the normal drop experienced between the boiler’s temperature and that
read by the M4 sensor, the Last Stage Hold setting must be accounted for. The boiler limit must be set above the Set Point
PLUS the Last Stage Hold PLUS the normal drop experienced in the piping.
• Using the previous example of a 10°F Last Stage Hold with a 160°F Set Point, the boilers’ limits must be set enough over 170°F
to prevent the boilers’ internal limits being reached. In this situation, the boiler high limit should be set at approximately 180°F
to prevent the difference in boiler temperature vs. header temperature causing erratic operation.