S55770-T350 RDF800KN KNX Room Thermostat Touch Screen 230V Fan Coil Two Universal Input Channels

The RDF800KN is a mains-powered room thermostat that communicates over KNX — a combination that is less common than it sounds. Most KNX thermostats are bus-powered with a small display; this unit accepts AC 230 V, switches its own fan and valve loads through a 5 A relay, and still publishes every setpoint change, mode change and input reading onto the bus as objects.

The result is a device that can run a fan coil unit on its own if the bus is down, and that hands the same information to a supervisor when the bus is up. It covers 2-pipe fan coils, 2-pipe systems with an electric heater stage, and 4-pipe systems, plus universal heating or cooling applications where the thermostat is only providing the control loop.

Two universal inputs are the feature that makes the device useful beyond a plain thermostat. Each can accept an NTC temperature sensor, a 0 to 10 V signal or a potential-free contact depending on how it is configured. A single article can therefore serve as a room thermostat with a remote sensor, a thermostat with a condensate alarm input, or a thermostat that reads a window contact and shuts the valve when the window opens.

That flexibility is worth more than the input count suggests, because it removes the separate binary input module that would otherwise be needed at the room. The mechanical side is straightforward: it mounts on a round or square conduit box, which in a refurbishment means the existing box can be reused, and the touch screen removes the dial that collects dust in a hotel room.

Mains-powered KNX thermostats are specified where the fan coil is local and the control has to survive a bus fault. Hotel guest rooms use them because the fan and valve are wired to the thermostat in the room rather than back to a riser, and because housekeeping can still adjust a room when the bus segment is being serviced.

Small offices apply them to perimeter fan coil units where each room has its own thermostat and the KNX bus is used for scheduling and supervision rather than for local control. Clinics and consulting rooms specify them where a window contact must override the cooling, and laboratories use the 0 to 10 V input for a remote sensor when the thermostat has to be mounted outside the controlled space.