diff --git a/CHANGELOG.md b/CHANGELOG.md
index 655b70d..a85b651 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ All notable changes to `OWNd` are documented in this file.
 ### Fixed
 - **WHO 4 Dimension 22 is never sent**: Dimension 22 is the published split-unit IR control (`*#4*3#Z#N*22##` / `*#4*3#Z#N*#22*MOD*SP*VEL*SWING##`, WHO 4 specification pp. 66-68); in any spelling (`*#4*ZONE*22##`, `*#4*#0*22##`, leading zeros, whitespace or `+` tokens) it crashes the thermoregulation daemon (`bt_termo`) on Linux IP gateways and reboots an F461 2.0.11 about three minutes later (own-firmware-oracle `tools/expected_divergences.yaml` crash frames, live F461 2.0.11). `OWNCommandSession.send` (and so `AsyncTcpTransport.send` and `keepalive`), `OWNCommandSession.send_to_gateway` (before `connect()`: no socket, no handshake) and `AsyncSerialTransport.send` refuse it before anything is written, with the contract above and `reason="destructive_frame"`. The detector `is_who4_dimension_22()` is public, accepts `bytes`, and is anchored to the first `*` of each frame, so a mid-frame WHERE `#4` / DIMENSION `#4` such as `*#2*#4*#11#001*22##` or `*#1*14*#4*1*22##` stays sendable. Parsing is unchanged: the frame can still be read from the bus.
 - **`AsyncSerialTransport.send` validates outbound frames** like `OWNCommandSession.send`: an invalid or signaling frame (for example the single-`#` concatenation `*1*1*11#*#4*2*22##`) is refused with `reason="invalid_frame"` instead of being written to the port. The frame is serialised once, terminator included: the string checked is the string written.
+- **WHO 4 (`OWNHeatingCommand.set_central_mode`)**: raises `ValueError` unless `where` is a central-unit `#` address (`#0`; `#0#C` with `C` the 4-zone unit's configured zone 1..99 or `01`..`99`; `#Z` with `Z` 1..99 or `01`..`99`; `#Z#C` with central unit `C` 1..99 as in libqtdevices' `*4*311*#23#1##`, the frame `set_mode("23#1")` already builds; `#00`, `#0#0`, `#0#00` and three-digit values are refused, whitespace is stripped); the plain zone form `*4*311*Z##` is `nack`/`silent` on all 12 emulated gateways (own-firmware-oracle `thermo-zone-mode`, `ownd-pr82-heating`) and on a live F461 2.0.11.
 
 ## [2.0.0b11] - 2026-10-20
 
diff --git a/OWNd/message/heating.py b/OWNd/message/heating.py
index a9a38f9..f0fbb9f 100644
--- a/OWNd/message/heating.py
+++ b/OWNd/message/heating.py
@@ -109,6 +109,22 @@ def is_who4_dimension_22(frame: object) -> bool:
     return any(_WHO4_DIMENSION_22_REGEX.search(chunk) for chunk in raw.split("##"))
 
 
+# A central-unit address: ``#0`` (99-zone central unit), ``#0#C`` (a 4-zone
+# central unit, ``C`` its configured zone 1..99 or ``01``..``99``: MyHOME_Suite
+# OPEN.db ``#0#[ZA][ZB]``, MHCatalogue 4695 ``ZAZB_CENTRAL`` 1..99, oracle
+# ``thermo-central-mode`` ``*4*0*#0#2##`` -> ``D1 02``), ``#Z`` (zone 1..99 or
+# ``01``..``99`` addressed through the central unit) or ``#Z#C`` (zone Z
+# controlled through 4-zone central unit C, 1..99: the form ``set_mode("23#1")``
+# builds and libqtdevices sends as ``*4*311*#23#1##``, forwarded by all 12
+# emulated gateways in the firmware oracle ``thermo-ts10`` suite; see
+# ``_zone_address_and_name``). ``#00``, ``#0#0`` and ``#0#00`` name no zone or
+# central unit, and three digits are out of range. ASCII digits only.
+_WHO4_CENTRAL_WHERE_REGEX = re.compile(
+    r"#(?:0(?:#(?:0[1-9]|[1-9][0-9]?))?|(?:0[1-9]|[1-9][0-9]?)(?:#(?:0[1-9]|[1-9][0-9]?))?)",
+    re.ASCII,
+)
+
+
 def who4_temperature(raw: str) -> float | None:
     """Decode WHO 4 temperature ``SXXX`` (tenths of a degree)."""
     if not _WHO4_TEMP_REGEX.match(raw):
@@ -1082,7 +1098,25 @@ def set_central_mode(
         would also switch the plant to summer. 311 is the generic automatic
         command libqtdevices sends to zones; on ``#0`` it puts every zone back
         on the central unit's program.
+
+        ``where`` must be a central-unit address (``#0``; ``#0#C`` with
+        ``C`` the 4-zone unit's configured zone 1..99 or ``01``..``99``;
+        ``#Z`` with ``Z`` 1..99 or ``01``..``99``; ``#Z#C`` with central
+        unit ``C`` 1..99, the form ``set_mode`` builds for ``23#1``;
+        whitespace around it is stripped): central-unit WHATs address the
+        central unit by API contract (OWNd#77 fix 3). ``#00``, ``#0#0``,
+        ``#0#00`` and three-digit values (``#999``, ``#0#100``) raise too.
+        The plain zone form ``*4*311*Z##`` is ``nack``/``silent`` on
+        all 12 emulated gateways (firmware oracle ``thermo-zone-mode`` and
+        ``ownd-pr82-heating``), and a live F461 2.0.11 NACKs ``*4*311*2##``
+        while it ACKs ``*4*311*#2##``. ``*4*1*Z##`` and ``*4*303*Z##`` are
+        forwarded by some gateways, but as zone commands, not central ones.
         """
+        where = str(where).strip() if where is not None else ""
+        if not _WHO4_CENTRAL_WHERE_REGEX.fullmatch(where):
+            raise ValueError(
+                f"Central unit mode needs a central unit address such as '#0': {where!r}"
+            )
         mode_map = {
             CLIMATE_MODE_OFF: 303,
             CLIMATE_MODE_HEAT: 1,
diff --git a/tests/test_who4_central_mode_address.py b/tests/test_who4_central_mode_address.py
new file mode 100644
index 0000000..0c513e6
--- /dev/null
+++ b/tests/test_who4_central_mode_address.py
@@ -0,0 +1,111 @@
+"""``set_central_mode`` only on a central-unit ``#`` address.
+
+Central-unit WHATs address the central unit by API contract (OWNd#77 fix 3).
+The plain zone form ``*4*311*Z##`` is ``nack``/``silent`` on all 12 emulated
+gateways (firmware oracle ``thermo-zone-mode`` and ``ownd-pr82-heating``), and
+a live F461 2.0.11 NACKs ``*4*311*2##`` while it ACKs ``*4*311*#2##``.
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from OWNd.message import CLIMATE_MODE_AUTO, CLIMATE_MODE_HEAT, CLIMATE_MODE_OFF, OWNHeatingCommand
+
+
+@pytest.mark.parametrize(
+    "where",
+    [
+        "1",
+        "2",
+        "0",
+        "23#1",
+        "",
+        " ",
+        "#",
+        "##0",
+        "#0*22",
+        "#a",
+        "#0\n1",
+        "#\u0661",
+        "#00#1",
+        "#0#",
+        # Zones and central selectors are 1..99 (or 01..99); ``#00``, ``#0#0``
+        # and ``#0#00`` name nothing, three digits are out of range.
+        "#00",
+        "#100",
+        "#999",
+        "#0#0",
+        "#0#00",
+        "#0#100",
+        "#0#999",
+        "#1#0",
+        "#1#00",
+        "#1#100",
+        "#1#2#3",
+        None,
+        True,
+        -1,
+    ],
+)
+def test_set_central_mode_requires_a_central_address(where: str) -> None:
+    with pytest.raises(ValueError, match="central unit"):
+        OWNHeatingCommand.set_central_mode(where, CLIMATE_MODE_AUTO)
+
+
+@pytest.mark.parametrize(
+    "mode", [CLIMATE_MODE_AUTO, CLIMATE_MODE_HEAT, CLIMATE_MODE_OFF]
+)
+def test_set_central_mode_refuses_a_plain_zone_for_every_mode(mode: str) -> None:
+    """The guard is per address, not per WHAT: 311, 1 and 303 alike."""
+    with pytest.raises(ValueError, match="central unit"):
+        OWNHeatingCommand.set_central_mode("1", mode)
+
+
+@pytest.mark.parametrize(
+    ("where", "frame"),
+    [
+        ("#0", "*4*311*#0##"),
+        ("#0#1", "*4*311*#0#1##"),
+        ("#0#4", "*4*311*#0#4##"),
+        ("#1", "*4*311*#1##"),
+        ("#23", "*4*311*#23##"),
+        ("#99", "*4*311*#99##"),
+        # ``#Z#C``: probe C of zone Z under the central unit, the address
+        # ``set_mode("23#1")`` builds and libqtdevices sends
+        # (``*4*311*#23#1##``, forwarded by all 12 emulated gateways in the
+        # firmware oracle ``thermo-ts10`` suite).
+        ("#23#1", "*4*311*#23#1##"),
+        ("#1#2", "*4*311*#1#2##"),
+        # ``#0#C``: the 4-zone central unit's configured zone, 1..99 or 01..99
+        # (MyHOME_Suite ``#0#[ZA][ZB]``, MHCatalogue 4695 ``ZAZB_CENTRAL``).
+        ("#0#5", "*4*311*#0#5##"),
+        ("#0#05", "*4*311*#0#05##"),
+        ("#0#12", "*4*311*#0#12##"),
+        ("#0#99", "*4*311*#0#99##"),
+        # Two-digit virtual spellings of zones and central units.
+        ("#01", "*4*311*#01##"),
+        ("#23#12", "*4*311*#23#12##"),
+        # Surrounding whitespace is stripped, as ``actuators_status`` does.
+        (" #0 ", "*4*311*#0##"),
+        ("#0\n", "*4*311*#0##"),
+    ],
+)
+def test_set_central_mode_keeps_central_addresses(where: str, frame: str) -> None:
+    command = OWNHeatingCommand.set_central_mode(where, CLIMATE_MODE_AUTO)
+
+    assert str(command) == frame
+    assert command.human_readable_log == (
+        f"Setting Central Unit {where.strip()} mode to 'auto' (code 311)."
+    )
+
+
+def test_set_central_mode_agrees_with_set_mode_on_probe_addresses() -> None:
+    """``set_mode("23#1")`` and ``set_central_mode("#23#1")`` build one frame."""
+    assert str(OWNHeatingCommand.set_mode("23#1", CLIMATE_MODE_AUTO)) == str(
+        OWNHeatingCommand.set_central_mode("#23#1", CLIMATE_MODE_AUTO)
+    )
+
+
+def test_set_central_mode_default_is_the_central_unit() -> None:
+    assert str(OWNHeatingCommand.set_central_mode(mode=CLIMATE_MODE_HEAT)) == "*4*1*#0##"