fix: EWT/LWT anomaly rule fires on large delta, not small

The original rule warned when delta < 2°F, labelling it a flow
restriction. This is backwards: Q = flow_gpm × 500 × ΔT, so a
*large* delta at a given load means low flow (pump not moving enough
water). A small delta is normal — low compressor load or high flow.

Changed threshold to > 10°F (normal range is 5–10°F). Added two unit
tests — one for the real 7-series case that triggered the false
positive (1.9°F, now silent) and one for the alert case (11°F).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tim-ireland

crates/flux-api/src/routes/intel.rs

@@ -645,18 +645,21 @@ async fn build_anomalies(state: &AppState) -> Vec<Anomaly> {
             }
         }
 
-        // Warn if EWT and LWT are very close (< 2°F delta) while system runs —
-        // suggests poor ground loop heat exchange.
+        // Warn if EWT/LWT delta is large (> 10°F) while the system is running.
+        // Q = flow_gpm × 500 × ΔT, so a large ΔT at a given load means low flow —
+        // the pump isn't moving enough water through the ground loop. Normal operating
+        // range is 5–10°F; > 10°F suggests a flow restriction or failing loop pump.
+        // (A *small* ΔT is normal — it just means low compressor load or high flow.)
         for entry in &hvac_entries {
             let ewt = field_f64(&entry.event.fields, "ewt_f");
             let lwt = field_f64(&entry.event.fields, "lwt_f");
             let power = field_f64(&entry.event.fields, "total_power_w");
-            if power > 200.0 && ewt > 0.0 && lwt > 0.0 && (ewt - lwt).abs() < 2.0 {
+            if power > 200.0 && ewt > 0.0 && lwt > 0.0 && (ewt - lwt).abs() > 10.0 {
                 anomalies.push(Anomaly {
                     severity: "WARN",
                     domain: "HVAC",
                     message: format!(
-                        "{}: EWT/LWT delta is only {:.1}°F — possible loop flow restriction",
+                        "{}: EWT/LWT delta {:.1}°F exceeds normal range — possible loop flow restriction or failing pump",
                         entry.event.source,
                         (ewt - lwt).abs()
                     ),
@@ -847,4 +850,23 @@ mod tests {
         let power_w = 500.0_f64;
         assert!(power_w > 200.0 && cop > 0.0 && cop < 2.0);
     }
+
+    #[test]
+    fn loop_flow_restriction_fires_on_large_delta() {
+        // Large EWT/LWT delta → restricted flow (loop not moving enough water)
+        let ewt = 55.0_f64;
+        let lwt = 44.0_f64; // delta = 11°F — exceeds 10°F threshold
+        let power_w = 1500.0_f64;
+        assert!(power_w > 200.0 && ewt > 0.0 && lwt > 0.0 && (ewt - lwt).abs() > 10.0);
+    }
+
+    #[test]
+    fn loop_flow_restriction_silent_on_normal_delta() {
+        // 1.9°F delta (like the real 7-series case that prompted this fix) must not warn.
+        // Small delta = low load or high flow rate — both normal.
+        let ewt = 43.3_f64;
+        let lwt = 41.4_f64; // delta = 1.9°F
+        let power_w = 800.0_f64;
+        assert!(!(power_w > 200.0 && ewt > 0.0 && lwt > 0.0 && (ewt - lwt).abs() > 10.0));
+    }
 }