fix: keep auto-mode setpoint band on the unit grid (Celsius)

nexia/zone.py

@@ -558,12 +558,18 @@ async def set_heat_cool_temp(
                     self.round_temp(heat_temperature) + deadband,
                 )
             else:
-                cool_temperature = self.round_temp(set_temperature) + math.ceil(
-                    deadband / 2,
-                )
-                heat_temperature = self.round_temp(set_temperature) - math.ceil(
-                    deadband / 2,
-                )
+                # Split the deadband symmetrically around the target. The
+                # offset must land on the unit's setpoint grid (0.5° on
+                # Celsius, 1° on Fahrenheit) and be wide enough that the
+                # resulting band still satisfies the deadband, so round the
+                # half-deadband up to the next grid step. Using math.ceil on
+                # the raw half over-widened the band on Celsius systems
+                # (deadband 1.0 yielded ±1.0 instead of the minimal ±0.5).
+                step = 0.5 if self.thermostat.get_unit() == UNIT_CELSIUS else 1.0
+                half_deadband = math.ceil(deadband / 2 / step) * step
+                target = self.round_temp(set_temperature)
+                cool_temperature = target + half_deadband
+                heat_temperature = target - half_deadband
 
         await self._set_setpoints(cool_temperature, heat_temperature)
 

tests/test_home.py

@@ -2490,3 +2490,50 @@ async def test_check_heat_cool_setpoints_accepts_in_range(
     zone.check_heat_cool_setpoints(heat_temperature=69, cool_temperature=78)
     # Boundary values (exactly at the limits) are allowed.
     zone.check_heat_cool_setpoints(heat_temperature=55, cool_temperature=99)
+
+
+async def test_set_heat_cool_temp_auto_mode_fahrenheit_band(
+    aiohttp_session: aiohttp.ClientSession,
+) -> None:
+    """Auto-mode set_temperature splits the deadband on the Fahrenheit grid.
+
+    The zone is in AUTO mode, so a single set_temperature is expanded into a
+    heat/cool band centered on the target. On Fahrenheit (deadband 3) the
+    half-deadband rounds up to 2, giving a ±2° band.
+    """
+    nexia = NexiaHome(aiohttp_session)
+    devices_json = json.loads(await load_fixture("single_zone_xl1050.json"))
+    nexia.update_from_json(devices_json)
+    zone = nexia.get_thermostat_by_id(345678).get_zone_by_id(234567)
+    assert zone.get_current_mode() == "AUTO"
+
+    with patch.object(zone, "_set_setpoints", new=AsyncMock()) as set_setpoints:
+        await zone.set_heat_cool_temp(set_temperature=70)
+
+    cool, heat = set_setpoints.call_args.args
+    assert (cool, heat) == (72, 68)
+
+
+async def test_set_heat_cool_temp_auto_mode_celsius_half_degree_grid(
+    aiohttp_session: aiohttp.ClientSession,
+) -> None:
+    """Auto-mode set_temperature stays on the 0.5° grid for Celsius systems.
+
+    With a 1.0° Celsius deadband the half-deadband is exactly 0.5°, so the band
+    is the minimal ±0.5° around the target rather than the over-wide ±1.0° that
+    integer ceil produced.
+    """
+    nexia = NexiaHome(aiohttp_session)
+    devices_json = json.loads(await load_fixture("single_zone_xl1050.json"))
+    nexia.update_from_json(devices_json)
+    zone = nexia.get_thermostat_by_id(345678).get_zone_by_id(234567)
+
+    with (
+        patch.object(zone.thermostat, "get_unit", return_value="C"),
+        patch.object(zone.thermostat, "get_deadband", return_value=1.0),
+        patch.object(zone, "_set_setpoints", new=AsyncMock()) as set_setpoints,
+    ):
+        await zone.set_heat_cool_temp(set_temperature=21.5)
+
+    cool, heat = set_setpoints.call_args.args
+    assert (cool, heat) == (22.0, 21.0)