How a Radiax radiant floor system thinks, seen from the bus
This page was translated automatically from Italian. Read the original
A Radiax radiant floor system, as seen from home, is a wall display, a few probes in the rooms, and a box in the manifold compartment. Seen from its bus, it is something else, and many choices that seem strange, such as the pump staying on or the system not restarting without a display, become logical. This guide summarizes what we understood by listening for weeks to the bus of a system in use with rehumatism. It is intended for those who have such a system and want to understand it, for those who have to repair it, and for those who want to connect something to it, as we did with ORFHAn.
The cable
All devices sit on a four-wire cable, marked A B − + on the terminal blocks. A and B are an RS485 half-duplex bus; + and − carry approximately 20 V DC, which power the display and the probes. Those 20 V are produced by the transformer on the manifold board, inside the apartment.
The line runs at 9600 baud, 8 bits, no parity, one stop bit. It is not Modbus: no message has a valid CRC. It is a proprietary protocol, simple and unencrypted.
Who is in charge
The discovery that changed everything else: the bus master is not in the apartment. It is the board in the shared plant room, next to the pump and the heat meters, and it has its own power supply. We realized this by switching off everything in the house, including the manifold board: someone on the bus was still polling. Everything located in the apartment is a slave, which responds when queried:
| Address | Device | What it does |
|---|---|---|
01 | wall display | manages mode, setpoint, corrections, configuration, and device list; contains the zone 1 probe |
50 | manifold board | controls the zone valves and powers the bus |
0b–0f | wall probes | temperature and humidity for zones 2 through 6 |
54 | dehumidifier box | receives mode, thresholds, and setpoints |
How a message is structured
Every message has the same format:
02 ADDR LEN CMD [data] XOR 03
02 and 03 open and close. ADDR is the slave address, which is the same in both directions: the bus master does not have an address and never appears. LEN is the length of the entire message, from 02 to 03 inclusive. CMD is the command, and the response repeats the same number. XOR is the XOR of all bytes from the address to the last data byte.
An example, the shortest request: 02 54 06 04 56 03. Address 54, length 6, command 04, no data; and indeed 54 ⊕ 06 ⊕ 04 = 56. Temperatures travel in tenths of a degree over two bytes: 22.0 °C is 00 dc (220), 38.0 °C is 01 7c (380).
The bus master’s cycle
The bus master polls the slaves in turn, several times per second, without ever stopping: it reads the probes, writes the system status to the probes and the manifold relay board, and asks the touch panel for the settings. To those that do not respond, it repeats the question, usually three times in a row.
The cycle has practical consequences. A setpoint changed on the touch panel reaches the probes after 8-25 seconds, depending on their turn. And a disconnected touch panel slows everything down: the bus master continues to call it, waits in vain for a response, and the rest of the cycle drops to about one-third of the speed.
The display is the system’s memory
In a Radiax, the display is not a simple screen. It holds the mode, the setpoint, the corrections for each room, the configuration, and the list of devices in the system. The bus master remembers nothing on its own: everything it does, it does because the display tells it to.
This is clearly visible upon power-up. After a complete shutdown, the bus master asks the display for the list of devices—four pages containing the address and type of every board and probe—and until it has received it, it commands nothing. Once, it took more than four minutes. This explains two things: why a replacement board works as soon as it is mounted, without configuration, and why a faulty display is not just an aesthetic failure but the end of the regulation.
The display does not even have a battery for the clock. Every time it is powered on, it starts from Friday, January 1st, 2010, at midnight, and after a couple of minutes, the bus master writes the correct time and date to it.
The probes
A wall probe says little and always the same things. Upon a reading request, it responds with the temperature in tenths, the humidity in percent (ff if it does not measure it), and the current room offset. The bus master writes the system status to it: whether the zone is requesting, the setpoint, the offset, and the mode. The probe simply confirms.
The probes are also the weak point of the bus: they are far away, powered by the cable, and when the line is loaded, they are the first to go unheard. To a zone that does not respond, the bus master writes a setpoint of 51.0 °C, and in winter, that zone remains in request even when the system is switched off. In our case, the kitchen probe remained silent for three weeks: the fault lay with the USB-RS485 dongle used for listening, which had been left connected to the bus. Once disconnected, the probe returned on its own.
The season, and the two “off” states
The season is decided by the bus master, not the display: it is the building’s controller that switches from cooling to heating. The bus master communicates this to the display when it finds it, with a different value in summer and winter, and the display adjusts accordingly.
The interesting part is the off state. In these systems, turning off is not a command: it is a setpoint chosen so that no room requests anything. In summer, it is 38.0 °C, and no room requests cooling. In winter, it is 7.0 °C, and no room requests heat. A display that sends the “off” value for the wrong season achieves the opposite effect: with 38.0 °C in the middle of heating season, every room is “cold”, all valves open, and the pump starts. This actually happened to us, on the day the building switched to heating, with the ORFHAn panel acting as the display. The panel’s firmware has been corrected: it now takes the “off” value from the season reported by the bus master.
The pump and consumption
The circulator pump only has phase, neutral, and earth, and it is switched on by a relay from the bus master. There is no command for the pump: the bus master turns it on when the display tells it that the system is on and a zone is requesting. And since it continues with the last settings read, if the display turns off while the system is on, the pump stays on until someone tells it “off” again.
The bus master is located next to the heat meter and the water meter: it reads them and sends the consumption to the display once an hour, with the history of the last eighteen periods.
What follows
- Everything inside the apartment can be replaced, because they are all slaves. The bus master cannot: it belongs to the building.
- Whoever replaces the display controls the system. This is the method used by ORFHAn, without touching anything in the shared plant room.
- Two devices on the same address will conflict: whoever takes the place of the display must disconnect the original one.
- Everything connected to the bus loads it: once a listening session is finished, the key is disconnected.
What is not yet known
The dehumidifier box responds rarely and must be studied on the bench. Some message fields remain meaningless. Summer operation has been thoroughly observed; winter operation has been observed in its essential parts (the off state, the season, the pump stopped), but not yet in every detail, for example the water temperature with the pump running.
How it was done
Everything comes from listening to the bus of a single system, owned and in use, with a USB-RS485 dongle connected in parallel. No firmware was read or copied. Transmission tests came later, one by one, starting with those that could not cause interference. Different systems and versions may behave differently. The recordings are not published, because they reveal the consumption and habits of a real home; the complete protocol documentation, listening tools, and legal notes are in the rehumatism repository.