When battery voltage drops to or below 12.0 volts, the system exclusively uses recirculation mode for any intermittent sprinkler operation. The pump is commanded on when intermittent sprinkler mode first activates and remains on continuously throughout the intermittent period. The P1 recirculation valve cycles between pass-through (delivering water to sprinklers) and recirculation (returning water to tank) to create the intermittent pattern. This minimizes battery drain by avoiding repeated diesel starter motor activation—each pump start might draw 200 amperes momentarily, while continuous pump operation draws perhaps 10 amperes. With a compromised battery unable to reliably restart the pump, continuous operation with valve cycling becomes the only viable strategy.
When battery voltage exceeds 12.0 volts and the sprinkler cycle's off time exceeds 15 minutes, the system switches to pump cycling mode. The P1 valve remains in pass-through position continuously, and the main pump itself turns on during sprinkler-on phases and off during sprinkler-off phases. This mode is more efficient for very long off periods (such as the three minutes on, 30 minutes off cycle that would apply to very low external temperatures during cooldown), allowing the diesel engine to cool and the battery to recover between sprinkler cycles. However, it requires adequate battery capacity to reliably restart the pump after each off interval.
For cycles with off times of 15 minutes or less, recirculation mode is used even with good battery voltage. The frequent cycling that would result from pump mode with short intervals would cause excessive diesel starter wear and wouldn't provide meaningful battery recovery time. The 15-minute threshold represents a balance point where pump cycling begins providing efficiency benefits that outweigh the disadvantages.
Continuous sprinkler operation always uses the main pump directly without valve cycling. When external temperature exceeds 80 degrees Celsius triggering continuous mode, the pump is commanded on and P1 is set to pass-through, providing maximum flow to the sprinklers for the duration of the fire front passage. No battery-based decisions apply to continuous mode—protection takes priority over battery conservation when fire is imminent.
The high flow valve P3 activates whenever external temperature exceeds 80 degrees Celsius, the same threshold that triggers continuous sprinkler operation. This valve, when energized, increases hydraulic pressure and flow to the sprinklers, improving coverage and cooling effectiveness. The choice to tie P3 exclusively to the 80-degree threshold rather than to TRISE (temperature rise rate) represents a simplification from earlier designs—temperature rise rate proved less reliable as a predictor of immediate fire threat than absolute temperature level.
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