diff --git a/internal/mqttingest/latest.go b/internal/mqttingest/latest.go
index 7109344..d29d726 100644
--- a/internal/mqttingest/latest.go
+++ b/internal/mqttingest/latest.go
@@ -121,20 +121,13 @@ func (l *Latest) computeRainIncrement(rainMM float64) float64 {
 	}
 
 	// Decide mode (unknown):
-	// If delta is consistently positive when rainMM > 0, cumulative is likely.
-	// If delta is ~0 while rainMM occasionally > 0, incremental is likely.
-	//
-	// Single-sample heuristic:
-	// - if rainMM > 0 and delta > 0 => lean cumulative
-	// - if rainMM > 0 and delta ~ 0 => lean incremental
+	// Default to cumulative once we see any non-zero rain value. This avoids
+	// wildly overcounting when the service starts and rain_mm is a cumulative counter.
 	if rainMM > 0 {
+		l.mode = rainModeCumulative
 		if delta > 0.0009 {
-			l.mode = rainModeCumulative
 			return delta
 		}
-		// delta near zero but rainMM nonzero suggests incremental
-		l.mode = rainModeIncremental
-		return rainMM
 	}
 
 	return 0