diff --git a/backend/services/telemetry_collector.py b/backend/services/telemetry_collector.py
index 7469d7e..7f75721 100644
--- a/backend/services/telemetry_collector.py
+++ b/backend/services/telemetry_collector.py
@@ -517,7 +517,7 @@ MIM_OFF_WATER_IN = 15      # °C×10 signed
 MIM_OFF_WATER_OUT = 16     # °C×10 signed
 MIM_OFF_DHW_ONOFF = 22
 MIM_OFF_DHW_TEMP = 25      # °C×10 (zásobník TUV)
-MIM_REG_DEFROST = 2        # modulový registr: 0=off, jinak defrost
+MIM_REG_DEFROST = 2        # modulový registr: 0 i 0xFF = off, jinak defrost
 MIM_MODE_NAMES = {0: "auto", 1: "cool", 2: "dry", 3: "fan", 4: "heat"}
 
 
@@ -585,7 +585,8 @@ async def poll_heat_pump(site_id: int, db: asyncpg.Connection) -> None:
         dhw_temp_c = _mim_temp_c(iu[MIM_OFF_DHW_TEMP])
         water_in_c = _mim_temp_c(iu[MIM_OFF_WATER_IN])
         room_temp_c = _mim_temp_c(iu[MIM_OFF_ROOM_TEMP])
-        defrost = bool(defrost_raw)
+        # manuál MIM: 0 i 0xFF = defrost OFF, ostatní hodnoty = ON
+        defrost = int(defrost_raw) not in (0, 0xFF)
         # Idle-skip: TČ vypnuté a teploty (na 0.2 °C) beze změny → heartbeat;
         # pomalý drift teplot zachytí heartbeat 840 s (TUV delta se normalizuje
         # per minutu ve fn_update_tuv_usage_stats).