heat-timer Mini-MOD-CNC Installation And Operation Manual page 5

01 MINI-MOD-CNC OVERVIEW
MODULATING LOGIC OVERVIEW
The Modulation PID logic provides the capability of controlling multiple modulating current or voltage boilers. When heat is
required, the control PID will activate the lead boiler and start its purge. See "PURGE DELAY" on page 32. This will be followed
by the initiation of its modulation at the Ignition %. See "IGNITION %" on page 43. After the Purge period and when additional
heat is needed, the control shall start to increase the lead boiler modulation until the Modulation Start % has been reached. See
"MODULATION START %" on page 43. That shall be followed by the lag boiler purge initiation. The lead boiler shall resume its
modulation while the lag boiler is in purge until the lead boiler reaches full fire (100% modulation). Any additional heat requirements
will trigger the control to increase the lag boiler modulation.
When the Mini-MOD-CNC requires reduced output, it will reduce the modulation of the lag boiler until it reaches its Ignition %. That
shall be followed by the reduction of modulation of the lead boiler until it reaches 40% percent of the Modulation Start %. This shall
trigger the control to turn off the lag boiler. The control will keep reducing the lead boiler modulation until it reaches its Ignition %.
If the Last Stage Hold was activated, the control will hold the lead boiler at the Ignition % until the System Temperature exceeds the
Target Set Point by the Last Stage hold setting. See "LAST STAGE HOLD" on page 34.
STAGING LOGIC OVERVIEW
The Staging PID control logic is primarily used with multi-stage boilers. The logic will utilize two primary settings to add or subtract
stages. The Reaction Time is used to turn on/energize stages. See "REACTION TIME" on page 36. The Minimum Runtime is used
to turn off/de-energize stages. See "MINIMUM RUNTIME" on page 36. A call for heat, by either closing the TSTAT input or opening
the SHUTDOWN input while the outdoor temperature is below the Outdoor Cutoff, will trigger the Mini-MOD-CNC to turn on/
energize the lowest firing stage of the Lead Boiler to start its Purge. See "PURGE DELAY" on page 32. After the elapse of the purge
period, the Mini-MOD-CNC will start calculating the Reaction Period. If after a full Reaction Time the control logic foresees the need
for additional stages, the Mini-MOD-CNC will energize the following stage. If that stage was another boiler, that boiler has to go
through a full Purge Delay before starting to calculate the Reaction Time for that stage. Otherwise, if the next stage was the higher
firing stage on the same boiler, the Reaction Time will start from the moment the higher firing stage relay is energized.
When the Mini-MOD-CNC logic foresees that the system will overshoot, regardless of the current system and target values, it will
make sure that the last stage turned on/energized has elapsed a full Minimum Runtime before it is turned off/de-energized. Except
for the lead stage, no additional stages will be turned off/de-energized until another full Minimum Runtime is elapsed. On the other
hand, if the last stage is a lead stage, it will remain energized until the system reading exceeds the target set point by the Last Stage
Hold value in addition to satisfying the Minimum Runtime condition. That is, if the Set Point was 150°F and the Last Stage Hold was
set to 10°F, the lead stage will remain energized until the system reaches 160°F and a full Minimum Runtime elapses. This is useful in
protecting the lead stages from short cycling.
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