An electric radiator with a built-in display, sold by Atlantic mainly in France. It speaks io-homecontrol two-way, so the hub sets its temperature and mode and reads back the room temperature and whether it is heating. Three radiators are confirmed on a Heltec V3 (SX1262) with the climate: platform; see Heating and climate for every option.

What works

Capability	Status
Discover & Pair	✅ Confirmed on three radiators; the hub pulls each radiator's own key
Target temperature	✅ Confirmed — the display follows, and the hub reads the value back
Off, Heat, Auto and the Program preset	✅ Confirmed — the display follows, and the hub reads the mode back
Room temperature	✅ Confirmed
Heating / idle	✅ Confirmed; follows a change from Home Assistant at the poll 45 seconds later, other changes at the regular poll (every 5 minutes by default)
Maximum setpoint	✅ Confirmed — the radiator reports 28.0 °C
home / away presets	⚠️ The radiator acknowledges both; no visible change on the display yet (in AUTO, the manual describes a gradual reduction after half an hour away)
Window sensor	✅ Confirmed — turns on when a window near the radiator is opened
Open Window Detection switch	⚠️ Reads back the radiator's setting; switching detection off from Home Assistant is not tested yet

Modes in Home Assistant

The radiator has three operating modes, described in its installation manual. Home Assistant has no mode for a weekly program, so the program mode appears as auto with the Program preset:

Radiator display	Home Assistant
BASIC — holds the set temperature, smart functions off	heat
AUTO — learns when the room is used and adjusts the temperature	auto, no preset
PROG — follows the radiator's weekly program	auto with the Program preset
Off	off

Choosing the Program preset switches the radiator to PROG; choosing heat or off leaves it. The weekly program itself is edited on the radiator.

How a radiator appears on the radio

Each radiator answers as two io-homecontrol nodes:

The pairing node answers Discover & Pair. It presents itself as a remote controller (node type 1023), refuses a key from the hub, and hands over its own key instead.
The heater is an electrical heater with an adjustable setpoint (node type 52, subtype 1), and it is the node your climate: entry controls. On a radiator as delivered it sits at the next address up (pairing node + 1).

Each radiator has a key of its own. The hub handles both points for you: Discover & Pair pulls the key, reads the heater at the next address under that key, and prints a block for the heater with that key in it. If the heater does not answer that read, the hub looks for it with a roll-call instead, which takes about 20 seconds.

Resetting a radiator's connections

The radiator's Expert menu (hold both arrow keys for more than 3 seconds) can delete all its connections. That gives the radiator a new key and new addresses for both nodes, and the new heater address is unrelated to the new pairing address. Your existing climate: entry for it stops working: pair the radiator again and replace the entry with the new block. The hub finds the heater with its roll-call, so this pairing takes about 20 seconds.

Pairing a radiator

Pair one radiator at a time. The menus are in French.

On the radiator, press the round button to open the menu.
Scroll down to Paramètres and press the round button.
Choose Connectivité and press the round button.
Choose Connexion and press the round button. The radiator shows an hourglass while it waits.
In Home Assistant, press Discover & Pair on the hub.

The pairing takes a few seconds, or about 15 seconds when the hub has to fall back to a roll-call to find the heater. The radiator then reports that the device was added ("Appareil ajouté avec succès"). The hub's log prints a report between DEVICE KEY RECEIVED -- DO NOT SHARE IT and END OF DEVICE KEY REPORT, with a climate: block for the heater that includes the radiator's system_key:.

Put the key into secrets.yaml, not into your main YAML.
Paste the block under climate:, replace the key with !secret ..., and give it a name.
Flash the hub.

Until you flash, the hub controls the heater with the pulled key; after a reboot it needs the system_key: in your YAML.

When no block is printed

If neither the direct read nor the roll-call reaches the heater, the log says outcome=key_received and prints the key without a block for the heater. Run Discover & Pair again with the radiator in Connexion mode, or put the key into a climate: entry's system_key:, flash, and press Scan Paired Devices. The heater answers only some roll-call requests, so repeat the scan if it finds nothing.

A radiator that already holds the hub's own key (for example, one you paired before by setting its key as the hub's system_key:) gets a block without system_key:, because its heater uses the hub's key. If the hub cannot reach the heater, the attempt ends as hub_key_returned: press Scan Paired Devices, and again if nothing answers.

Pairing the same radiator again before you flash prints its block again. Press Discover & Pair once per attempt: a press made while an attempt runs is ignored, with a log line saying so.

Working YAML

climate:
  - platform: home_io_control
    name: "Bedroom radiator"
    io_device_id: "986111"
    io_device_type: "electrical_heater"
    io_subtype: 1
    system_key: !secret bedroom_radiator_key

io_device_id is the heater's address as the printed block shows it. Give every radiator its own entry with its own system_key:.

Known quirks

Heating / idle trails the radiator. The hub reads the regulator when it polls: 45 seconds after a change from Home Assistant, and every status_poll_interval otherwise. A change made at the radiator, or a failed poll, can leave the card behind until the next poll.
A radiator far from the hub misses some replies. A radiator received at about -60 dBm lost several replies to the hub's challenge answer, so its Unconfirmed Exchanges counter rose and a setpoint change sometimes had to be repeated. A radiator at about -40 dBm had no such failures.
The pairing can lose the radiator's proof of its key. The hub challenges up to three times. If the log still says not proven: no 0x3D received, press Discover & Pair again. The radiator hands over the same key every time.
The device name is generic. The heater reports its name as I2G_Actuator.

Evidence

Six corpus captures from issue #165, on a Heltec V3 with the SX1262 driver:

An earlier capture shows the radiator refusing a pushed key: atlantic_agilia_pairing_key_transfer_command_unknown.